GB2358954A

GB2358954A - Detritiation method

Info

Publication number: GB2358954A
Application number: GB0002271A
Authority: GB
Inventors: John Richards Jones; Shui-Yu Lu
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 2000-02-02
Filing date: 2000-02-02
Publication date: 2001-08-08
Also published as: GB0002271D0

Abstract

A method for detritiating a tritiated material, the method comprising contemporaneously: <SL> <LI>a) contacting of the material with a hydrogen-containing liquid; and <LI>b) irradiating the material with microwave radiation in the presence of a catalyst; </SL> wherein the hydrogen-containing liquid comprises molecules having a non-zero dipole moment and at least one labile hydrogen atom.

Description

Detritiation method This invention relates to the field of decontamination of tritiated materials.

Waste from nuclear fuels often comprises tritiated materials (i.e. those having some tritium (3 H) instead of the 1H isotope of hydrogen), such as oils. These materials have to be stored since they cannot be incinerated or released into the environment by other means. In order to be able to dispose of such materials, they must first be detritiated (tritium replaced by the 1H isotope of hydrogen). This is conventionally achieved by heating the tritiated material with a catalyst and a hydrogen- containing solvent, such as water. The solvent becomes tritiated, while the formerly tritiated material can be readily disposed of. The conventional method is, however, relatively slow and relies on the use of relatively high temperatures. The invention in suit overcomes these problems and enables the cheap and fast detritiation of tritiated materials.

Unless otherwise indicated, references herein to `hydrogen' are to the 1H isotope.

In accordance with the present invention a method for detritiating tritiated material, the method comprising contemporaneously: a) contacting of the material with a hydrogen- containing liquid; and b) irradiating the material with microwave radiation in the presence of a catalyst; wherein the hydrogen- containing liquid comprises molecules having a non-zero dipole moment and at least one labile hydrogen atom.

This provides for a quick method for the detritiation of tritiated material.

The material can be any one of a tritiated liquid, a tritiated liquid mixed with a solvent (solvent being used to express a liquid other than the tritiated liquid) and a tritiated solid dissolved in a solvent. The solvent may comprise the hydrogen- containing liquid. Putting the tritiated material into the liquid phase maximises areal contact between tritiated molecules and the molecules of the hydrogen-containing liquid.

The catalyst can be any one of Raney Nickel, a Lewis acid, a zeolite having super-acid characteristics and a polysulphonic acid catalyst. The Lewis acid may be any one of AlC13, SbF5, BBr3 and EtAlC12. The optimum catalyst for a given system will depend on the contaminated material and the hydrogen- containing liquid.

The material and hydrogen- containing liquid can be miscible. If the material and hydrogen- containing liquid are immiscible, then it is preferable to agitate the material and hydrogen- containing liquid to increase contact between the two.

In order to ensure a rapid reaction when dealing with immiscible phases, it is preferable to use a phase-transfer catalyst. This will improve the efficiency of hydrogen exchange.

A phase-transfer catalyst may be polymer-supported or sol- gel encapsulated. This allows easy extraction of the catalyst.

The hydrogen- containing liquid may be one or more alcohols, or water, or a mixture thereof. Water is preferred since it has a high density of reactive hydrogen atoms. Furthermore, tritiated water can carry a large amount of tritium, which makes storage cheaper. Tritiated water can be recycled.

The microwave radiation may be polychromatic or substantially monochromatic and the characteristics of the radiation (principally the f requency)may be selected to improve the efficiency of the method. The radiation may be pulsed or continuous.

The invention will now be described by the way of example only.

One type of tritiated oil is used for a11 of the experiments mentioned below. NMR analysis of the oil suggests the presence of methine groups, cyclic saturated groups, methylene groups in terminal and branched hydrocarbon chains, but an absence of aromatic groups.

One of the most widely used conventional methods of detritiation uses thermal heating. Examples 1 to 4 give details of typical results obtained by the use of this known detritiation technique.

Example 1 A Pyrex tube containing tritiated oil (55mg), Raney nickel (150mg) and water (150A) was sealed and the contents heated to 120 C for 48 hours. After cooling, the oil was extracted in diethyl ether (15m1), washed with water (3 x 5m1) and dried over anhydrous NaZS04. Liquid scintillation counting of the oil in an Ecosint A scintillator showed that the radioactivity had been reduced by 62% (from 140 KBq/mg to 53 KBq/mg). Example 2 The methodology of Example 1 was repeated, but substituting ethanol for water. A 54% reduction in radioactivity was observed (from 140 KBq/mg to 65 KBq/mg).

Example 3 The methodology of Example 1 was repeated, but the reagents were heated to 160 C, instead of 120 C. An80% reduction in radioactivity was observed (from 140 KBq/mg to 28 KBq/mg). Example 4 The methodology of Example 1 was repeated, but the ratio of water to oil was 20:1 #L1/mg. The radioactivity was reduced by 710 (from 140 KBq/rng to 40 KBq/mg).

The examples above illustrate that the described known method is very slow.

Reaction scheme 1 below shows a reaction methodology in accordance with the present invention.

IA1c:R0%,A,1'ZS , 1) RT + R H ':4 RH + R T CA-FA lY wherein RT is the material to be detritiated and R'H is the hydrogen- containing liquid. R represents any chemical moiety which has become tritiated and R'H is any molecule that has a non-zero dipole moment and at least one labile hydrogen atom (comprising water, or an alcohol, for example). The hydrogen- containing liquid is contacted with the tritiated material while being irradiated by microwave radiation in the presence of a catalyst. A large percentage of the tritium atoms in the tritiated material are replaced by 1H atoms (forming RH), while the tritium atoms replace 1H atoms in the hydrogen- containing liquid (forming R'T). The exact mechanism of this substitution process is unknown, as is the role that microwaves play in producing such an effective detritiation method.

