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Detritiation of heavy water
1. DETRITIATION OF HEAVY WATER
USED AS THE MODERATOR IN POWER REACTORS
Gheorghe VĂSARU
Aleea Tarnita, Nr. 7, Apt. 11,
400659 CLUJ-NAPOCA, ROMANIA
E-mail: gvasaru@hotmail.com
1. Introduction
Recently has been developed a wet proof catalyst for LPCD liquid phase catalytic
exchange which was used for detritiation. A pilot plant based on LPCE cryogenic
distillation with about 90 per cent removal from heavy water has been commissioned at
Bhabha Atomic Research Centre, Bombay, India, now under experimental. This facility
seems to be the only operating LPCE-based detritiation facility in the world. A
commercial detritiation plant based on this process is being set up at one of their nuclear
power stations at Kalpakkam near Madras. According to technical estimates, 2400 curies
of tritium could be produced for every MW of electricity produced in heavy water
reactors.
According to BARC scientists, the new technology is aimed at lowering the
tritium content in heavy water circulating around the moderator circuit. They argue that
the project is being executed to prevent the many health hazards associated with the
leakage of tritium from reactors.
2. Tritium
Tritium is a radioactive isotope of hydrogen with a half-life of 12.3 years,
meaning that 5.5 percent of tritium will decay into 3He every year. Deuterium, another
isotope of hydrogen, along with the elementary gas itself, is stable and non-radioactive.
Tritium decays and is converted into a non-radioactive form of helium.
Although tritium is present naturally in the environment, this amount is too small
for practical recovery. Therefore, tritium required for strategic purposes has to be
produced artificially, and there are two ways to do this, both involving nuclear reactions
with neutrons: in the first method, neutrons are made to strike a target of lithium or
aluminium metal, which gives tritium and another by-products; the second method
involves a neutron reaction with 3He which gives tritium and hydrogen as by-products.
The first method is widely used and was employed for several years at the
Savannah River Site (SRS) in the USA before it was shut own in 1988. The production of
tritium requires the generation of energetic neutrons, the source of which can be either
power reactors or accelerators. In reactors, neutrons are produced as a result of fission,
while in accelerators they occur as a result of spallation, where proton strike a metallic
target and ”kick off” neutron from the metal.
Tritium finds peripherical use in medical diagnostics, but it is mainly used in the
construction of hydrogen bombs and to boost the yield of both fission and thermonuclear
weapons. Contained in removable and refillable reservoirs in nuclear arsenals, it boosts
the efficiency of the nuclear materials. Although no official data is available on inventory
2. amounts of tritium, each thermonuclear warhead is said to contain 4 g of the isotope.
However, neutron bombs designed to release more radiation will require 10-30 g of
tritium, according to a status report prepared by U.S. Department of Energy’s Science
Policy Research Division and an assessment made by the Institute for Energy and
Environment Research (IEER) in Maryland, USA.
Authoritative US reports put the USA’s total tritium production since 1955 at 225
kg. After decay, it is now left with 75 kg of tritium, which is sufficient to take the country
through the first quarter of the next millenium
Even in low levels, tritium has been linked to developmental problems,
reproductive problems, genetic and neurological abnormalities and other health problems.
Additionally, there is evidence of adverse health effects on population living near tritium
facilities. Tritium contamination has been reported at the Savannah River site in ground
water soil from operational releases and accidents. No figures are available relating to the
Indian stockpile of tritium, however. The pilot plant at BARC was set up, according to
well-placed sources in the department, in 1992.
The tritium build-up in the PHWR (CANDU-type power reactors) increases with
the number of years of plant in operation. The pilot plant is called the detritiation plant
because the process involves lowering tritium levels in heavy water, but the fact remains
that the by-product is highly enriched tritium. The reason why BARC developed new
technology was to reduce radioactive levels by lowering the tritium content in heavy
water.
3. The process
The presence of tritium in heavy water has been a major concern of reactor
engineers in India for a long time. During operation of a PHWR, tritium is produced as a
result of fission and irradiation of reactor components with neutrons. This tritium remains
in the fuel and later passes into the effluents in the fuel reprocessing plants. The BARC
pilot plant produces tritium using moderator heavy water, where tritium is produced due
to the capture of neutrons by deuterium atoms in the water. This reaction, as reported in
scientific literature, is known to yields maximum tritium.
