Detritiation of heavy water


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A short survey on a indian heavy water detritiation plant

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Detritiation of heavy water

  1. 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: 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. 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. 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. 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