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Smarter Use of
           Fast-neutron reactors
               could extract
            much more energy
               from recycled
                nuclear fuel,
            minimize the risks
         of weapons proliferation
           and markedly reduce
          the time nuclear waste
             must be isolated

         By William H. Hannum,
          Gerald E. Marsh and
            George S. Stanford




               D                       espite long-standing
public concern about the safety of nuclear energy, more and
more people are realizing that it may be the most environmen-
tally friendly way to generate large amounts of electricity.
Several nations, including Brazil, China, Egypt, Finland, In-
dia, Japan, Pakistan, Russia, South Korea and Vietnam, are
building or planning nuclear plants. But this global trend has
not as yet extended to the U.S., where work on the last such
                                                                     If developed sensibly, nuclear power could be truly sustain-
                                                                 able and essentially inexhaustible and could operate without
                                                                 contributing to climate change. In particular, a relatively new
                                                                 form of nuclear technology could overcome the principal
                                                                 drawbacks of current methods — namely, worries about reac-
                                                                 tor accidents, the potential for diversion of nuclear fuel into
                                                                 highly destructive weapons, the management of dangerous,
                                                                 long-lived radioactive waste, and the depletion of global re-
                                                                                                                                    JANA BRENNING




facility began some 30 years ago.                                serves of economically available uranium. This nuclear fuel

84   SCIENTIFIC A MERIC A N                                                                                     DECEMBER 2005
                                             COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
NUCLEAR
                            WASTE
                                                                   ar power plants contain what are called thermal reactors,
                                                                   which are driven by neutrons of relatively low speed (or ener-
                                                                   gy) ricocheting within their cores. Although thermal reactors
                                                                   generate heat and thus electricity quite efficiently, they cannot
                                                                   minimize the output of radioactive waste.
                                                                       All reactors produce energy by splitting the nuclei of heavy-
                                                                   metal (high-atomic-weight) atoms, mainly uranium or elements
                                                                   derived from uranium. In nature, uranium occurs as a mixture
                                                                   of two isotopes, the easily fissionable uranium 235 (which is
                                                                   said to be “fissile”) and the much more stable uranium 238.
                                                                       The uranium fire in an atomic reactor is both ignited and
                                                                   sustained by neutrons. When the nucleus of a fissile atom is hit
                                                                   by a neutron, especially a slow-moving one, it will most likely
                                                                   cleave (fission), releasing substantial amounts of energy and
                                                                   several other neutrons. Some of these emitted neutrons then
                                                                   strike other nearby fissile atoms, causing them to break apart,
                                                                   thus propagating a nuclear chain reaction. The resulting heat
                                                                   is conveyed out of the reactor, where it turns water into steam
                                                                   that is used to run a turbine that drives an electric generator.
                                                                       Uranium 238 is not fissile; it is called “fissionable” be-
                                                                   cause it sometimes splits when hit by a fast neutron. It is also
                                                                   said to be “fertile,” because when a uranium 238 atom ab-
                                                                   sorbs a neutron without splitting, it transmutes into plutoni-
                                                                   um 239, which, like uranium 235, is fissile and can sustain a
                                                                   chain reaction. After about three years of service, when tech-
cycle would combine two innovations: pyrometallurgical pro-        nicians typically remove used fuel from one of today’s reac-
cessing (a high-temperature method of recycling reactor waste      tors because of radiation-related degradation and the deple-
into fuel) and advanced fast-neutron reactors capable of burn-     tion of the uranium 235, plutonium is contributing more than
ing that fuel. With this approach, the radioactivity from the      half the power the plant generates.
generated waste could drop to safe levels in a few hundred             In a thermal reactor, the neutrons, which are born fast, are
years, thereby eliminating the need to segregate waste for tens    slowed (or moderated) by interactions with nearby low-atomic-
of thousands of years.                                             weight atoms, such as the hydrogen in the water that flows
    For neutrons to cause nuclear fission efficiently, they must     through reactor cores. All but two of the 440 or so commercial
be traveling either slowly or very quickly. Most existing nucle-   nuclear reactors operating are thermal, and most of them— in-

w w w. s c ia m . c o m                                                                                 SCIENTIFIC A MERIC A N   85
                                           COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
cluding the 103 U.S. power reactors —         left over from the enrichment process.         about 1 percent of the spent fuel, they
 employ water both to slow neutrons and            The spent fuel consists of three class-    constitute the main source of today’s nu-
 to carry fission-created heat to the asso-     es of materials. The fission products,         clear waste problem. The half-lives (the
 ciated electric generators. Most of these     which make up about 5 percent of the           period in which radioactivity halves) of
 thermal systems are what engineers call       used fuel, are the true wastes— the ashes,     these atoms range up to tens of thousands
 light-water reactors.                         if you will, of the fission fire. They com-      of years, a feature that led U.S. govern-
     In any nuclear power plant, heavy-        prise a mélange of lighter elements cre-       ment regulators to require that the
 metal atoms are consumed as the fuel          ated when the heavy atoms split. The mix       planned high-level nuclear waste reposi-
 “burns.” Even though the plants begin         is highly radioactive for its first several     tory at Yucca Mountain in Nevada iso-
 with fuel that has had its uranium 235        years. After a decade or so, the activity is   late spent fuel for over 10,000 years.
 content enriched, most of that easily fis-     dominated by two isotopes, cesium 137
 sioned uranium is gone after about three      and strontium 90. Both are soluble in          An Outdated Strategy
 years. When technicians remove the de-        water, so they must be contained very se-      e a r ly n u c l e a r engineers expected
 pleted fuel, only about one twentieth of      curely. In around three centuries, those       that the plutonium in the spent fuel of
 the potentially fissionable atoms in it       isotopes’ radioactivity declines by a fac-     thermal reactors would be removed and
 (uranium 235, plutonium and uranium           tor of 1,000, by which point they have         then used in fast-neutron reactors,
 238) have been used up, so the so-called      become virtually harmless.                     called fast breeders because they were
 spent fuel still contains about 95 percent        Uranium makes up the bulk of the           designed to produce more plutonium
 of its original energy. In addition, only     spent nuclear fuel (around 94 percent);        than they consume. Nuclear power pio-
 about one tenth of the mined uranium          this is unfissioned uranium that has lost       neers also envisioned an energy econo-
 ore is converted into fuel in the enrich-     most of its uranium 235 and resembles          my that would involve open commerce
 ment process (during which the concen-        natural uranium (which is just 0.71 per-       in plutonium. Plutonium can be used to
 tration of uranium 235 is increased con-      cent fissile uranium 235). This compo-          make bombs, however. As nuclear tech-
 siderably), so less than a hundredth of the   nent is only mildly radioactive and, if        nology spread beyond the major super-
 ore’s total energy content is used to gen-    separated from the fission products and         powers, this potential application led to
 erate power in today’s plants.                the rest of the material in the spent fuel,    worries over uncontrolled proliferation
     This fact means that the used fuel        could readily be stored safely for future      of atomic weapons to other states or
 from current thermal reactors still has the   use in lightly protected facilities.           even to terrorist groups.
 potential to stoke many a nuclear fire. Be-        The balance of the material— the tru-          The Nuclear Non-Proliferation
 cause the world’s uranium supply is finite     ly troubling part — is the transuranic         Treaty partially addressed that problem
 and the continued growth in the num-          component, elements heavier than ura-          in 1968. States that desired the benefits
 bers of thermal reactors could exhaust        nium. This part of the fuel is mainly a        of nuclear power technology could sign
 the available low-cost uranium reserves       blend of plutonium isotopes, with a sig-       the treaty and promise not to acquire
 in a few decades, it makes little sense to    nificant presence of americium. Although        nuclear weapons, whereupon the weap-
 discard this spent fuel or the “tailings”     the transuranic elements make up only          ons-holding nations agreed to assist the
                                                                                              others with peaceful applications. Al-
Overview/Nuclear Recycling                                                                    though a cadre of international inspec-
                                                                                              tors has since monitored member adher-
   ■   To minimize global warming, humanity may need to generate much of its                  ence to the treaty, the effectiveness of
       future energy using nuclear power technology, which itself releases                    that international agreement has been
       essentially no carbon dioxide.                                                         spotty because it lacks effective author-
   ■   Should many more of today’s thermal (or slow-neutron) nuclear power plants             ity and enforcement means.
       be built, however, the world’s reserves of low-cost uranium ore will be tapped             Nuclear-weapons designers require
       out within several decades. In addition, large quantities of highly radioactive        plutonium with a very high plutonium
       waste produced just in the U.S. will have to be stored for at least 10,000             239 isotopic content, whereas plutonium
       years — much more than can be accommodated by the Yucca Mountain                       from commercial power plants usually
       repository in Nevada. Worse, most of the energy that could be extracted from           contains substantial quantities of the
       the original uranium ore would be socked away in that waste.                           other isotopes of plutonium, making it
   ■   The utilization of a new, much more efficient nuclear fuel cycle — one based on         difficult to use in a bomb. Nevertheless,
       fast-neutron reactors and the recycling of spent fuel by pyrometallurgical             use of plutonium from spent fuel in
       processing — would allow vastly more of the energy in the earth’s readily              weapons is not inconceivable. Hence,
       available uranium ore to be used to produce electricity. Such a cycle would            President Jimmy Carter banned civilian
       greatly reduce the creation of long-lived reactor waste and could support              reprocessing of nuclear fuel in the U.S.
       nuclear power generation indefinitely.                                                  in 1977. He reasoned that if plutonium
                                                                                              were not recovered from spent fuel it

