CH647480A5 - PROCESS FOR EXTRACTING TRITIUM FROM A LIQUID HEAVY WATER STREAM. - Google Patents

Description

This invention relates to a process for the simultaneous extraction of protium and tritium from a stream of liquid heavy water.

Nuclear power reactors of the type using heavy water (D20) as coolant and moderator undergo a progressive formation of tritiated heavy water (DTO) in D20, and this can lead to problems of controlling exposure to radiation in nuclear center. This impurity in D20 is produced continuously in the reactor as D20 is subjected to neutron irradiation. In current Canadian nuclear power plants. the average levels of tritium are of the order of 1 Ci / kg of D20 in the primary heat transport systems and greater than 10 Ci / kg of D20 in the moderating systems. and these levels are increasing. Thus, tritium, while present in relatively small amounts, due to its radioactivity, should be extracted from reactor systems to maintain concentrations at normal or lower levels.

Tritium oxide (or tritiated water) can be concentrated by various processes such as vacuum distillation or the electrolytic cascade (several stages of water electrolysis). However, these methods have limited utility because of the high toxicity of tritium in its oxide form, the low separation factor for water distillation, and the high energy consumption of electrolytic systems. A more practical method is either to convert the water: tritiated urea to the elementary form, for example by electrolysis ie water, or to extract the tritium from the water by catalytic exchange with a current of deuterium. The elemental form, which is much toxic, can then be enriched by known methods, such as distillation at cryogenic temperatures.

A process for extracting protium and tritium from heavy water) by vapor phase catalytic exchange is described in the US patent No. 3505017 of E. Roth, published on April 7, 1980. Although the> process describes and claims in this invention comprises the steps of deriving heavy water contained in a nuclear reactor t of subjecting this heavy water derivative to an exchange reaction [isotopes with deuterium gas, it is obvious from the invention that the derived heavy water is heavy water vapor. Being that the exchange takes place between water vapor and gas, the two streams flow to the column simultaneously and the process must take place high temperatures (80 to 400 C using catalysts). This process involves the use of multiple stages and evaporators and condensers at each equilibrium exchange step, which is extremely disadvantageous, both in which relates to energy consumption and the complexity of the pr oceded.

A process for the concentration of hydrogen isotopes between liquid water and hydrogen gas is described in US Patent No. 3,981,976 to W.H. Stevens, published September 21, 1976 and assigned to Atomic Energy of Canada Limited. This patent notes that the method can be used to reduce the concentration of tritium, present as DTO, in heavy water which has been used in a nuclear reactor. This is achieved by increasing the concentration of tritium in liquid water by donation from deuterium gas derived from liquid water. Deuterium is produced from heavy water in a deuterium gas generator.

The following are patents relating to the extraction or removal of tritium from light and heavy water: Nos 4173620, from M. Shi-mizu, published on November 6, 1979, 4190515, from Butler et al., Published on February 26 1980, and 4191626, of Hammerli et al., Published on March 4, 1980. These three patents relate to systems using a catalytic exchange column and an electrolysis cell. In these systems, all of the liquid water passing through the catalytic exchange column passes into the electrolytic cell to convert it into hydrogen and oxygen gas. The electrolysis stage therefore needs to be large and therefore consumes a large amount of electrical energy.

In US patent application No. 06/079808, filed September 28, 1979, the applicant describes a system for extracting tritium from a stream of liquid heavy water. The present invention is an extension of this invention which allows a wider field of application.

It is an object of the present invention to provide a method for the simultaneous extraction of protium and tritium from liquid heavy water, which method is applicable at ambient temperatures and pressures or close to ambient or near pressures. ambient conditions.

Another object of the present invention is to provide a process for removing protium and tritium from liquid heavy water extracts, rather than concentrates of protium and tritium from heavy water. These and other objectives of the invention are achieved by a process for extracting protium and tritium from a stream of liquid heavy water, which process involves bringing the heavy liquid water into contact with a stream of deuterium gas countercurrent in a column packed with a catalyst, so that the tritium 45 is transferred by isotopic exchange from the stream of liquid heavy water to the stream of deuterium gas, to pass the gas enriched in tritium from the column through means for removing the tritium and returning the tritium-depleted gas to the column, taking part of the stream of liquid heavy water after it has left the column and passing it through d '' an electrolysis stage to produce an outlet of deuterium gas which is returned to the column and an outlet of oxygen gas which is rejected or can be used for other purposes, to obtain the remaining fraction of the outlet of heavy water column fluid, t their heavy water being reduced in tritium.

