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GB2039866A - Process for the extraction of tritium from heavy water - Google Patents

Process for the extraction of tritium from heavy water Download PDF

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Publication number
GB2039866A
GB2039866A GB8000902A GB8000902A GB2039866A GB 2039866 A GB2039866 A GB 2039866A GB 8000902 A GB8000902 A GB 8000902A GB 8000902 A GB8000902 A GB 8000902A GB 2039866 A GB2039866 A GB 2039866A
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tritium
column
heavy water
gas
stream
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GB2039866B (en
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Atomic Energy of Canada Ltd AECL
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Atomic Energy of Canada Ltd AECL
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    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B4/00Hydrogen isotopes; Inorganic compounds thereof prepared by isotope exchange, e.g. NH3 + D2 → NH2D + HD

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Health & Medical Sciences (AREA)
  • General Health & Medical Sciences (AREA)
  • Inorganic Chemistry (AREA)
  • Catalysts (AREA)
  • Separation By Low-Temperature Treatments (AREA)
  • Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
  • Physical Or Chemical Processes And Apparatus (AREA)

Abstract

A process for the extraction of tritium from a liquid heavy water stream comprising contacting the liquid heavy water with a countercurrent gaseous deuterium in a column packed with a water-repellent catalyst, preferably a Group VIII metal having a water- repellent gas and water-permeable coating of an organic polymer or resin, such that tritium is transferred by isotopic exchange from the liquid heavy water stream to the gaseous deuterium stream, passing the gas enriched in tritium from the column through for example a cryogenic distillation column for removing tritium therefrom, returning the gas lean in tritium to the column and obtaining a liquid heavy water output from the column, said heavy water being reduced in tritium content. Feed water is preferably purified before passing through the column, and the gas stream from the column is purified prior to separation of the tritium.