 86    SCIENTIFIC A MERIC A N                                                                                         DECEMBER 2005
                                                  COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
NEW TYPE OF NUCLEAR REACTOR
              A safer, more sustainable                     which rely on relatively slow             would burn fuel made by                   like this: Nuclear fire burning in
              nuclear power cycle could be                  moving neutrons to propagate              recycling spent fuel from                 the core would heat the
              based on the advanced liquid-                 chain reactions in uranium and            thermal reactors.                         radioactive liquid sodium
              metal reactor (ALMR) design                   plutonium fuel. An ALMR-based                 In most thermal-reactor               running through it. Some of the
              developed in the 1980s by                     system, in contrast, would use            designs, water floods the core             heated sodium would be pumped
              researchers at Argonne                        fast-moving (energetic)                   to slow (moderate) neutrons               into an intermediate heat
              National Laboratory. Like all                 neutrons. This process permits            and keep it cool. The ALMR,               exchanger (2), where it would
              atomic power plants, an ALMR-                 all the uranium and heavier               however, employs a pool of                transfer its thermal energy to
              based system would use                        atoms to be consumed, thereby             circulating liquid sodium as the          nonradioactive liquid sodium
              nuclear chain reactions in the                allowing vastly more of the               coolant (1). Engineers chose              flowing through the adjacent but
              core to produce the heat                      fuel’s energy to be captured. In          sodium because it does not                separate pipes (3) of a
              needed to generate electricity.               the near term, the new reactor            slow down fast neutrons                   secondary sodium loop. The
                  Current commercial nuclear                                                          substantially and because it              nonradioactive sodium (4) would
              plants feature thermal reactors,                                                        conducts heat very well, which            in turn bring heat to a final heat
                                                                                                      improves the efficiency of heat            exchanger/steam generator (not
                                                                                                      delivery to the electric                  shown), where steam would be
                                                                                                      generation facility.                      created in adjacent water-filled
                                           Warm air
                                                                                                          A fast reactor would work             pipes. The hot, high-pressure
              Cool air                                                                                                                          steam would then be used to turn
                                                                                                                                                steam turbines that would drive
                                                                                                                                                electricity-producing generators
                                                                                           Cooling-system air inlet                             (not shown).
                                             To steam             Top of reactor silo      and exhaust stack
                                             generator
                                                                                           Freestanding
                                                                                           reactor housing
                                                                                                                                                              l
                                                                                                                                                nd   le v e
                                                                                                              Reactor foundation       G   r ou




                                                 4                 Secondary
                                                                   sodium loop




             Intermediate
                                                                                                              Sodium pump
             heat exchanger                                   3

                                                                  2                                                                                     REACTOR SAFEGUARDS
                                                                                                                                                        ■         During operation, powerful
                                                                                                                                                                  pumps would force sodium
             Sodium pump                                                                                                                                          coolant through the core. If the
                                                                                                                      Sodium coolant                              pumps failed, gravity would
                                                                                                                      pumped through                              circulate the coolant.
                                                                        1                                             core
                                                                                                                                                        ■         If coolant pumps malfunctioned
                                                                                                                      Sodium cycling                              or stopped, special safety
                                                                                                                      through heat                                devices would also permit
                                                                                                                      exchanger                                   extra neutrons to leak out
                                                                                                                                                                  of the core, lowering its
                                                                                                                      Nonradioactive                              temperature.
                                                                                                                      sodium cycling                    ■         In an emergency, six neutron-
                                                                                                                      through steam                               absorbing control rods would
             Liquid-sodium pool                                                           Seismic isolator            generator
                                                                                                                                                                  drop into the core to shut it
                                                                                                                                                                  down immediately.
                                                                                                                                                        ■         Should chain reactions
                              Hot reactor core             Reactor vessel                                                                                         continue, thousands of
                              (uranium fuel rods)                                                                                                                 neutron-absorbing boron
                                                                                                                                                                  carbide balls would be
DON FOLE Y




                                                         Base of reactor silo                                                                                     released into the core,
                                                                                                                                                                  guaranteeing shutdown.



                 w w w. s c ia m . c o m                                                                                                                           SCIENTIFIC A MERIC A N      87
                                                                                COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
could not be used to make bombs. Car-          below]. Several such reactors have been            transuranic atoms will do so as well.
ter also wanted America to set an ex-          built and used for power generation— in                Water cannot be employed in a fast
ample for the rest of the world. France,       France, Japan, Russia, the U.K. and the            reactor to carry the heat from the core —
Japan, Russia and the U.K. have not,           U.S.— two of which are still operating             it would slow the fast neutrons. Hence,
however, followed suit, so plutonium re-       [see “Next-Generation Nuclear Power,”              engineers typically use a liquid metal
processing for use in power plants con-        by James A. Lake, Ralph G. Bennett and             such as sodium as a coolant and heat
tinues in a number of nations.                 John F. Kotek; Scientific American,                 transporter. Liquid metal has one big ad-
                                               January 2002].                                     vantage over water. Water-cooled sys-
An Alternative Approach                            Fast reactors can extract more energy          tems run at very high pressure, so that a
w h e n t h e b a n was issued, “repro-        from nuclear fuel than thermal reactors            small leak can quickly develop into a
cessing” was synonymous with the               do because their rapidly moving (higher-           large release of steam and perhaps a seri-
PUREX (for plutonium uranium extrac-           energy) neutrons cause atomic fissions              ous pipe break, with rapid loss of reactor
tion) method, a technique developed to         more efficiently than the slow thermal              coolant. Liquid-metal systems, however,
meet the need for chemically pure pluto-       neutrons do. This effectiveness stems              operate at atmospheric pressure, so they
nium for atomic weapons. Advanced              from two phenomena. At slower speeds,              present vastly less potential for a major
fast-neutron reactor technology, how-          many more neutrons are absorbed in                 release. Nevertheless, sodium catches fire
ever, permits an alternative recycling         nonfission reactions and are lost. Second,          if exposed to water, so it must be man-
strategy that does not involve pure plu-       the higher energy of a fast neutron makes          aged carefully. Considerable industrial
tonium at any stage. Fast reactors can         it much more likely that a fertile heavy-          experience with handling the substance
thus minimize the risk that spent fuel         metal atom like uranium 238 will fission            has been amassed over the years, and
from energy production would be used           when struck. Because of this fact, not             management methods are well devel-
for weapons production, while provid-          only are uranium 235 and plutonium                 oped. But sodium fi res have occurred,
ing a unique ability to squeeze the maxi-      239 likely to fission in a fast reactor, but        and undoubtedly there will be more. One
mum energy out of nuclear fuel [see box        an appreciable fraction of the heavier             sodium fire began in 1995 at the Monju