In the drawings which illustrate an embodiment of the invention,

fig. 1 is a flow diagram of the process in its simplest form, and FIG. 2 is a flow sheet of a complete system incorporated in a moderate nuclear reactor and cooled with heavy water.

With reference to fig. 1, a supply of liquid heavy water passes through a purification stage 10. Depending on the quality of the supply water, the purification stage of the feed water will include a system filtration system to remove suspended solids, an ion exchange system to remove ionic compounds, and a standard degassing system to remove dissolved gases such as 02 and N2. If the feed water is contaminated

3

647,480

born with oil or other organic materials, it will be purified by adsorption with carbon or by chemical processes. Normally, the heavy water removed from the reactor systems is relatively clean and will only pass through a single filtration and ion exchange system. The ratio of atoms between tritium and deuterium (T / D) in the heavy water stream is of the order of parts per million (typically 0.1-1.10 ppm) and the species tritium and deuterium are DTO and D20. After purification, the liquid stream is supplied to the upper part of a catalytic isotope exchange column 11 in which the tritium is extracted from the liquid stream by being brought into contact with a gaseous stream flowing in the opposite direction of DT / D2 in the column packed with a catalyst repelling or rejecting water. The process can be applied with any type of water repellant catalyst, but the preferred type is that described in U.S. Patent No. 3,888,974 to WH Stevens, issued June 10, 1975 and assigned to Atomic Energy of Canada Limited. This catalyst consists of at least one cataiytically active metal, chosen by group VIII of the periodic table, having on it a polymer or organic resin coating, repellent to liquids and water, which is permeable to water vapor and hydrogen gas. This type of catalyst is also described in the aforementioned patent No. 3981976 and in US Patent No. 4025560, by John H. Rolston et al., Published May 24, 1977 and attributed to Atomic Energy of Canada Limited. having passed through the column, a major proportion of the detrimented heavy liquid water is returned to the nuclear reactor or other source.

The deuterium gas entering the bottom of column 11 is poor in tritium (component DT) and, after leaving the column, it is enriched in tritium (DT). This gas is purified in the gas purification stage 12 and it is sent to a cryogenic distillation stage 13 which lowers the concentration of DT / T in the gas, after which it is returned to the bottom of the column 11. The feed gas purification system for the cryogenic unit is designed to remove traces of impurities which condense and solidify as the temperature of the feed stream drops (humidity C02, N2, 02, CO). Typically, the feed gas purification train includes molecular sieve dryers, regenerative heat exchangers, and cryogenic carbon or silica gel absorbers. The distillation stage 13 gives as output a concentrated gas stream DT / T2 which would normally be withdrawn in suitable containers. The cryogenic distillation stage of D2 13 can be replaced by other isotopic separation methods such as thermal diffusion or gas chromatography.

A proportion of heavy water of high concentration from the catalytic exchange column 11 is passed through the electrolysis stage 14, and deuterium gas is returned to the recycling loop of the distillation column, ie to the current of gas entering the exchange column, or to the current leaving the column, as illustrated by the broken line 14a.

The process transfers protium and tritium from heavy water to the cryogenic unit by contact of the feed water with a carrier gas D2 on the catalysts:

HDO + D2 <=> D, 0 + HD! _. ,.

DTO 4- D2 ^ D20 4- DT} Spange of catalysts

D; 0 / HDO / DTO v'- »D, / HD / DT (0,) Electrolysis

The method, in this case, performs a three-component transfer (D20 / HD0 / DT0 to D2 / HD / DT), as illustrated above.

For the extraction of tritium, two gas-liquid reactions occur simultaneously:

D: 0 ") + DT ,,., <H20 |", + HT ,.

