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Nuclear Familiarisation - Reprocessing and Recycling
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FAMILIARISATION WITH
NUCLEAR TECHNOLOGY
REPROCESSING AND RECYCLING
Peter D. Wilson
DURATION ABOUT 40 MINUTES
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WHY REPROCESS?
Originally
– To obtain plutonium for military use
Currently
– To ease storage problems
especially Magnox - cladding corrodes easily
– To concentrate high-level waste
– To recover clean plutonium and uranium
– As a business opportunity
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DISCHARGED FUEL HAS -
Diminished reactivity owing to
– substantially reduced fissile content
much of initial enrichment consumed
not entirely compensated by new plutonium
– neutron-absorbing fission products
Somewhat weakened structure
Possible pressurisation by fission gases
Nearly all original fertile content (U-238)
Minor actinide content (Np, Am, Cm) super-proportional to irradiation
Continuing heat release from decay of fission products & minor actinides
Potential for much greater energy generation than already realised
(by up to 2 orders of magnitude)
Reasons for
discharge
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MANAGEMENT OPTIONS
(after decay storage)
Direct Disposal
Minimises operations and cost
Minimises immediate risk of
illicit diversion, but
Leaves Pu content intact with
gradually rising quality and
decaying radioactive defence -
“plutonium mine”
Minimises secondary wastes
Abandons all remaining energy
potential after at best ca. 1%
utilisation of mined uranium
(including enrichment tails)
Reprocessing
Major industrial operations
Recovers fissile and fertile materials
for further use
In principle permits near-elimination
of fissile content
Minimises HLW volume, but
Generates more ILW & LLW
Operational radiation exposure
Permits recycling
– potentially 50 - 100% utilisation
– but without fast reactors only
~15-30% improvement over once-
though
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PROCEDURE - CLOSED CYCLE
Local storage for decay of heat release
Transport to reprocessing site
Further decay storage to limit radiation
Reprocessing
– separation of uranium & plutonium from each other
and from fission products
– finishing U & Pu products
purification and conversion to form for use or storage
– conditioning wastes for disposal
Refabrication of U and Pu into new fuel
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DELAY STORAGE
Wet
Water provides cooling and
shielding
Permits direct sight and
manipulation
Requires strong structure
Needs continual purification and
leak monitoring
Tends to cause corrosion
Liable to create uncomfortably
humid working environment -
needs good ventilation
Dry
Avoids corrosion especially of
Magnox
Avoids need for water
purification
Allows tighter packing
– less risk of criticality
Remote manipulation
Needs more complex building
and equipment
Requires guided convection or
forced-air cooling
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TRANSPORT FLASK REQUIREMENTS
Shielding appropriate
to radioactive content
(gamma, neutron)
Heat dispersion
adequate for maximum
thermal load
With customary water
coolant, robust
containment of
activated corrosion
products
Structural integrity
maintained against
worst credible impact
or fire Photo copyright BNFL (?)
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PROCESS REQUIREMENTS
Operational and environmental safety
– nuclear (avoiding criticality)
– against radiation & contamination
Product quality - decontamination by106 - 108
Manageable wastes
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BASIS OF SEPARATION PROCESS
Uranium and plutonium in their most stable chemical states
are readily soluble in both nitric acid and certain organic
solvents immiscible with it
Fission products generally are at most very much less so.