   NEW WAY TO REUSE NUCLEAR FUEL
     The key to pyrometallurgical recycling of nuclear fuel is the    plutonium fuel from fast reactors would go straight to the
     electrorefining procedure. This process removes the true          electrorefiner. Electrorefining resembles electroplating:
     waste, the fission products, from the uranium, plutonium and      spent fuel attached to an anode would be suspended in a
     the other actinides (heavy radioactive elements) in the spent    chemical bath; then electric current would plate out uranium
     fuel. The actinides are kept mixed with the plutonium so it      and other actinides on the cathode. The extracted elements
     cannot be used directly in weapons.                              would next be sent to the cathode processor to remove
         Spent fuel from today’s thermal reactors (uranium and        residual salts and cadmium from refining. Finally, the
     plutonium oxide) would first undergo oxide reduction to           remaining uranium and actinides would be cast into fresh fuel
     convert it to metal, whereas spent metallic uranium and          rods, and the salts and cadmium would be recycled.
         Oxide fuel from
         thermal reactors                 Most of the                Cathode                                                    Casting
                                          fission products                                               Heating                 molds
                                                                                                        elements
                                                                                  Uranium
                                                      Anode                       and actinides

                                                    Metal


                                                                                                               Crucible
                                                                                 Chopped
                                                                                 fuel


                                                    Metal                      Salts and cadmium

                                                                                                                          New metal
                                                                                                                          fuel rods
  Chopped metallic fuel
  from fast reactors
                                    OXIDE                                                   CATHODE                    INJECTION
                                                                                                                                               DON FOLE Y




         SPENT FUEL             REDUCTION UNIT              ELECTROREFINER                 PROCESSOR                 CASTING SYSTEM



88    SCIENTIFIC A MERIC A N                                                                                                DECEMBER 2005
                                                 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
fast reactor in Japan. It made a mess in      full batch is amassed, operators remove
                                     the reactor building but never posed a        the electrode. Next they scrape the ac-
                                     threat to the integrity of the reactor, and   cumulated materials off the electrode,
                                     no one was injured or irradiated. Engi-       melt them down, cast them into an ingot
                                     neers do not consider sodium’s flamma-         and pass the ingot to a refabrication line
                                     bility to be a major problem.                 for conversion into fast-reactor fuel.
                                         Researchers at Argonne National           When the bath becomes saturated with
                                     Laboratory began developing fast-reac-        fission products, technicians clean the
                                     tor technology in the 1950s. In the 1980s     solvent and process the extracted fission
                                     this research was directed toward a fast      products for permanent disposal.
                                     reactor (dubbed the advanced liquid-              Thus, unlike the current PUREX
                                     metal reactor, or ALMR), with metallic        method, the pyroprocess collects virtu-
                                     fuel cooled by a liquid metal, that was to    ally all the transuranic elements (includ-
                                     be integrated with a high-temperature         ing the plutonium), with considerable
                                     pyrometallurgical processing unit for re-     carryover of uranium and fission prod-
                                     cycling and replenishing the fuel. Nucle-     ucts. Only a very small portion of the
                                     ar engineers have also investigated sev-      transuranic component ends up in the fi-
                                     eral other fast-reactor concepts, some        nal waste stream, which reduces the
                                     burning metallic uranium or plutonium         needed isolation time drastically. The
                                     fuels, others using oxide fuels. Coolants     combination of fission products and
                                     of liquid lead or a lead-bismuth solution     transuranics is unsuited for weapons or
                                     have been used. Metallic fuel, as used in     even for thermal-reactor fuel. This mix-
                                     the ALMR, is preferable to oxide for sev-     ture is, however, not only tolerable but
                                     eral reasons: it has some safety advan-       advantageous for fueling fast reactors.
                                     tages, it will permit faster breeding of          Although pyrometallurgical recy-                     EX TR ACTED UR ANIUM and actinide elements
                                     new fuel, and it can more easily be paired    cling technology is not quite ready for                  from spent thermal-reactor fuel are plated out
                                     with pyrometallurgical recycling.             immediate commercial use, researchers                    on the cathode of an electrorefiner during the
                                                                                   have demonstrated its basic principles. It               pyroprocessing procedure. After further
                                     Pyroprocessing                                has been successfully demonstrated on a                  processing, the metallic fuel can be burned in
                                                                                                                                            fast-neutron reactors.
                                     t h e p y rom e ta l lu rgic a l process      pilot level in operating power plants,
                                     (“pyro” for short) extracts from used         both in the U.S. and in Russia. It has not               with the same electrical capacity, in con-
                                     fuel a mix of transuranic elements in-        yet functioned, however, on a full pro-                  trast, is a little more than a single ton of
                                     stead of pure plutonium, as in the            duction scale.                                           fission products, plus trace amounts of
                                     PUREX route. It is based on electroplat-                                                               transuranics.
                                     ing— using electricity to collect, on a       Comparing Cycles                                             Waste management using the ALMR
                                     conducting metal electrode, metal ex-         t h e op e r at i ng c a pa bi l i t i e s of            cycle would be greatly simplified. Be-
                                     tracted as ions from a chemical bath. Its     thermal and fast reactors are similar in                 cause the fast-reactor waste would con-
                                     name derives from the high tempera-           some ways, but in others the differences                 tain no significant quantity of long-lived
                                     tures to which the metals must be sub-        are huge [see box on next page]. A                       transuranics, its radiation would decay
                                     jected during the procedure. Two simi-        1,000-megawatt-electric thermal-reac-                    to the level of the ore from which it came
                                     lar approaches have been developed,           tor plant, for example, generates more                   in several hundred years, rather than
                                     one in the U.S., the other in Russia. The     than 100 tons of spent fuel a year. The                  tens of thousands.
                                     major difference is that the Russians         annual waste output from a fast reactor                      If fast reactors were used exclusively,
                                     process ceramic (oxide) fuel, whereas
                                     the fuel in an ALMR is metallic.                            WILLIAM H. HANNUM, GERALD E. MARSH and GEORGE S. STANFORD are physicists who
                                                                                   THE AUTHORS




                                         In the American pyroprocess [see                        worked on fast-reactor development before retiring from the U.S. Department of Energy’s
                                     box on opposite page], technicians dis-                     Argonne National Laboratory. Hannum served as head of nuclear physics development
                                     solve spent metallic fuel in a chemical
A RGONNE N ATION A L L A BOR ATORY




                                                                                                 and reactor safety research at the DOE. He was also deputy director general of the Nucle-
                                     bath. Then a strong electric current se-                    ar Energy Agency of the Organization for Economic Co-operation and Development in
                                     lectively collects the plutonium and the                    Paris. Marsh, a fellow of the American Physical Society, worked as a consultant to the U.S.
                                     other transuranic elements on an elec-                      Department of Defense on strategic nuclear technology and policy in the Reagan, Bush
                                     trode, along with some of the fission                        and Clinton administrations and is co-author of The Phantom Defense: America’s Pursuit
                                     products and much of the uranium.                           of the Star Wars Illusion (Praeger Press). Stanford, whose research focused on experi-
                                     Most of the fission products and some of                     mental nuclear physics, reactor physics and fast-reactor safety, is co-author of Nucle-
                                     the uranium remain in the bath. When a                      ar Shadowboxing: Contemporary Threats from Cold War Weaponry (Fidlar Doubleday).


