GB2032165A - Disposal of Tritium-containing Effluents by Solidifciation - Google Patents

Abstract

Tritium-containing effluents are converted to a solid disposable form by incorporation as setting water in a solid produced using a magnesia- based binding agent, preferably a Sorel cement. Suitably, magnesium chloride is dissolved in the effluent until the density is 18 to 24 DEG Be, and then 2 to 3.5 parts by weight of a magnesia (e.g. a combustion product of particle size < 100 mu m) per part of MgCl2 are stirred in, and stirring continues until the material thickens. Viscosity-increasing and water- absorbing additives may be incorporated. The product may be given an impermeable coat, e.g. of resin. The product can have several times the water content (w/w) of conventional gypsum or Portland cement, and bind it more securely.

Description

SPECIFICATION Disposal of Tritium-containing Effluents by Solidification The invention relates to a process for the disposal of tritium-containing effluents by binding them as "setting water" to solids.

In the production of energy by nuclear fission, a large number of solid and liquid fission and activation waste products are formed and must be kept safely away from the biocycle. In this respect, special problems are presented by the volatile products, of which the three main types are noble gases, effluents containing iodine, and effluents containing tritium. In particular, material of this last type which occurs in large quantities as effluents having a low tritium content, is difficult to remove in a biologically safe manner.

Proposals have been made to process these effluents in the plant by the concentration or by isotope separation methods. Means under discussion for subsequently keeping them in a biologically safe form are: storage in tanks; solidification with gypsum or Portland cement; and final storage in worked-out underground salt beds (in solidified form), aquiferous fissures (liquid), at sea (solidified) or in ice (frozen) in the arctic regions. The proposal to electrolyse the water and to bind the hydrogen isotopes as metallic hydrides or to incorporate them in organic compounds may be mentioned in passing.

Storage in a solidified condition in underground cavities or at sea appears to be preferable and possible at acceptable cost, but the materials, Portland cement and gypsum, which have hitherto been contemplated as binders for the solidification have the disadvantage that they bind relatively small quantities of water and liberate them again relatively readily. Thus, this method is attended by a considerable increase in the volume of the waste, and there is no guarantee of permanence of fixing.

According to the invention, a process for converting tritium-containing effluent to a disposable solid form comprises incorporation of the effluent as setting-water in a solid prepared using a magnesia-based binding agent, preferably a Sorel cement.

Magnesia-based binding agents are generally formed from a calcined magnesium oxide material with salts of diva lent metals, of which copper and zinc salts, magnesium sulphate and, most preferably, magnesium chloride, have been found to be suitable. Reaction of magnesium oxide, water and magnesium chloride leads to oxychlorides of the assumed composition MgCl2. 5 Mg(OH)2. 7-8 H 20, and mixtures corresponding substantially to this stoichiometric proportion give dense solid products.

The mixing water necessary for producing a plastic or pourable consistency suitable for the processing is completely absorbed by the mass as setting-water. It is convenient for this (effluent) water to contain the magnesium chloride (or other divalent salt), in which case there may be little difference in density between this solution and the relatively light magnesium oxide, and this will reduce the tendency of the mixture to separate into its constituents so that this can be less of a problem than with, for example, Portland cement suspensions. The products which are formed in the hardening are hard, elastic and of low porosity and they exhibit relatively little liberation of water.

A Sorel cement for solidifying radioactive muds (as an alternative to vitrification, coating with plastics or gelling and hardening by organic binders) has been considered by R. Bonniaud and P. Cohen, but in that work the binding of water was neither aimed at nor specifically investigated, and in fact the comparative results give the impression that Sorel cements are far less suitable than other materials for safe disposal of waste. Thus there is no suggestion that Sorel cements might be suitable for binding tritium-containing effluents.

In contrast thereto, however, it has now been found that magnesia binders, especially Sorel cements, exhibit far better behaviour, particularly in regard to the setting quantities and the setting strength, than the Portland cements and gypsums at present considered suitable. Thus, it has been found possible to produce mechanically reliable Sorel cement blocks having a water content of up to 60%. By means of the invention it may be possible to bind and dispose of tritium-containing effluents with almost complete safety, and at acceptable cost. The effluents may be bound in such a way that the increase in volume of material to be stored or otherwise disposed of is minimised.

The following table shows for gypsum, Portland cement and Sorel cement the quantities of water which are: (a) required by the stoichiometry; (b) necessary or desirable for mixing and (c) contained in fixed form in the end product. It clearly shows the advantages of the use of magnesia binders in a process according to the invention for the fixing of tritium-containing effluents: Percentage of water content Binder (a) ('b) (c) Gypsum 15.7% 2 2-29% 15.7% Portland cement Ca 27% 20 44% 15% Sorel cement Ca 44%*) 44% 44% *) This water content corresponds to the above-indicated formula, but in practice it was found possible to produce solid blocks containing up to 60% of water.By admixing water-absorbing additives such as kieselguhr, for example, it is possible to increase this value.

Examination of the change of weight in air shows that these magnesia-based materials give up relatively little water.

Leaching tests with tritium-labelled Sorel blocks showed losses of activity of which the absolute values were of the same order of magnitude as in the case of Portland cement blocks, which is to be regarded as very much more favourable behaviour having regard to the very much (e.g. 2 to 3 times) higher water content of the Sorel blocks.