Reaction scheme 1 illustrates the exchange of one tritium atom per molecule. It will be appreciated by those skilled in the art that this is not a limitation of the present invention. Many tritium atoms on one molecule may be replaced by hydrogen atoms using the method in accordance with the present invention. Examples 5 and 6 below are in accordance with the present invention and demonstrate the relative high speed of the new method. Examples 5 and 6 are by way of example only. Example 5 A mixture of tritiated oil (6.7mg), water (1651) and Raney Nickel (65mg) was placed in a pear-shaped glass flask (25m1) fitted with a silicone rubber septum disc as a pressure release device. The contents were under nitrogen gas protection. The vessel was irradiated in a conventional microwave oven (700W for 5 pulses, each pulse lasting 2 minutes, approx. 2 minutes between each pulse). Since the oil and water do not mix, this reaction is a heterogeneous reaction. After cooling, the oil was extracted into diethyl ether (15m1), washed with water (3x5ml) and dried over anhydrous Na2S04. The solvent was removed from the detritiated oil using a rotary evaporator. Liquid scintillation counting of the oil showed an87% reduction in radioactivity (from 61.8 KBq/mg to 7.9 KBq/mg).

Example 6 The methodology of Example 5 was applied to a larger sample of the same tritiated oil (14.8mg). Liquid scintillation counting showed a 75% decrease in radioactivity (from 61.8 KBq/mg to 15.7 KBq/mg).

Examples 5 and 6 illustrate the unexpected utility of the present invention. Furthermore, it is extremely unlikely that the rapid detritiation observed can be explained by the heating effect of the microwave radiation alone. Other effects must be present which are currently not understood. In Examples 5 and 6 above, water is used as the hydrogen- containing liquid. It is expected that the liquid could comprise other liquids having a non-zero dipole moment and at least one labile hydrogen atom, such as alcohols (or mixtures thereof), or mixtures of water and on or more alcohol.

In Examples 5 and 6 it was found not to be necessary to agitate the reagents. It is anticipated that it may be advantageous to agitate larger volumes of liquid such that detritiation is effective.

While this method has only been demonstrated with Raney nickel as a catalyst, it is anticipated that many other catalysts (such as Lewis acids, zeolites having super-acid characteristics, boron tribromide and polysulphonic acid catalysts) will produce the same effect. The catalysts may be supported on a polymer or in a sol-gel matrix. Furthermore, it is expected that tritiated solids could be dissolved in suitable solvents to enable detritiation to be achieved by the given method.

It is anticipated that the tritiated material (be it a tritiated liquid, a solution of a tritiated solid or a mixture of a tritiated liquid and solvent) may be immiscible with the hydrogen- containing liquid. In such a case, phase- transfer catalysis may be preferable.

The microwave oven used is of the multimodal (or polychromatic) kind. It is anticipated that a microwave source of the monomodal (or monochromatic) kind could also be used. Whilst the experiments described herein use short pulses of radiation it is expected that continuous application of microwave radiation may be beneficial provided a high pressure vessel is used to contain the reagents. It is further expected that the characteristics of the microwave radiation (principally, but not exclusively, the frequency characteristics) could be selected to increase the rate of detritiation and/or decrease the amount of tritiated material that remains after treatment.

It is also anticipated that although the method in accordance with the present invention has only been exemplified using tritiated materials, it (or a trivial variant) may be used by a person skilled in the art to treat deuterated materials in order to replace deuterium (Z H) atoms with hydrogen (1H) atoms.

Claims

Claims 1. A method for detritiating a tritiated material, the method comprising contemporaneously: a) contacting of the material with a hydrogen- containing liquid; and b) irradiating the material with microwave radiation in the presence of a catalyst; wherein the hydrogen- containing liquid comprises molecules having a non-zero dipole moment and at least one labile hydrogen atom.
2. A method according to claim 1 wherein the material comprises any one of a tritiated liquid; a mixture of a tritiated liquid and a solvent; and a solution of a tritiated solid in a solvent.
3. A method as claimed in claim 2 wherein the solvent comprises the hydrogen- containing liquid.
4. A method according to any one of claims 1 to 3 wherein the catalyst comprises any one of Raney Nickel, a Lewis acid, a zeolite having super-acid characteristics and a polysulphonic acid catalyst.
5. A method according to claim 4 wherein the Lewis acid comprises any one of AlC13, SbFs, BBr3 and EtAlC12.
6. A method according to any preceding claim wherein the material is miscible with the hydrogen- containing liquid.
7. A method according to any one of claims 1 to 5 wherein the material is immiscible with the hydrogen- containing liquid.
8. A method according to claim 7 wherein the material and hydrogen- containing liquid are agitated to increase contact between the material and hydrogen- containing liquid.
9. A method according to any one of claims 7 to 8 wherein the catalyst comprises a phase-transfer catalyst, such as tributyl (hexa)phosphonium bromide.
10. A method according to claim 9 wherein the catalyst is polymer-supported.
11. A method according to claim 9 wherein the catalyst is sol-gel encapsulated.
12. A method according to any preceding claim wherein the hydrogen-containing liquid comprises one or more alcohols, or water, or a mixture thereof.
13. A method according to any preceding claim wherein the microwave radiation is substantially monochromatic.
14. A method according to any one of claims 1 to 12 wherein the microwave radiation is substantially polychromatic.
15. A method according to any preceding claim wherein the microwave radiation is pulsed.
16. A method according to any one of claims 1 to 14 wherein the microwave radiation is continuous.
17. A method according to any preceding claim wherein the characteristics of the microwave radiation are selected to improve the efficiency of the method.
18. A method for detritiating a tritiated material as hereinbefore described.