                                     w w w. s c ia m . c o m                                                                                                 SCIENTIFIC A MERIC A N     89
                                                                                   COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
COMPARING THREE NUCLEAR FUEL CYCLES
     Three major approaches to burning nuclear fuel and handling its wastes can be employed; some of their features are noted below.

           ONCE-THROUGH ROUTE                              PLUTONIUM RECYCLING                                      FULL RECYCLING
 Fuel is burned in thermal reactors and is not    Fuel is burned in thermal reactors, after which    Recycled fuel prepared by pyrometallurgical
         reprocessed; occurs in the U.S.           plutonium is extracted using what is called      processing would be burned in advanced fast-
                                                         PUREX processing; occurs in other             neutron reactors; prototype technology
                                                                developed nations

                                                                 FUEL UTILIZATION

                                                                                                       5 percent
                                                                                                          used in
  5 percent                                         6 percent                                            thermal
     is used                                           is used                                            reactor

 95 percent                                                                                           Somewhat
                                                  94 percent                                           more than
  is wasted                                        is wasted                                          94 percent
                                                                                                       is used in
                                                                                                     fast reactor                        Less than
                   Initial fuel supply                                                                                                   1 percent
                                                                                                                                         is wasted

Uses about 5 percent of energy in thermal-       Uses about 6 percent of energy in original         Can recover more than 99 percent of energy
 reactor fuel and less than 1 percent             reactor fuel and less than 1 percent of            in spent thermal-reactor fuel
 of energy in uranium ore (the original           energy in uranium ore                             After spent thermal-reactor fuel runs out,
 source of fuel)                                 Cannot burn depleted uranium or uranium              can burn depleted uranium to recover more
Cannot burn depleted uranium (that part           in spent fuel                                       than 99 percent of the rest of the energy
 removed when the ore is enriched) or                                                                 in uranium ore
 uranium in spent fuel
                                                    REQUIRED FACILITIES AND OPERATIONS
     Red: requires rigorous physical safeguards Orange: needs only moderate physical safeguards Blue: potential risks for future generations

Uranium mines                                    Uranium mines                                      On-site fuel fabrication
Fuel enrichment to concentrate fissile            Fuel enrichment                                    On-site pyrometallurgical processing
 uranium                                         Plutonium blending (mixing)                         (prompt recycling of spent fuel)
Fuel fabrication                                 Off-site fuel fabrication                          Power plants
Power plants                                     Off-site PUREX reprocessing                        On-site waste processing
Interim waste storage (until waste can be        Power plants                                       Storage able to segregate waste for less
  permanently disposed of)                                                                            than 500 years
                                                 Interim waste storage
Permanent storage able to                                                                           (No mining needed for centuries; no uranium
 securely segregate waste for 10,000 years       Off-site waste processing                            enrichment needed, ever)
(Needs no plutonium handling or waste            Permanent storage able to securely
  processing operations)                          segregate waste for 10,000 years

                                                                 PLUTONIUM FATE

Increasing inventories of plutonium              Increasing inventories of plutonium                Inventories eventually shrink to only what is
  in used fuel                                     in used fuel and available for economic trade      in use in reactors and in recycling
Excess weapons-grade plutonium degraded          Excess weapons-grade plutonium degraded            Existing excess weapons-grade plutonium can
 only slowly by mixing into fresh fuel            only slowly by mixing into fresh fuel              be degraded rapidly
                                                                                                    Plutonium in the fuel is too impure for diversion
                                                                                                      to weapons

                                                                 TYPES OF WASTE

Energy-rich used fuel isolated in containers     Energy-rich, highly stable glassy waste            Tailored waste forms that would only have to
 and underground storage facility                Waste is radioactive enough to be defined as          remain intact for 500 years, after which
Waste is radioactive enough to be defined as       “self-protected” for a few hundred years            material would no longer be hazardous
 “self-protected” for a few hundred years         against most groups wanting to obtain             Lacking plutonium, waste would not be useful
                                                                                                                                                        DON FOLE Y




 against most groups wanting to obtain            plutonium 239 for building nuclear weapons         for making weapons
 plutonium 239 for building nuclear weapons



90   SCIENTIFIC A MERIC A N                                                                                                       DECEMBER 2005
                                                    COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
transportation of highly radioactive ma-     nalities,” the hard-to-quantify costs of        overnight, of course. If we were to begin
terials would occur only under two cir-      outside effects resulting from using the        today, the first of the fast reactors might
cumstances — when the fission product         technology. When we burn coal or oil to         come online in about 15 years. Notably,
waste was shipped to Yucca Mountain          make electricity, for example, our soci-        that schedule is reasonably compatible
or an alternative site for disposal and      ety accepts the detrimental health effects      with the planned timetable for shipment
when start-up fuel was shipped to a new      and the environmental costs they entail.        of spent thermal-reactor fuel to Yucca
reactor. Commerce in plutonium would         Thus, external costs in effect subsidize        Mountain. It could instead be sent for
be effectively eliminated.                   fossil-fuel power generation, either di-        recycling into fast-reactor fuel.
    Some people are advocating that the      rectly or via indirect effects on the soci-         As today’s thermal reactors reach the
U.S. embark on an extensive program of       ety as a whole. Even though they are dif-       end of their lifetimes, they could be re-
PUREX processing of reactor fuel, mak-       ficult to reckon, economic comparisons           placed by fast reactors. Should that oc-
ing mixed oxides of uranium and pluto-       that do not take externalities into ac-         cur, there would be no need to mine any
nium for cycling back into thermal reac-     count are unrealistic and misleading.           more uranium ore for centuries and no
tors. Although the mixed oxide (MOX)                                                         further requirement, ever, for uranium
method is currently being used for spoil-    Coupling Reactor Types                          enrichment. For the very long term, re-
ing excess weapons plutonium so that it      if a dva nced fast r e ac tors come             cycling the fuel of fast reactors would be
cannot be employed in bombs — a good         into use, they will at first burn spent          so efficient that currently available ura-
idea— we think that it would be a mis-       thermal-reactor fuel that has been recy-        nium supplies could last indefinitely.
take to deploy the much larger PUREX         cled using pyroprocessing. That waste,              Both India and China have recently
infrastructure that would be required to     which is now “temporarily” stored on            announced that they plan to extend their
process civilian fuel. The resource gains    site, would be transported to plants that       energy resources by deploying fast reac-
would be modest, whereas the long-term       could process it into three output              tors. We understand that their first fast
waste problem would remain, and the          streams. The first, highly radioactive,          reactors will use oxide or carbide fuel
entire effort would delay for only a short   stream would contain most of the fission         rather than metal— a less than optimum
time the need for efficient fast reactors.    products, along with unavoidable traces         path, chosen presumably because the
    The fast-reactor system with pyro-       of transuranic elements. It would be            PUREX reprocessing technology is ma-
processing is remarkably versatile. It       transformed into a physically stable            ture, whereas pyroprocessing has not yet
could be a net consumer or net producer      form — perhaps a glasslike substance —          been commercially demonstrated.
of plutonium, or it could be run in a        and then shipped to Yucca Mountain or               It is not too soon for the U.S. to com-
break-even mode. Operated as a net           some other permanent disposal site.             plete the basic development of the fast-
producer, the system could provide               The second stream would capture             reactor/pyroprocessing system for me-
start-up materials for other fast-reactor    virtually all the transuranics, together        tallic fuel. For the foreseeable future, the
power plants. As a net consumer, it          with some uranium and fission prod-              hard truth is this: only nuclear power
could use up excess plutonium and            ucts. It would be converted to a metallic       can satisfy humanity’s long-term energy
weapons materials. If a break-even           fast-reactor fuel and then transferred to       needs while preserving the environment.
mode were chosen, the only additional        ALMR-type reactors.                             For large-scale, sustainable nuclear en-
fuel a nuclear plant would need would            The third stream, amounting to              ergy production to continue, the supply
be a periodic infusion of depleted ura-      about 92 percent of the spent thermal-          of nuclear fuel must last a long time.
nium (uranium from which most of the         reactor fuel, would contain the bulk of         That means that the nuclear power cycle
fissile uranium 235 has been removed)         the uranium, now in a depleted state. It        must have the characteristics of the
to replace the heavy-metal atoms that        could be stashed away for future use as         ALMR and pyroprocessing. The time
have undergone fission.                       fast-reactor fuel.                              seems right to take this new course to-
    Business studies have indicated that         Such a scenario cannot be realized          ward sensible energy development.
this technology could be economically
competitive with existing nuclear power        MORE TO EXPLORE
technologies [see the Dubberly paper in        Breeder Reactors: A Renewable Energy Source. Bernard L. Cohen in American Journal of
                                               Physics, Vol. 51, No. 1; January 1983.
“More to Explore,” on this page]. Cer-
                                               The Technology of the Integral Fast Reactor and Its Associated Fuel Cycle. Edited by
tainly pyrometallurgical recycling will        W. H. Hannum. Progress in Nuclear Energy, Special Issue, Vol. 31, Nos. 1–2; 1997.
be dramatically less expensive than            Integral Fast Reactors: Source of Safe, Abundant, Non-Polluting Power. George Stanford.
PUREX reprocessing, but in truth, the          National Policy Analysis Paper #378; December 2001. Available at www.nationalcenter.org/
economic viability of the system cannot        NPA378.html
be known until it is demonstrated.             LWR Recycle: Necessity or Impediment? G. S. Stanford in Proceedings of Global 2003.
                                               ANS Winter Meeting, New Orleans, November 16–20, 2003. Available at www.nationalcenter.org/
    The overall economics of any energy        LWRStanford.pdf
source depend not only on direct costs         S-PRISM Fuel Cycle Study. Allen Dubberly et al. in Proceedings of ICAPP ’03. Córdoba, Spain,
but also on what economists call “exter-       May 4–7, 2003, Paper 3144.