Description

SPECIFICATION Process for the extraction of tritium from heavy water This invention relates to a process for the extraction of tritium from a liquid heavy water stream.
Nuclear power reactors of the type using heavy water (D20) as coolant and moderator incur a progressive build-up of tritiated heavy water (DTO) in the D2O) and this can lead to problems of controlling radiation exposure at the nuclear power stations. This D2) impurity is produced continuously in the reactor as the D20 is subjected to neutron irradiation. In present Canadian nuclear generating stations, the average tritium levels are the order of 1 curie per kg of D20 in the primary heat transport systems and over 10 curies per kg of D20 in the moderator systems and these levels are rising. Thus the tritium, while present in comparatively minute quantities, because of its radioactivity should desirably be extracted from reaction systems to maintain concentrations at current levels or lower.
Tritium oxide (or "tritiated water") can be concentrated by various processes such as vacuum distillation or electrolytic cascade (several stages of water electrolysis). However, these processes are of limited usefulness because of high toxicity of tritium in the oxide form, the low separation factor for water distillation, and the high power consumption for the electrolysers. A more practical method is to either convert the tritiated heavy water to the elemental form, for example, by water electrolysis or to extract tritium from water by catalytic exchange with a deuterium stream. The much less toxic elemental form can then be enriched by known processes such as distillation at cryogenic temperatures.
A process for removing protium and tritium from heavy water by vapour-phase catalytic exchange is described in United States Patent No. 3,505,017. Although the process described and claimed in this patent includes the steps of tapping the heavy water contained in a nuclear reactor and subjecting said tapped heavy water to an isotope exchange reaction with gaseous deuterium, it is obvious from the disclosure that the "tapped heavy water" is heavy water vapour. Because the exchange is between water vapour and gas, the two streams flow to the exchange column concurrently and the process must operate at elevated temperatures (80 to 4000C using catalysts). This process involves the use of evaporators and condensers at each equilibirum exchange step and this is most disadvantageous. Both in energy consumption and the complexity of the process.
A process for hydrogen isotope concentration between liquid water and hydrogen gas is described in United States Patent No. 3,981,976. This patent points out that the process may be used to reduce the tritium concentration, present as DTO, in heavy water that has been used in an operating nuclear reactor. This is achieved by increasing the concentration of tritium in liquid water by donation from gaseous deuterium derived from the liquid water. The deuterium is produced from heavy water in a deuterium gas generator.
In accordance with this invention tritium is extracted from a liquid heavy water stream by contacting the heavy water with a countercurrent gaseous deuterium stream in a column packed with a catalyst such that tritium is transferred by isotopic exchange from the liquid heavy water stream to the gaseous deuterium stream. The catalyst must of course, be able to function in the presence of liquid water. One such catalyst is described in United States Patent No. 3,888,975.
The above process has the advantages of being operable at ambient temperatures and on liquid heavy water extracts, rather than concentrates. Moreover evaporation and condensation of the heavy water is not required, nor is a deuterium gas generator.
The process of the invention will be further described with reference to the accompanying drawing, in which: Figure lisa flow diagram of the process; Figure 2 is a representative equilibrium diagram for the process, and Figure 3 shows protium extraction.
Referring to Figure 1, a liquid heavy water feed is passed through a purification stage 10. Depending on the quality of the feedwater, the feedwater purification stage will include a filtering system to remove suspended solids, an ion-exchange system to remove ionic compounds and a standard degassing system to remove dissolved gases such as O2 and N2. If the feedwater is contaminated with oil or other organic materials, it will be purified by charcoal adsorption or chemical methods. Normally, the heavy water withdrawn from reactor systems is relatively clean and will be passed only through a filtering and an ion-exchange system.The tritium to deuterium (nod) atom ratio (XO) in the heavy water stream is in the order of parts per million (typically 0.1 - 110 ppm) and the principle tritium-deuterium species, are DTO and D20. After purification, the liquid stream is fed to the top of a catalytic isotope exchange column 11 in which the tritium is extracted from the liquid stream by contacting it with a counter-flowing gaseous stream of DT - D2 in the column packed with a water-repellent catalyst. The process is operative with any type of catalyst that is water-repellent bu the preferred type is that described in United States Patent No. 3,888,974.This catalyst consists of at least one catalytically active metal selected from Group VIII of the Periodic Table having a substantially liquid-water-repellent organic resin or polymer coating thereon which is permeable to water vapour and hydrogen gas. This type of catalyst is also described in the aforementoned U.S. Patent No.