In the practical application of the process of the invention, the following procedure is preferably followed: Anhydrous magnesium chloride is dissolved in the tritium-containing effluent until the density corresponding to the desired concentration is obtained (this should preferably be in the range from about 180 to 300 Be, more preferably approximately 250Be). This solution is then mixed with the magnesium oxide material in appropriate proportions (e.g. 2 to 3.5, and preferably about 2.2 to 2.5, parts by weight of MgO per part by weight of MgCl2), and the slurry formed is stirred until it thickens, and is allowed to harden in moulds.Any tendency of the freshly prepared mass to settle or to separate into its constituents can be reduced by admixing viscosity-increasing additives, carboxymethyl cellulose being a preferred example. Any microbicides present in commercial products of such nature may also be of advantage in the present case.

Particular importance in regard to the quality of the end products attaches to the nature of the MgO-material as well as to the optimum adjustment of the composition of the MgCI2 liquor and of the mixing proportions. It has been found that for the magnesia material, finely ground reactive combustion products having particle sizes of less than about 100,um are particularly suitable. Such products can give masses which harden particularly slowly and have a high water-retaining capacity, which properties are desirable.For example, use of the type "K" magnesia of Steirische Magnesit-lndustrie AG, Vienna [a caustically calcined natural magnesite containing 86-89% of MgO with 4 6% loss on ignition, and having a grain size corresponding to a screening residue on a sieve having 4900 apertures per cm2 of 6-8% and a weight per litre according to DIN 273 of about 650 g/l (run-in) and of about 1200--1 300 g/l (shaken in)j gave (after final hardening for 12 hours) products which did not soften on moist storage and did not harden completely (so as to produce cracking) on dry storage.

Finished blocks can be given as a vapour and diffusion barrier an appropriate coating, for example of a cold-setting synthetic resin which is as far as possible impermeable to water vapour. Such a coating may be applied in a number of layers, and there may also be metal layers (preferably intermediate resin layers). In one experiment, the blocks were dipped into a thin liquid singlecomponent urethane resin (G4, Voss-Chemie, Uetersen) to provide one or more layers of plastics. This resin condenses under the influence of the moisture of the air and is thus well compatible with the moist block surface. Other suitable plastics products, such as ones having a high CH content, are well known to the plastics specialist.

Some embodiments of the invention will now be described in greater detail with reference to examples: Example 1 1000 g of magnesium oxide was mixed with 1000 ml of aqueous, tritium-containing 22.2% (250Be) magnesium chloride solution with stirring. The mixture was stirred for a further 3 4 hours until the increasing viscosity completely prevented separation of the constituents of the mixture. The mass was introduced into moulds, the air was extracted therefrom, and the mass was left to harden for 12-24 hours forming blocks.After release of the blocks from the moulds, the outermost surfaces were left exposed to the air for a short time and the blocks were dipped one or more times--depending upon the required vapour-tightness-into a single-component urethane sealant resin, in the form of a thin liquid. For conveyance into the final store (salt cavern or deep sea) there is provided an additional sheet-metal or plastics container.

The water present in the mixture was totally retained by the solidifying mass to form solid blocks having a water content of 43%.

Example 2 1000 g of MgO was mixed with 2200 ml of a 16.4% (190Be) aqueoustritium-containing magnesium chloride solution. In order to prevent settling or separation into constituent parts due to the high water content, 10 g of carboxymethyl cellulose (CMC, Zelleim) was added to the magnesium chloride solution in order to increase the viscosity. In other respects, the procedure was the same as in Example 1. Solid blocks having a water content of 60% were obtained.

Example 3 1000 g of MgO was mixed with 100 g of copper hydroxycarbonate before the addition to the magnesium chloride solution in order to improve the resistance to lixiviation (by formation of atacamite). The mixture was mixed with 2200 ml of tritium-containing magnesium chloride solution (at 1 90Be). In other aspects the procedure was the same as in Example 1. Solid blocks having a water content of 58% were obtained.

Example 4 1000 g of MgO was mixed with a tritium-containing 27% aqueous, magnesium sulphate solution with stirring. In other respects the procedure was the same as in Example 1. Blocks having a water content of 48% were obtained.

The parts, percentages and proportions indicated in the foregoing description are by weight.

Claims (12)

Claims

1. Process for converting tritium-containing effluent to a disposable solid form by incorporation as setting-water in a solid prepared using a magnesia-based binding agent.

2. Process according to claim 1 wherein said binding agent is a Sorel cement.

3. Process according to claim 1 or claim 2 wherein said binding agent is prepared using as the magnesia component a magnesium oxide material such that the binding agent is capable of absorbing a large proportion of water to give a material which displays slow setting behaviour.

4. Process according to any one of the preceding claims wherein said binding agent is prepared using as the magnesia component a magnesium oxide material which is a reactive combustion product of particle size generally less than 100 ym.

5. Process according to any one of the preceding claims wherein substantially anhydrous magnesium chloride is dissolved in the effluent which is thereafter reacted with a magnesium oxide material to produce said disposable solid.

6. Process according to claim 5, wherein a magnesium chloride/effluent solution of density 1 8- 240Be is mixed with the magnesium oxide material in a proportion by weight of 2 to 3.5 parts of magnesium oxide material per part of magnesium chloride (w/w).

7. Process according to claim 6 wherein the mixture is stirred until it thickens.

8. Process according to any one of the preceding claims wherein one or more viscosity-increasing additives are incorporated during the preparation of said disposable solid.

9. Process according to any one of the preceding claims wherein one or more water-absorbing additives are incorporated during the preparation of said disposable solid.

10. Process according to any one of the preceding claims wherein the solid product is enveloped in a coating impermeable to water vapour.

11. Process according to claim 10 wherein the coating is applied in a plurality of layers, of which at least one is impermeable to water vapour.

12. A process for converting tritium-containing effluent to a disposable solid form substantially as described herein with reference to any of the Examples.