w w w. s c ia m . c o m                                                                                      SCIENTIFIC A MERIC A N    91
                                             COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.

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Nuclear Fast Reactors Sa1205

  • 1. Smarter Use of Fast-neutron reactors could extract much more energy from recycled nuclear fuel, minimize the risks of weapons proliferation and markedly reduce the time nuclear waste must be isolated By William H. Hannum, Gerald E. Marsh and George S. Stanford D espite long-standing public concern about the safety of nuclear energy, more and more people are realizing that it may be the most environmen- tally friendly way to generate large amounts of electricity. Several nations, including Brazil, China, Egypt, Finland, In- dia, Japan, Pakistan, Russia, South Korea and Vietnam, are building or planning nuclear plants. But this global trend has not as yet extended to the U.S., where work on the last such If developed sensibly, nuclear power could be truly sustain- able and essentially inexhaustible and could operate without contributing to climate change. In particular, a relatively new form of nuclear technology could overcome the principal drawbacks of current methods — namely, worries about reac- tor accidents, the potential for diversion of nuclear fuel into highly destructive weapons, the management of dangerous, long-lived radioactive waste, and the depletion of global re- JANA BRENNING facility began some 30 years ago. serves of economically available uranium. This nuclear fuel 84 SCIENTIFIC A MERIC A N DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 2. NUCLEAR WASTE ar power plants contain what are called thermal reactors, which are driven by neutrons of relatively low speed (or ener- gy) ricocheting within their cores. Although thermal reactors generate heat and thus electricity quite efficiently, they cannot minimize the output of radioactive waste. All reactors produce energy by splitting the nuclei of heavy- metal (high-atomic-weight) atoms, mainly uranium or elements derived from uranium. In nature, uranium occurs as a mixture of two isotopes, the easily fissionable uranium 235 (which is said to be “fissile”) and the much more stable uranium 238. The uranium fire in an atomic reactor is both ignited and sustained by neutrons. When the nucleus of a fissile atom is hit by a neutron, especially a slow-moving one, it will most likely cleave (fission), releasing substantial amounts of energy and several other neutrons. Some of these emitted neutrons then strike other nearby fissile atoms, causing them to break apart, thus propagating a nuclear chain reaction. The resulting heat is conveyed out of the reactor, where it turns water into steam that is used to run a turbine that drives an electric generator. Uranium 238 is not fissile; it is called “fissionable” be- cause it sometimes splits when hit by a fast neutron. It is also said to be “fertile,” because when a uranium 238 atom ab- sorbs a neutron without splitting, it transmutes into plutoni- um 239, which, like uranium 235, is fissile and can sustain a chain reaction. After about three years of service, when tech- cycle would combine two innovations: pyrometallurgical pro- nicians typically remove used fuel from one of today’s reac- cessing (a high-temperature method of recycling reactor waste tors because of radiation-related degradation and the deple- into fuel) and advanced fast-neutron reactors capable of burn- tion of the uranium 235, plutonium is contributing more than ing that fuel. With this approach, the radioactivity from the half the power the plant generates. generated waste could drop to safe levels in a few hundred In a thermal reactor, the neutrons, which are born fast, are years, thereby eliminating the need to segregate waste for tens slowed (or moderated) by interactions with nearby low-atomic- of thousands of years. weight atoms, such as the hydrogen in the water that flows For neutrons to cause nuclear fission efficiently, they must through reactor cores. All but two of the 440 or so commercial be traveling either slowly or very quickly. Most existing nucle- nuclear reactors operating are thermal, and most of them— in- w w w. s c ia m . c o m SCIENTIFIC A MERIC A N 85 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 3. cluding the 103 U.S. power reactors — left over from the enrichment process. about 1 percent of the spent fuel, they employ water both to slow neutrons and The spent fuel consists of three class- constitute the main source of today’s nu- to carry fission-created heat to the asso- es of materials. The fission products, clear waste problem. The half-lives (the ciated electric generators. Most of these which make up about 5 percent of the period in which radioactivity halves) of thermal systems are what engineers call used fuel, are the true wastes— the ashes, these atoms range up to tens of thousands light-water reactors. if you will, of the fission fire. They com- of years, a feature that led U.S. govern- In any nuclear power plant, heavy- prise a mélange of lighter elements cre- ment regulators to require that the metal atoms are consumed as the fuel ated when the heavy atoms split. The mix planned high-level nuclear waste reposi- “burns.” Even though the plants begin is highly radioactive for its first several tory at Yucca Mountain in Nevada iso- with fuel that has had its uranium 235 years. After a decade or so, the activity is late spent fuel for over 10,000 years. content enriched, most of that easily fis- dominated by two isotopes, cesium 137 sioned uranium is gone after about three and strontium 90. Both are soluble in An Outdated Strategy years. When technicians remove the de- water, so they must be contained very se- e a r ly n u c l e a r engineers expected pleted fuel, only about one twentieth of curely. In around three centuries, those that the plutonium in the spent fuel of the potentially fissionable atoms in it isotopes’ radioactivity declines by a fac- thermal reactors would be removed and (uranium 235, plutonium and uranium tor of 1,000, by which point they have then used in fast-neutron reactors, 238) have been used up, so the so-called become virtually harmless. called fast breeders because they were spent fuel still contains about 95 percent Uranium makes up the bulk of the designed to produce more plutonium of its original energy. In addition, only spent nuclear fuel (around 94 percent); than they consume. Nuclear power pio- about one tenth of the mined uranium this is unfissioned uranium that has lost neers also envisioned an energy econo- ore is converted into fuel in the enrich- most of its uranium 235 and resembles my that would involve open commerce ment process (during which the concen- natural uranium (which is just 0.71 per- in plutonium. Plutonium can be used to tration of uranium 235 is increased con- cent fissile uranium 235). This compo- make bombs, however. As nuclear tech- siderably), so less than a hundredth of the nent is only mildly radioactive and, if nology spread beyond the major super- ore’s total energy content is used to gen- separated from the fission products and powers, this potential application led to erate power in today’s plants. the rest of the material in the spent fuel, worries over uncontrolled proliferation This fact means that the used fuel could readily be stored safely for future of atomic weapons to other states or from current thermal reactors still has the use in lightly protected facilities. even to terrorist groups. potential to stoke many a nuclear fire. Be- The balance of the material— the tru- The Nuclear Non-Proliferation cause the world’s uranium supply is finite ly troubling part — is the transuranic Treaty partially addressed that problem and the continued growth in the num- component, elements heavier than ura- in 1968. States that desired the benefits bers of thermal reactors could exhaust nium. This part of the fuel is mainly a of nuclear power technology could sign the available low-cost uranium reserves blend of plutonium isotopes, with a sig- the treaty and promise not to acquire in a few decades, it makes little sense to nificant