3,981,976 and United States Patent No. 4,025,560. After passing through the column the detritiated liquid heavy water (T/D = Xn) is returned to the nuclear reactor or other source.
The deuterium gas (T/D = yO) entering the bottom of the column 11 is lean in tritium (DT component) and after leaving the column is enriched in tritium (DT). This gas (T/D = y1) is purified in gas purification stage 12 and sent to a cryogenic distillation stage 13 that lowers the concentration of the DT-T2 in the gas after which it is returned to the bottom of the column 11. The feedgas purification system for the cryogenic unit is designed to remove traces of impurities which condense and soiidify as the temperature of the feedstream drops (moisture CO2, N2,O2, CO). Typically, the feedgas purification train includes molecular sieve driers, regenerative heat exchangers and cryogenic silica gel or charcoal adsorbers.Distillation stage 13 gives as output a concentrated DT-T2 gas stream which would normally be withdrawn into suitable containers. The cryogenic D2 distillation stage 13 may be replaced with other isotopic separation processes such as thermal diffusion or gas chromatography.
The overall reaction in the catalytic isotope exchange column can be represented as follows:
The equilibrium constant K for the above reaction (expressed in terms of equilibrium mole fractions of the reaction species) is as follows: K=[D2cs,] [DTO(f)I (2) [DT(g)] [D20(f)] Since at low concentrations of DT and DTO, the mole fractions of D2 and D20 are nearly constant and approach unity, the above equation (2) can be simplified as follows: K = nD atom ratio in the liquid DTO-D20 phase (3) T/D atom ratio in the gaseous DP-D2 phase provided the liquid phase is in equilibrium with the gas phase.
Equation (3) is also used commonly to define the separation factor aforthe exchange of isotopic species between the liquid and gas phases. Therefore, K = a in this case.
The equation for K based on partition function ratios given by Bron J et al in the paper "Isotopic Partition Function Ratios Involving H2, H20, H2S, H2Se, and NH3", Z. Naturforsch, Vol.
28a, pgs 129-136, 1973 is as follows: 1 n k = 0.05352 + 0.29362 (300/T) + 0.46241 (300/T)2 - 0.27574 (300iT)3 + 0.06280 (300/T)4 (4) where T = Temperature, kelvins.
This shows that the separation factor at ambient temperature is approximately 1.6 (a = 1.6387 at 250C).
Thus very useful separation factor levels are obtained in the present process provided that: (1) water-repellent catalysts are used, (2) the atom ratio nD in gas entering the column, yO, is smaller than the atom ratio T/D in the liquid entering the column, XO, that is X0 > ayO.
As shown in Figure 2, the atom ratio T/D in liquid entering the column, xO, is greater than the atom ratio nD in the gas entering the column, yO, that is xO > ayO the atom ratio nD in the gas leaving the column, y1, is greater than in the gas entering the column, that is, y1 > ayO. As tritium is stripped from the liquid phase x0 > on, where isTID ratio in the liquid leaving the column.Figure 2 represents a simplified graphical determination of the number of equilibrium stages required to achieve a given extraction of tritium from the liquid-phase (frequently referred to as the McCabe-Thiele diagram). It involves alternative use of two plots: one representing a materials balance for each equilibrium stage, referrred to as the operating line, and the other representing the equilibrium T/D ratio in the liquid phase (x) and in the gas phase (y), referred to as the equilibrium curve.A material balance for the exchange tower can be expressed by the following equation representing the terminal points of the operating line, that is, xO and yea at the top of the tower and xn, y0 at the bottom of the tower: L(xO - x) = G(y1 - yO) where Land G are molar liquid and gas flow rates, respectively.
To achieve the transfer of tritium from the liquid to the gas phase, the operating line must always be below the qulibirium curve (xO > yO, y > y0 and x0 > xn).
In Figure 1, the cryogenic D2 distillation unit is designed to remove tritium (DT) from the circulating gas is enriched in the bottom section of the cryogenic distillation column 13.
As the gas leaving the top of the cryogenic distillation column has the same concentration of protium (HD) as the gas entering it, the concentration of protium in the gas entering and leaving the catalytic isotope exchange column 11 are also the same, and protium present as HDO in the feedwater. The process will also "extract HD" from feedwater, the same way it extracts DT, if gas returned column 11 is HD lean. The overall reaction is:
(The large arrow indicates the net result of the exchange reaction.) To achieve this, the cryogenic distillation column must be "extended" to provide a top HD enrichment stage (as HD is more volatile than D2 and distills to the top of the column).The new arrangement is shown in Figure 3 with distillation column 13 divided into a lower portion 13a for tritium extraction and upper portion 13b for protium extraction.
The protium-rich gas team (HD, H2) taken at the top of the protium enrichment section 13b is relatively small, less than 1/10th of that of the circulating gas, depending on the concentration of protium in the feed-water (HDO).
Two practical alternatives for using this gas are: (1) To burn it with a supply of oxygen. This produces approx. 40t 60% HID water vUhich can then gas be upgraded to reactor grade by common D20 upgrading methods.
(2) To use a small reverse catalytic isotope exchange column to transfer the remaining D2 in the gas to a natural feedwater stream. The gas leaving the column contains only a very small amount of D2 and can be discarded.