: D2lg) 4- DTO ",; H2 (gl + HTO ,,,

(1) K, = 1.62 (25 C)

(2) K, = 6.8 (25 C)

JU) "H 11 Mg)

These are in equilibrium with the gas phase reactions D, + H, <= i2HD ... (3) Kj = 3.26

D, + HTî ^ HD + DT (4) K4 = 2.20

H, + DTï ^ HD + HT ... (5) K, = 1.48

and with the liquid phase reactions:

D20 + H20 2HDO ... (6) K6 = 3.8 (25'C)

D20 4- HTO HDO + DTO ... (7) K7 H, 0 + DTO HDO -I- HTO ... (8) Ks 5 K6 = K7K3

where K „K2, K„ are the appropriate equilibrium constants.

x (l-y),

The overall separation factor a T T

-and y y (1 -x) '

, can be calculated for

(H- + D + T) '(H + D-t-T)

the balance of the above reactions, but this is difficult due to the large number of simultaneous reactions taking place. In protium extraction, the equilibrium reactions are:

HDO (<) + D

2 <S> <

DzOï-O 4 "HD, e

D, 0 4- H20 U 2HDO

Dz 4- H, ^ 2HD

x (1 - y). a = -, or x =

K, = 3.23 at 25'C K, = 3.846 at 25JC K3 = 3.26 at 25 * C

D

20

y (1 - x) "D + H D

y = t in the gas phase.

in the liquid phase and

D + H

The reaction, however, can only be sustained temporarily. because the deuterium is depleted in the cryogenic unit. As illustrated in fig. 1.1 mol of D2 is transferred from the gas to the liquid per 1 mol of H, removed from the liquid; Therefore. the product leaving the column contains an additional 1 mol of D, 0 compared to that which enters the column with the feed water.

To maintain these reactions indefinitely, the additional D20 produced by the column is decomposed continuously and the D is returned to the cryogenic unit to avoid the depletion of D: this takes place in stage d electrolysis 14. For applications on reactors, the best process for this partial decomposition of product D20 is electrolysis. However, chemical decomposition of water (such as Mg -I- D20 - "MgO 4-D2) or other methods could be used.

As illustrated in fig. 2, two exchange columns 1 la and 1 lb would be used in a system in connection with a nuclear reactor having a moderating system 15 and a heat transport system 16. The leaks recovered from the two systems are stored in the tank 17 and pass to column IIb. The column called detritiation removes the large part of tritium and a relatively small quantity of protium. while the column called high content removes the large part of protium and relatively small quantities of tritium. Although theoretically a column, as illustrated in fig. 1, can be used, such a device would be expensive. The coupling between the cryogenic unit and the catalytic exchange must be designed with care, since it constitutes the bulk of the installation work to a much greater extent than in the other processes. The key function of the cryogenic unit is to produce relatively pure currents D :, H, and T2. The current D2 is returned to the catalytic exchange column (to take more protium and tritium), the current H2 can be discharged as waste, and the current T, is stored.

55 Each catalytic column supplies the gas to a separate cryogenic distillation column: the protium 18 distillation column (fed by the high-content exchange column) is designed to produce a H2 stream of high concentration, but it does not concentrate tritium; the gas coming from the bottom of this column, with the feed coming from the detritiation column, is sent to the deuterium distillation column 19 to produce a current D2 poor in protium / tritium in the middle part, and the currents very protium and tritium concentrates in the upper part and at the bottom of the column, respectively. The upper parts of this column are returned to the protium column and the bottoms are sent to a smaller tritium distillation column 20 for further separation.

1 sheet of drawings

Claims (3)

647,480 1. Process for the simultaneous extraction of protium and tritium from a liquid stream of heavy water, comprising: a) bringing the liquid heavy water into contact with a deuterium gas stream against the current in a column packed with a catalyst, so that the tritium is transferred, by isotopic exchange, from the liquid heavy water stream to the deuterium gas stream; b) passing the tritium-enriched gas from the column through means for extracting the tritium therefrom and returning the tritium-depleted gas to the column; e) take part of the stream of liquid heavy water after it has left the column and pass this part through an electrolysis stage to produce a gaseous outlet of deuterium, which is returned to the gaseous stream passing through through the column and towards the tritium extraction means. and an outlet for gaseous oxygen which is rejected or used for other purposes, and d) obtaining the remaining fraction of the liquid heavy water outlet from the column, the tritium content of the heavy water being reduced. 2. The method of claim 1, wherein the outlet of deuterium gas from the electrolysis stage is returned to the gas stream entering through the bottom of the column. 2 3. The method of claim 1, wherein the outlet of deuterium gas from the electrolysis stage is returned to the gas stream leaving the column.