presence of americium. Although nuclear weapons, whereupon the weap- discard this spent fuel or the “tailings” the transuranic elements make up only ons-holding nations agreed to assist the others with peaceful applications. Al- Overview/Nuclear Recycling though a cadre of international inspec- tors has since monitored member adher- ■ To minimize global warming, humanity may need to generate much of its ence to the treaty, the effectiveness of future energy using nuclear power technology, which itself releases that international agreement has been essentially no carbon dioxide. spotty because it lacks effective author- ■ Should many more of today’s thermal (or slow-neutron) nuclear power plants ity and enforcement means. be built, however, the world’s reserves of low-cost uranium ore will be tapped Nuclear-weapons designers require out within several decades. In addition, large quantities of highly radioactive plutonium with a very high plutonium waste produced just in the U.S. will have to be stored for at least 10,000 239 isotopic content, whereas plutonium years — much more than can be accommodated by the Yucca Mountain from commercial power plants usually repository in Nevada. Worse, most of the energy that could be extracted from contains substantial quantities of the the original uranium ore would be socked away in that waste. other isotopes of plutonium, making it ■ The utilization of a new, much more efficient nuclear fuel cycle — one based on difficult to use in a bomb. Nevertheless, fast-neutron reactors and the recycling of spent fuel by pyrometallurgical use of plutonium from spent fuel in processing — would allow vastly more of the energy in the earth’s readily weapons is not inconceivable. Hence, available uranium ore to be used to produce electricity. Such a cycle would President Jimmy Carter banned civilian greatly reduce the creation of long-lived reactor waste and could support reprocessing of nuclear fuel in the U.S. nuclear power generation indefinitely. in 1977. He reasoned that if plutonium were not recovered from spent fuel it 86 SCIENTIFIC A MERIC A N DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 4. NEW TYPE OF NUCLEAR REACTOR A safer, more sustainable which rely on relatively slow would burn fuel made by like this: Nuclear fire burning in nuclear power cycle could be moving neutrons to propagate recycling spent fuel from the core would heat the based on the advanced liquid- chain reactions in uranium and thermal reactors. radioactive liquid sodium metal reactor (ALMR) design plutonium fuel. An ALMR-based In most thermal-reactor running through it. Some of the developed in the 1980s by system, in contrast, would use designs, water floods the core heated sodium would be pumped researchers at Argonne fast-moving (energetic) to slow (moderate) neutrons into an intermediate heat National Laboratory. Like all neutrons. This process permits and keep it cool. The ALMR, exchanger (2), where it would atomic power plants, an ALMR- all the uranium and heavier however, employs a pool of transfer its thermal energy to based system would use atoms to be consumed, thereby circulating liquid sodium as the nonradioactive liquid sodium nuclear chain reactions in the allowing vastly more of the coolant (1). Engineers chose flowing through the adjacent but core to produce the heat fuel’s energy to be captured. In sodium because it does not separate pipes (3) of a needed to generate electricity. the near term, the new reactor slow down fast neutrons secondary sodium loop. The Current commercial nuclear substantially and because it nonradioactive sodium (4) would plants feature thermal reactors, conducts heat very well, which in turn bring heat to a final heat improves the efficiency of heat exchanger/steam generator (not delivery to the electric shown), where steam would be generation facility. created in adjacent water-filled Warm air A fast reactor would work pipes. The hot, high-pressure Cool air steam would then be used to turn steam turbines that would drive electricity-producing generators Cooling-system air inlet (not shown). To steam Top of reactor silo and exhaust stack generator Freestanding reactor housing l nd le v e Reactor foundation G r ou 4 Secondary sodium loop Intermediate Sodium pump heat exchanger 3 2 REACTOR SAFEGUARDS ■ During operation, powerful pumps would force sodium Sodium pump coolant through the core. If the Sodium coolant pumps failed, gravity would pumped through circulate the coolant. 1 core ■ If coolant pumps malfunctioned Sodium cycling or stopped, special safety through heat devices would also permit exchanger extra neutrons to leak out of the core, lowering its Nonradioactive temperature. sodium cycling ■ In an emergency, six neutron- through steam absorbing control rods would Liquid-sodium pool Seismic isolator generator drop into the core to shut it down immediately. ■ Should chain reactions Hot reactor core Reactor vessel continue, thousands of (uranium fuel rods) neutron-absorbing boron carbide balls would be DON FOLE Y Base of reactor silo released into the core, guaranteeing shutdown. w w w. s c ia m . c o m SCIENTIFIC A MERIC A N 87 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 5. could not be used to make bombs. Car- below]. Several such reactors have been transuranic atoms will do so as well. ter also wanted America to set an ex- built and used for power generation— in Water cannot be employed in a fast ample for the rest of the world. France, France, Japan, Russia, the U.K. and the reactor to carry the heat from the core — Japan, Russia and the U.K. have not, U.S.— two of which are still operating it would slow the fast neutrons. Hence, however, followed suit, so plutonium re- [see “Next-Generation Nuclear Power,” engineers typically use a liquid metal processing for use in power plants con- by James A. Lake, Ralph G. Bennett and such as sodium as a coolant and heat tinues in a number of nations. John F. Kotek; Scientific American, transporter. Liquid metal has one big ad- January 2002]. vantage over water. Water-cooled sys- An Alternative Approach Fast reactors can extract more energy tems run at very high pressure, so that a w h e n t h e b a n was issued, “repro- from nuclear fuel than thermal reactors small leak can quickly develop into a cessing” was synonymous with the do because their rapidly moving (higher- large release of steam and perhaps a seri- PUREX (for plutonium uranium extrac- energy) neutrons cause atomic fissions ous pipe break, with rapid loss of reactor tion) method, a technique developed to more efficiently than the slow thermal coolant. Liquid-metal systems, however, meet the need for chemically pure pluto- neutrons do. This effectiveness stems operate at atmospheric pressure, so they nium for atomic weapons. Advanced from two phenomena. At slower speeds, present vastly less potential for a major fast-neutron reactor technology, how- many more neutrons are absorbed in release. Nevertheless, sodium catches fire ever, permits an alternative recycling nonfission reactions and are lost. Second, if exposed to water, so it must be man- strategy that does not involve pure plu- the higher energy of a fast neutron makes aged carefully. Considerable industrial tonium at any stage. Fast reactors can it much more likely that a fertile heavy- experience with handling the substance thus minimize the risk that spent fuel metal atom like uranium 238 will fission has been amassed over the years, and from energy production would be used when struck. Because of this fact, not management methods are well devel- for weapons production, while provid- only are uranium 235 and plutonium oped. But sodium fi res have occurred, ing a unique ability to squeeze the maxi- 239 likely to fission in a fast reactor, but and undoubtedly there will be more. One mum energy out of nuclear fuel [see box an appreciable fraction of the heavier sodium fire began in 1995 at the Monju NEW WAY TO REUSE NUCLEAR FUEL The key to pyrometallurgical recycling of nuclear fuel is the plutonium fuel from fast reactors would go straight to the electrorefining procedure. This process removes the true electrorefiner. Electrorefining resembles electroplating: waste, the fission products, from the