Claims (8)

1. A process for the extraction of tritium from a liquid heavy water stream comprising: (a) contacting the liquid heavy water with a counter-current gaseous deuterium stream in a column packed with a water-repellent catalyst effective to catalyse the transfer of tritium by isotopic exchange from the liquid heavy water stream to the gaseous deuterium stream; (b) passing the gas enriched in tritium from the column through means for removing tritium therefrom and returning the gas lean in tritium to the column, and (c) obtaining a liquid heavy water output from the column, said heavy water being reduced in tritium content.
2. A process according to claim 1,wherein the liquid heavy water from which tritium is to be extracted is passed through a purification stage prior to contact with said gaseous deuterium stream.
3. A process according to claim 1 or 2, wherein the tritium enriched gas stream from the column is passed through a gas purification stage prior to removal of the tritium therefrom.
4. A process according to claim 1, 2 or 3, wherein the tritium is separated from the tritium enriched gas stream by cryogenic distillation.
5. A process according to any one of the preceding claims, wherein said catalyst comprises at least one metal from Group VIII of the Periodic Table having formed thereon a water-repellent, gas and water vapour permeable coating of an organic polymer or resin.
6. A process according to any one of the preceding claims, wherein protium is also extracted from the said heavy water stream by passing the tritium enriched gas from the column through means for removing protium therefrom as well as through said means for removing tritium.
7. A process according to claim 6, wherein said protium is extracted by cryogenic distillation.
8. A process according to claim 1, substantially as hereinbefore described with reference to Figure 1 or Figure 3 of the accompanying drawings.
GB8000902A 1979-01-22 1980-01-10 Process for the extraction of tritium from heavy water Expired GB2039866B (en)

Applications Claiming Priority (1)

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CA000320154A CA1160428A (en) 1979-01-22 1979-01-22 Process for the extraction of tritium from heavy water

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GB2039866A true GB2039866A (en) 1980-08-20
GB2039866B GB2039866B (en) 1982-12-22

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JP (1) JPS55132622A (en)
BE (1) BE881265A (en)
CA (1) CA1160428A (en)
DE (1) DE3001967A1 (en)
FR (1) FR2446798A1 (en)
GB (1) GB2039866B (en)
IL (1) IL58977A (en)
IT (1) IT8067086A0 (en)
SE (1) SE439476B (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10381121B2 (en) 2013-11-13 2019-08-13 Savannah River Nuclear Solutions, Llc Decontamination of tritiated water
US11058994B2 (en) 2019-01-18 2021-07-13 Savannah River National Solutions, LLC Tritium cleanup system and method

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0198940B1 (en) * 1985-04-25 1989-07-12 GebràœDer Sulzer Aktiengesellschaft Process for the separation and enrichment of tritium from tritiated fluids, especially from cooling water of the primary cycle and the deuterium/tritium streams of a nuclear fusion plant
US5468462A (en) * 1993-12-06 1995-11-21 Atomic Energy Of Canada Limited Geographically distributed tritium extraction plant and process for producing detritiated heavy water using combined electrolysis and catalytic exchange processes

Family Cites Families (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR1526867A (en) * 1966-08-09 1968-05-31 Commissariat Energie Atomique Improvements in means for removing protonium and tritium from heavy water
JPS5544786B2 (en) * 1972-05-04 1980-11-14
JPS4918680A (en) * 1972-06-05 1974-02-19
JPS5132800A (en) * 1974-09-11 1976-03-19 Shigeki Awata KAIRYOTABAKOTSUKI FUIRUTAA
DE2905585A1 (en) * 1978-04-13 1979-10-18 Sulzer Ag PROCESS FOR SEPARATION OF THE HYDROGEN ISOTOPES H, D, T IN ORDER TO SELECTIVELY REMOVE INDIVIDUAL ISOTOPES FROM A MIXTURE

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US10381121B2 (en) 2013-11-13 2019-08-13 Savannah River Nuclear Solutions, Llc Decontamination of tritiated water
US11087897B2 (en) 2013-11-13 2021-08-10 Savannah River Nuclear Solutions, Llc Decontamination of tritiated water
US11058994B2 (en) 2019-01-18 2021-07-13 Savannah River National Solutions, LLC Tritium cleanup system and method

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Publication number Publication date
FR2446798A1 (en) 1980-08-14
SE439476B (en) 1985-06-17
SE8000447L (en) 1980-07-23
BE881265A (en) 1980-05-16
IL58977A (en) 1982-12-31
IT8067086A0 (en) 1980-01-22
CA1160428A (en) 1984-01-17
JPS55132622A (en) 1980-10-15
DE3001967A1 (en) 1980-07-31
GB2039866B (en) 1982-12-22

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