– iodine (a major exception) is largely boiled off during
dissolution
Equilibrium distribution depends on e.g. acidity
Uranium and plutonium can therefore be extracted from a
fuel solution and then taken back into clean dilute acid
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Separation of fuel from cladding
Dissolution of fuel substance
Extraction of uranium and plutonium into solvent
– 1st Sellafield plant Butex,
since 1964 tributyl phosphate (TBP) diluted with e.g kerosene
Separate backwashing of plutonium and uranium
– plutonium backwash assisted by chemical reduction
Concentration and storage of wastes (fission products etc)
Waste conditioning for eventual disposal
REPROCESSING STAGES
Magnox, peel & dissolve;
Oxide, chop & leach
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PUREX PROCESS OUTLINE
U, Pu,
FPs
U, Pu
FPs
Highly-active
waste
Pu
Plutonium
purification
U
U
Uranium
purification
Solvent purification
(alkali wash)
Extraction
Reductive
backwash
Dilute acid
backwash
Dissolution
Aqueous
Solvent
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COUNTERCURRENT OPERATION
Fresh solvent
Aqueous
feed
Loaded solvent
Depleted
aqueous
Required separation factors need many stages of equilibrium or
equivalent in partial equilibrations
Loaded solvent meets the most concentrated aqueous solution
Fresh solvent meets depleted aqueous feed
Thus extraction and loading are maximised
Similar principles apply in reverse to backwashing
Design challenge is to maximise local inter-phase contact without
excessive longtitudinal mixing
Contact between solvent and aqueous may be continuous or stagewise
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MIXER-SETTLER
Physical & theoretical stages
very nearly equivalent
Simple to design and operate
– can be set up effectively with
beakers and bent tubes on a bench
Tolerates variable throughput
BUT
Large settler volume at each
stage
Therefore long residence time,
high process inventory and
solvent degradation
Poor geometry for high
plutonium content
NEVERTHELESS
Adequate for uranium and low-
irradiated fuel
Part of mixer-settler bank
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PULSED COLUMN
Multiple stage equivalence with settler
volumes only at top and bottom
Tall, thin profile - good for nuclear safety
Gamma loss & short residence time reduce
solvent degradation
Therefore satisfactory for plutonium and
fairly high-irradiated fuel
BUT
Performance depends on conditions
– limited range of throughput
Prediction largely empirical and approximate
Needs sophisticated operational control
Height requires tall buildings, seismic
qualification expensive
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REDUCTIVE BACKWASH
Necessary for clean separation of plutonium from uranium
– Pu(III) very much less extractable than Pu(IV)
Magnox plant uses ferrous sulphamate
– leaves salt residue (ferric sulphate)
corrosive
limits volume reduction - intended for discharge after
decay storage, so
must be kept free from major contamination
– therefore U/Pu split in second cycle
Thorp uses uranous nitrate
– waste contains no residual salts
– can be greatly concentrated by evaporation
– therefore acceptable in first cycle (early split)
nearly didn’t work - unexpected complications from technetium
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SOLVENT DEGRADATION
Combination of radiolysis and acid attack
Short-term, i.e. within cycle (chiefly TBP extractant)
– forms (a) dibutyl and (b) monobutyl phosphates
– (a) impairs backwash
– (b) forms precipitates
– removed by alkaline wash
Long-term (largely diluent)
– forms acids, alcohols, ketones, nitro-compounds etc.
– impair decontamination and settling
– only partly removed by washing
– require gradual or complete solvent change
– waste solvent needs disposal
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WASTE MANAGEMENT PRINCIPLES
Absolute separation of radioactive from inactive material
impossible
– most fission products etc. confined to small volume
– some inevitably emerge in other streams
Radioactive content confined as far as practicable to
eventually solid forms for disposal
Some very difficult to confine reliably, e.g. iodine, krypton
– very small dose to everyone preferred to risk of local
accidental high dose
– therefore dilution & dispersion rather than concentration
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SOLID WASTE CLASSIFICATION
High level (HLW) - sufficiently radioactive for heat release to
be significant in storage or disposal
Low level (LLW) - no more than
4 GBq alpha per tonne or
12 GBq beta/gamma per tonne
Intermediate level (ILW) - higher than LLW but not
significantly heat-releasing
Very low level (VLWW) - disposable with ordinary rubbish
bulk less than 4 GBq/m3 beta/gamma
no single item over 40 kBq beta/gamma
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RADIOACTIVE WASTES
HLW - vitrified fission products, minor actinides and
corrosion products mostly from the first cycle raffinate
ILW - cladding fragments, plutonium-contaminated
materials, resins & sludges from effluent treatment,
scrapped equipment
LLW - e.g. domestic-type rubbish from active areas, mildly
contaminated laboratory equipment
Low-level liquid - treated effluents from ponds,
condensate from evaporators, etc.