uranium, plutonium and spent fuel attached to an anode would be suspended in a the other actinides (heavy radioactive elements) in the spent chemical bath; then electric current would plate out uranium fuel. The actinides are kept mixed with the plutonium so it and other actinides on the cathode. The extracted elements cannot be used directly in weapons. would next be sent to the cathode processor to remove Spent fuel from today’s thermal reactors (uranium and residual salts and cadmium from refining. Finally, the plutonium oxide) would first undergo oxide reduction to remaining uranium and actinides would be cast into fresh fuel convert it to metal, whereas spent metallic uranium and rods, and the salts and cadmium would be recycled. Oxide fuel from thermal reactors Most of the Cathode Casting fission products Heating molds elements Uranium Anode and actinides Metal Crucible Chopped fuel Metal Salts and cadmium New metal fuel rods Chopped metallic fuel from fast reactors OXIDE CATHODE INJECTION DON FOLE Y SPENT FUEL REDUCTION UNIT ELECTROREFINER PROCESSOR CASTING SYSTEM 88 SCIENTIFIC A MERIC A N DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 6. fast reactor in Japan. It made a mess in full batch is amassed, operators remove the reactor building but never posed a the electrode. Next they scrape the ac- threat to the integrity of the reactor, and cumulated materials off the electrode, no one was injured or irradiated. Engi- melt them down, cast them into an ingot neers do not consider sodium’s flamma- and pass the ingot to a refabrication line bility to be a major problem. for conversion into fast-reactor fuel. Researchers at Argonne National When the bath becomes saturated with Laboratory began developing fast-reac- fission products, technicians clean the tor technology in the 1950s. In the 1980s solvent and process the extracted fission this research was directed toward a fast products for permanent disposal. reactor (dubbed the advanced liquid- Thus, unlike the current PUREX metal reactor, or ALMR), with metallic method, the pyroprocess collects virtu- fuel cooled by a liquid metal, that was to ally all the transuranic elements (includ- be integrated with a high-temperature ing the plutonium), with considerable pyrometallurgical processing unit for re- carryover of uranium and fission prod- cycling and replenishing the fuel. Nucle- ucts. Only a very small portion of the ar engineers have also investigated sev- transuranic component ends up in the fi- eral other fast-reactor concepts, some nal waste stream, which reduces the burning metallic uranium or plutonium needed isolation time drastically. The fuels, others using oxide fuels. Coolants combination of fission products and of liquid lead or a lead-bismuth solution transuranics is unsuited for weapons or have been used. Metallic fuel, as used in even for thermal-reactor fuel. This mix- the ALMR, is preferable to oxide for sev- ture is, however, not only tolerable but eral reasons: it has some safety advan- advantageous for fueling fast reactors. tages, it will permit faster breeding of Although pyrometallurgical recy- EX TR ACTED UR ANIUM and actinide elements new fuel, and it can more easily be paired cling technology is not quite ready for from spent thermal-reactor fuel are plated out with pyrometallurgical recycling. immediate commercial use, researchers on the cathode of an electrorefiner during the have demonstrated its basic principles. It pyroprocessing procedure. After further Pyroprocessing has been successfully demonstrated on a processing, the metallic fuel can be burned in fast-neutron reactors. t h e p y rom e ta l lu rgic a l process pilot level in operating power plants, (“pyro” for short) extracts from used both in the U.S. and in Russia. It has not with the same electrical capacity, in con- fuel a mix of transuranic elements in- yet functioned, however, on a full pro- trast, is a little more than a single ton of stead of pure plutonium, as in the duction scale. fission products, plus trace amounts of PUREX route. It is based on electroplat- transuranics. ing— using electricity to collect, on a Comparing Cycles Waste management using the ALMR conducting metal electrode, metal ex- t h e op e r at i ng c a pa bi l i t i e s of cycle would be greatly simplified. Be- tracted as ions from a chemical bath. Its thermal and fast reactors are similar in cause the fast-reactor waste would con- name derives from the high tempera- some ways, but in others the differences tain no significant quantity of long-lived tures to which the metals must be sub- are huge [see box on next page]. A transuranics, its radiation would decay jected during the procedure. Two simi- 1,000-megawatt-electric thermal-reac- to the level of the ore from which it came lar approaches have been developed, tor plant, for example, generates more in several hundred years, rather than one in the U.S., the other in Russia. The than 100 tons of spent fuel a year. The tens of thousands. major difference is that the Russians annual waste output from a fast reactor If fast reactors were used exclusively, process ceramic (oxide) fuel, whereas the fuel in an ALMR is metallic. WILLIAM H. HANNUM, GERALD E. MARSH and GEORGE S. STANFORD are physicists who THE AUTHORS In the American pyroprocess [see worked on fast-reactor development before retiring from the U.S. Department of Energy’s box on opposite page], technicians dis- Argonne National Laboratory. Hannum served as head of nuclear physics development solve spent metallic fuel in a chemical A RGONNE N ATION A L L A BOR ATORY and reactor safety research at the DOE. He was also deputy director general of the Nucle- bath. Then a strong electric current se- ar Energy Agency of the Organization for Economic Co-operation and Development in lectively collects the plutonium and the Paris. Marsh, a fellow of the American Physical Society, worked as a consultant to the U.S. other transuranic elements on an elec- Department of Defense on strategic nuclear technology and policy in the Reagan, Bush trode, along with some of the fission and Clinton administrations and is co-author of The Phantom Defense: America’s Pursuit products and much of the uranium. of the Star Wars Illusion (Praeger Press). Stanford, whose research focused on experi- Most of the fission products and some of mental nuclear physics, reactor physics and fast-reactor safety, is co-author of Nucle- the uranium remain in the bath. When a ar Shadowboxing: Contemporary Threats from Cold War Weaponry (Fidlar Doubleday). w w w. s c ia m . c o m SCIENTIFIC A MERIC A N 89 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 7. COMPARING THREE NUCLEAR FUEL CYCLES Three major approaches to burning nuclear fuel and handling its wastes can be employed; some of their features are noted below. ONCE-THROUGH ROUTE PLUTONIUM RECYCLING FULL RECYCLING Fuel is burned in thermal reactors and is not Fuel is burned in thermal reactors, after which Recycled fuel prepared by pyrometallurgical reprocessed; occurs in the U.S. plutonium is extracted using what is called processing would be burned in advanced fast- PUREX processing; occurs in other neutron reactors; prototype technology developed nations FUEL UTILIZATION 5 percent used in 5 percent 6 percent thermal is used is used reactor 95 percent Somewhat 94 percent more than is wasted is wasted 94 percent is used in fast reactor Less than Initial fuel supply 1 percent is wasted Uses about 5 percent of energy in thermal- Uses about 6 percent of energy in original Can recover more than 99 percent of energy reactor fuel and less than 1 percent reactor fuel and less than 1 percent of in spent thermal-reactor fuel of energy in uranium ore (the original energy in uranium ore After spent thermal-reactor fuel runs out, source of fuel) Cannot burn depleted uranium or uranium can burn depleted uranium to recover more Cannot burn depleted uranium (that part in spent fuel than 99 percent of the rest of the energy removed when the ore is enriched) or in uranium ore uranium in spent fuel REQUIRED FACILITIES AND OPERATIONS Red: requires rigorous physical safeguards Orange: needs only moderate physical safeguards Blue: potential risks for future generations Uranium mines Uranium mines On-site fuel