Although any method employed in the production and enrichment of isotopes can
also be used in the case of tritium, the BARC scientists’ choice of process was governed
by safe handling and economic reasons. BARC scientists first worked with the water
distillation and electrolytic method, which proved to be risky and inefficient. This
produces tritium in most hazardous form: liquid. They instead settled for the method of
chemical exchange followed by cryogenic distillation. In this method the tritium is in a
liquid phase only a short time during the chemical exchange process, with the final
product collected in gaseous form and kept in double containment to ensure safety. This
method yields 90 per cent enriched tritium.
4. The catalyst
The most important hurdle in producing tritium by this method is finding a
suitable catalyst for the process because heavy water from the moderator and pure
deuterium gas have to pass through the column containing catalyst. Besides, the exchange
reactions of deuterium between hydrogen and water require a slow and suitable catalyst,
taking into account the slow nature of these reactions. Nickel coated by chromium,
3. platinum or other noble metals supported on silica or activated charcoal have been found
effective for vapour phase exchange reactions, but BARC’s exchange reactions occur in
the liquid phase and require some other species of catalyst. All the catalysts mentioned
above lose their activity in contact with liquid water and prevent hydrogen from reaching
them.
Indian scientists have overcome this problem by imparting hydrophobic to the
catalysts. Since water in the liquid form wets and contaminates catalyst, the suitable
solution was a wet proof catalyst, which is what the BARC scientists opted for. A number
of technical snags associated with the proper choice of catalyst have been eliminated, and
experiments conducted to check the performance of the catalyst have shown positive
results. Although the department undertook this work in the early 1970s, it was only
recently that they perfected the technology.
5. Design
The pilot plant’s equipment is indigenously designed. Scientists, have taken into
consideration various aspects of handling inflammable gases like hydrogen, deuterium
and the radioactive tritium. Pipelines, fitting-valves and other equipment are made of
special steel, all suitable for cryogenic conditions. The entire cryogenic part of the plants
is housed inside a vacuum-insulated enclosure, which provides thermal insulation for its
components. The columns sections have been insulated with mylar to prevent any cold
leak.
Being a multi-component distillation system, it is not simple to operate. The
difficulties encountered include the decay heat of tritium (associated with the decay of
tritium into 3He), which would evaporate all the liquid. The pressure drop is minimised,
however, and temperature variations are kept to a minimum.
Scientists from the group say the philosophy of the plant’s operation is based on
fail-safe conditions. The operation of the entire distillation column takes place at
atmospheric pressure and a temperature of – 268 0 C. The whole plant has two sections: a
low tritium activity section and a high tritium activity section. Nearly 240 stages are
involved in the tritium enrichment process, and so it has to be carried out in three-stage
cascade distillation units.
The deuterium-tritium gas which emerges from the second stage is 100 per cent
enriched. After this the tritium is separated using an equilibrator, with the condensed
product serving as the reflux for the third stage. The highly concentrated tritium is drawn
off periodically from the bottom of the cryogenic column and immobilized in a matrix of
metal tritride, which would be compact, safe and stable at normal temperature. The gas
can be recovered at any time by heating the metal tritride. At this stage the pure tritium is
ready for stockpiling.
6. Appendix: Basic facts about tritium:
There are three isotopes of hydrogen:
a protium nucleus has one proton and no neutrons
a deuterium nucleus has one proton and one neutron
a tritium nucleus has one proton and two neutrons
Tritium decays to 3He + beta + neutrino
4. Half-Life:
(DOE 5630.9) = 12.323 ± 0.004 years = 4500.88 ± 1.46 days
(Mound) = 12.3232 ± 0.0043 years = 4500.96 ± 1.57 days
1 year = 365.2425 days
Q(T2 at t) = Q(T2 at start)e{[t(years) x ln 0.5]/12.323}
Tritium Decay Factor = 0.99984601/day
Energy of decay, dissociation, ionisation:
(Emax.) = 18.6 keV
(Emean.) = 5.69 keV
Dissociation energy, T2 to 2T = 4.59 eV
Ionisation energy, T to T+ + e- = 13.55 eV
Energy to break T bond = 3.858 eV/molecule
Miscellaneous:
1 Ci T2 gas at STP = 0.386 ml
1 gram T2 gas at STP = 9619 Ci
1 gram T2 gas at STP = 3.71579 litre
1 ml T2 gas at STP = 2.589 Ci
1 ml T2O (tritiated water) = 3,200 Ci
1 litre T2 gaz STP = 2,589 Ci
1 litre T2 gaz STP = 0.269122 gram
1 ppm of T2 gas STP = 2.589 Ci/m3
Atomic Weight = 3.01605
Molecular Weight = 6.0321
Boiling points:
H2: 20.39 K
HD: 22.14 K
HT: 22.92 K
D2: 23.66 K
DT: 24.38 K
T2: 25.04 K