Gaseous - filtered and treated ventilation air from cells
and working areas
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SELLAFIELD WASTE MANAGEMENT
Confine as much as possible of the heat-
releasing radionuclide waste to a small
volume of glass - HLW
Immobilise other substantially radioactive
waste (without troublesome heat release)
with cement - ILW
Pack and encapsulate low-level solid waste in secure
containers for near-surface burial
Discharge hard-to-confine species e.g. iodine, krypton
Otherwise discharge as little as reasonably achievable in
liquid and gaseous effluents
For eventual
deep disposal
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PRODUCT FINISHING
Finishing - conversion to a form suitable for sale, use or
storage
– Uranium
– thermal denitration to UO3
– Plutonium
– precipitation as oxalate
– calcination to PuO2
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WHY RECYCLE?
To make the most of a finite resource
To reduce short-term need for fresh mining
– Most environmentally damaging part of industry
To reduce storage or disposal requirements for materials
with little or no other legitimate use
– e.g. over a million tonnes depleted uranium world-wide
plutonium from decommissioned weapons
To put fissile material out of reach of potential terrorists
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Uranium
– recovered from oxide still has more than natural enrichment
could be used “as is” in CANDU
– also has U-232 (radiation hazard from daughters) and
– U-234 & U-236 (neutron absorbers) - though U-234 fertile
Plutonium
– contains
– Pu-238 (heat & neutron emission)
– Pu-240, Pu-241 (parent of Am-241 - radiation hazard) & Pu-242
– as well as desirable Pu-239
– only odd-numbered isotopes fissile
Current reactors take at most a partial load of plutonium-enriched fuel;
newer types designed for full load
Refabricating recycled civil material more expensive than fresh
but can be offset by avoiding isotopic enrichment of uranium
FACTORS RELEVANT TO RECYCLING
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DIFFICULTIES IN RECYCLING AS MOX
Deleterious isotopes in uranium
– U-236; unproductive neutron absorber
– U-232; extremely energetic - emitting daughter Tl-208
Requirement for intimate mixing, ideally solid solution
– to avoid hot spots weakening cladding
– achievable but difficult in solid state
– co-precipitation tends to some segregation
– sol-gel process may be preferable in future
Plutonium oxide very hard to dissolve in pure nitric acid
– a mixed product from a future reprocessing plant would
be more tractable
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PRACTICAL RECYCLING
Uranium
– 1600 te AGR fuel produced from re-enriched recovered
uranium
– manufacture essentially as from fresh material
– generally cheaper to use fresh - but for how long?
Plutonium
– used in about 2% of current fuel manufacture
– ~2000 tonnes fuel so far
– in UK as powder dry-blended with uranium dioxide,
formed into loose aggregates, pressed into pellets,
sintered, ground to size and packed into tubes
– elements distinguished only by identification markings
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FUTURE REPROCESSING
Aim to simplify, reduce waste arisings and costs at source
Single-cycle flowsheet?
– increased cycle decontamination, or
– reduced (more realistic) specification
Intensified process equipment
– continuous dissolver
– centrifugal solvent-extraction contactors
(essentially short-residence mixer-settlers)
Different (e.g. pyrochemical) processes for special fuels
Waste partitioning (e.g. for transmutation)
– currently seems an unjustifiable complication
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FUTURE RECYCLING
Near term
Reconstitution of oxide fuel for CANDU (Dupic)
– possibly with minimal process to remove volatiles
Sol-gel vibro-packing route
Distant
Molten salts
– as process medium
avoids large volumes of aqueous waste
generally poorer separations
– as fuel?
– symbiosis between pyrochemical reprocessing and molten-salt
reactors