fabrication Fuel enrichment to concentrate fissile Fuel enrichment On-site pyrometallurgical processing uranium Plutonium blending (mixing) (prompt recycling of spent fuel) Fuel fabrication Off-site fuel fabrication Power plants Power plants Off-site PUREX reprocessing On-site waste processing Interim waste storage (until waste can be Power plants Storage able to segregate waste for less permanently disposed of) than 500 years Interim waste storage Permanent storage able to (No mining needed for centuries; no uranium securely segregate waste for 10,000 years Off-site waste processing enrichment needed, ever) (Needs no plutonium handling or waste Permanent storage able to securely processing operations) segregate waste for 10,000 years PLUTONIUM FATE Increasing inventories of plutonium Increasing inventories of plutonium Inventories eventually shrink to only what is in used fuel in used fuel and available for economic trade in use in reactors and in recycling Excess weapons-grade plutonium degraded Excess weapons-grade plutonium degraded Existing excess weapons-grade plutonium can only slowly by mixing into fresh fuel only slowly by mixing into fresh fuel be degraded rapidly Plutonium in the fuel is too impure for diversion to weapons TYPES OF WASTE Energy-rich used fuel isolated in containers Energy-rich, highly stable glassy waste Tailored waste forms that would only have to and underground storage facility Waste is radioactive enough to be defined as remain intact for 500 years, after which Waste is radioactive enough to be defined as “self-protected” for a few hundred years material would no longer be hazardous “self-protected” for a few hundred years against most groups wanting to obtain Lacking plutonium, waste would not be useful DON FOLE Y against most groups wanting to obtain plutonium 239 for building nuclear weapons for making weapons plutonium 239 for building nuclear weapons 90 SCIENTIFIC A MERIC A N DECEMBER 2005 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.
  • 8. transportation of highly radioactive ma- nalities,” the hard-to-quantify costs of overnight, of course. If we were to begin terials would occur only under two cir- outside effects resulting from using the today, the first of the fast reactors might cumstances — when the fission product technology. When we burn coal or oil to come online in about 15 years. Notably, waste was shipped to Yucca Mountain make electricity, for example, our soci- that schedule is reasonably compatible or an alternative site for disposal and ety accepts the detrimental health effects with the planned timetable for shipment when start-up fuel was shipped to a new and the environmental costs they entail. of spent thermal-reactor fuel to Yucca reactor. Commerce in plutonium would Thus, external costs in effect subsidize Mountain. It could instead be sent for be effectively eliminated. fossil-fuel power generation, either di- recycling into fast-reactor fuel. Some people are advocating that the rectly or via indirect effects on the soci- As today’s thermal reactors reach the U.S. embark on an extensive program of ety as a whole. Even though they are dif- end of their lifetimes, they could be re- PUREX processing of reactor fuel, mak- ficult to reckon, economic comparisons placed by fast reactors. Should that oc- ing mixed oxides of uranium and pluto- that do not take externalities into ac- cur, there would be no need to mine any nium for cycling back into thermal reac- count are unrealistic and misleading. more uranium ore for centuries and no tors. Although the mixed oxide (MOX) further requirement, ever, for uranium method is currently being used for spoil- Coupling Reactor Types enrichment. For the very long term, re- ing excess weapons plutonium so that it if a dva nced fast r e ac tors come cycling the fuel of fast reactors would be cannot be employed in bombs — a good into use, they will at first burn spent so efficient that currently available ura- idea— we think that it would be a mis- thermal-reactor fuel that has been recy- nium supplies could last indefinitely. take to deploy the much larger PUREX cled using pyroprocessing. That waste, Both India and China have recently infrastructure that would be required to which is now “temporarily” stored on announced that they plan to extend their process civilian fuel. The resource gains site, would be transported to plants that energy resources by deploying fast reac- would be modest, whereas the long-term could process it into three output tors. We understand that their first fast waste problem would remain, and the streams. The first, highly radioactive, reactors will use oxide or carbide fuel entire effort would delay for only a short stream would contain most of the fission rather than metal— a less than optimum time the need for efficient fast reactors. products, along with unavoidable traces path, chosen presumably because the The fast-reactor system with pyro- of transuranic elements. It would be PUREX reprocessing technology is ma- processing is remarkably versatile. It transformed into a physically stable ture, whereas pyroprocessing has not yet could be a net consumer or net producer form — perhaps a glasslike substance — been commercially demonstrated. of plutonium, or it could be run in a and then shipped to Yucca Mountain or It is not too soon for the U.S. to com- break-even mode. Operated as a net some other permanent disposal site. plete the basic development of the fast- producer, the system could provide The second stream would capture reactor/pyroprocessing system for me- start-up materials for other fast-reactor virtually all the transuranics, together tallic fuel. For the foreseeable future, the power plants. As a net consumer, it with some uranium and fission prod- hard truth is this: only nuclear power could use up excess plutonium and ucts. It would be converted to a metallic can satisfy humanity’s long-term energy weapons materials. If a break-even fast-reactor fuel and then transferred to needs while preserving the environment. mode were chosen, the only additional ALMR-type reactors. For large-scale, sustainable nuclear en- fuel a nuclear plant would need would The third stream, amounting to ergy production to continue, the supply be a periodic infusion of depleted ura- about 92 percent of the spent thermal- of nuclear fuel must last a long time. nium (uranium from which most of the reactor fuel, would contain the bulk of That means that the nuclear power cycle fissile uranium 235 has been removed) the uranium, now in a depleted state. It must have the characteristics of the to replace the heavy-metal atoms that could be stashed away for future use as ALMR and pyroprocessing. The time have undergone fission. fast-reactor fuel. seems right to take this new course to- Business studies have indicated that Such a scenario cannot be realized ward sensible energy development. this technology could be economically competitive with existing nuclear power MORE TO EXPLORE technologies [see the Dubberly paper in Breeder Reactors: A Renewable Energy Source. Bernard L. Cohen in American Journal of Physics, Vol. 51, No. 1; January 1983. “More to Explore,” on this page]. Cer- The Technology of the Integral Fast Reactor and Its Associated Fuel Cycle. Edited by tainly pyrometallurgical recycling will W. H. Hannum. Progress in Nuclear Energy, Special Issue, Vol. 31, Nos. 1–2; 1997. be dramatically less expensive than Integral Fast Reactors: Source of Safe, Abundant, Non-Polluting Power. George Stanford. PUREX reprocessing, but in truth, the National Policy Analysis Paper #378; December 2001. Available at www.nationalcenter.org/ economic viability of the system cannot NPA378.html be known until it is demonstrated. LWR Recycle: Necessity or Impediment? G. S. Stanford in Proceedings of Global 2003. ANS Winter Meeting, New Orleans, November 16–20, 2003. Available at www.nationalcenter.org/ The overall economics of any energy LWRStanford.pdf source depend not only on direct costs S-PRISM Fuel Cycle Study. Allen Dubberly et al. in Proceedings of ICAPP ’03. Córdoba, Spain, but also on what economists call “exter- May 4–7, 2003, Paper 3144. w w w. s c ia m . c o m SCIENTIFIC A MERIC A N 91 COPYRIGHT 2005 SCIENTIFIC AMERICAN, INC.