GB2328784A

GB2328784A - Disposal of toxicant-, particularly radiotoxicant-, contaminated articles

Info

Publication number: GB2328784A
Application number: GB9818930A
Authority: GB
Inventors: Gerd Haag; Reinhard Odoj; Werner Von Lensa; Werner Schenk
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1997-08-29
Filing date: 1998-08-28
Publication date: 1999-03-03
Anticipated expiration: 2018-08-28
Also published as: DE19737891C2; FR2767957A1; GB9818930D0; DE19737891A1; GB2328784B; FR2767957B1

Abstract

A process for the disposal of an article contaminated by a toxicant, in particular a radiotoxicant involves removal of at least a part of the toxicant from the article by heating this, simultaneously to collect the removed portion of the toxicant and then to feed the decontaminated article and the collected toxicant separately to further disposal steps. If the toxicant is stably bound, even at high temperatures, to the, for example ceramic, base structure of the article, it can be converted into a thermally removable chemical compound, e.g. a halide, by chemical reaction with a suitable substance, e.g. a halogen. The decontaminated article may be protected by infiltration of suitable substances, chemical reaction and/or coating against chemical attack, leaching, diffusion of the residual contamination out of the article and/or combustion, so that either a special container for the transport and temporary or permanent storage is not necessary, or simplified containers can be used. The process is particularly suitable for treatment of carbon moderator elements.

Description

1 2328784 DISPOSAL OF TOMCANT -, PARTICULARLY RADIOTOMCANT COU_ATED

ARTICLES The invention relates to a process for the disposal of an article contaminated by a toxicant, in particular a radiotoxicant.

The term 'oxicant" is taken to mean not only a single toxic substance but also the totality of a plurality of toxic substances present in the article.

The disposal of contaminated articles is an important economic factor in conventional technology and, in particular, in connection nith the operation, decommissioning, and removal of nuclear plants. The disposal of nuclear plants or of their components is, in particular, complex and thus a considerable cost factor, because pollution of the environment with toxic, in particular radiotoxic, materials must be avoided or kept low.

Currently, and in the near future, for example, a plurality of reactors having moderators made of carbon, most of which are among the oldest nuclear plants, are being decommissioned. In particular in such nuclear plants, graphite, carbon brick and such materials having another ceramic basic structure are used in many ways, e.c. as moderators or as structural materials and insulation materials. Metallic articles, e.g. gas pipes made of high-temperature resistant alloy&, can also be contaminated and are therefore to be fed to an appropriate disposal. Rmfioactive contamination of these materials is due to neutron activation of existing chemical contaminants or by adsorption or diffusion of fission products.

It is known to dispose of weakly radioactive wastes by sinking these in the sea. This procedure is of ecological concern and therefore has no longer been practised for many years by western countries in accordance with the relevant London Convention.

Furthermore, it is knomm to deposit low-activity wastes in special repositories either immediately in a permanent store or for periods of 50 to 100 years in an intermediate store. Problems in this case are the high amounts arising: for example, in the case of a 2 decommissioned carbon-moderated reactor. depending on its size, several hundred tons of low-activity wastes arise.

In order to reduce the volume of waste, it is known to incinerate lowactivity wastes and only to store the combustion residues permanently. However, in the case of carbon structures, problematic radionuclides such as tritium and 14C are released into the atmosphere in this process, and it is questionable whether this corresponds to the requirement for minimization and whether the location of the incineration and the wind conditions prevailing there actually ensure any tolerable dilution of these radionuclides in the atmosphere.

For the immediate permanent storage of contaminated articles without prior reduction of volume, most countries lack adequate andlor cost- effective storage capacities. The problem of quantity is made considerably more acute by the fact that not only the quantity of the waste per se, but also the type of contamination is critical for the volume requirement of a store or of a transport andlor storage vessel, as well as for the relevant safety precautions. Thus, for example, in the case of the radiotoxicant 60Co, only a defined amount of this isotope may be stored per container, in order not to exceed legally specified limit values of radiation pollution outside the container and for the operating personnel. The extra costs caused by this arise not only for the provision of containers, but also for transport and permanent storage.

Tne long-term temporary storage of the radioactive wastes only puts off the problems of permanent storage and cannot therefore be considered as a final disposal alternative.

It is then an object of the present invention to provide a process of the type mentioned at the outset in which the abovementioned disadvantages are avoided or at least minimized. This object is achieved by a process of the type mentioned at the outset in such a manner that a) for the decontamination of the article, at least some of the toxicant is removed from the article by heating this, 3 b) the removed portion of the toxicant is collected and the decontaminated article and the collected toxicant are fed separately to further disposal steps.

In this manner, at least a considerable reduction of the toxicant in the article is achieved, and simultaneously by collecting the toxicant, its accumulation outside the article is achieved. The problem of quantity in disposal is thus markedly decreased: the toxicant present at high concentration is fed to a suitable device as early as on collection or subsequently and can then be sed of in a manner as is ay known for more highly radioactive wastes, e.g. filters from nuclear plants. In the decontaminated article, the concentration of toxicant is reduced. 7terefore, the material of the article can be stored temporarily or permanently under less stringent requirements. Thus, it is made possible, for example, by reduced shielding measures, in particular smaller container wall thicknesses, to decrease the volume required and thus simultaneously also the costs in permanent storage and during transport.

g The toxicant is removed on heating, depending on the type of the toxicant, either by simple outgassing of a toxicant (e.g. tritium or caesium) physically bound, e.g. by adsorption, to C> C the, for example ceramic structure, of the article or, if the toxicant is chemically bound (e.g. as carbide or oxide), by thermal decomposition.

The toxicant can be collected e.g. by condensation in cold traps or separation on suitable filter substances or in liquids.

If a toxicant, on account of the high bonding stability, cannot be expellec4 or can only be expelled over uneconomically long periods, by heating the article, it is advantageous to carry out the process according to the invention in such a manner that the toxicant is 0 converted by chemical reaction with a suitable substance, e.g. a halogen, j to a chemical C g n compound, e.g. a halide, which can be thermally removed from the article.

It can also be advantageous to carry out the process according to the invention in such a manner that, if the toxicant is present at an elevated concentration at the surface andlor in a surface-near region, in comparison with the interior of the article. this surface or the 4 surfacenear region is decontaminated by selecting for the heating a temperature at which a 0 further part of the toxicant can diffuse from the surface or from the surface-near region to the interior of the article.

The surface or the surface-near region is decontaminated in this case- via two processes proceeding simultaneously: the removal of the one part of the toxicant from the article and the diffusion of a further part into the interior of the article. This type of decontamination is useful in the case of those toxic substances which only act on the environment in a critical manner if they are bound in surface-near regions, e.g. (, and (eniitters. It is then not necessary to remove virtually the entire amount of the toxicant, as a result of which the decontamination could be carried out in a less complex manner, e.,o,,. at relatively low temperatures.

In the decontamination phase, the article is exposed to environmental conditions which highly probably no lono,,er occur during the further disposal, in particular during temporary 1 andlor permanent storage. A residual contamination which has withstood these conditions will therefore generally be bound with sufficient stability for the temporary andior permanent storage in the otherwise decontaminated article.

However, further protective measures may be appropriate, since unexpected changes of the storage conditions cannot be completely excluded. In particular, the decontaminated article should be stable to high ambient temperatures, high pressures, impact loads and the attack of physically and/or chcraically active substances. To this end, it can be advantageous to carry out the process according to the invention in such a manner that, to protect agairist the release of the residual contamination, at least one closed layer is formed to seal andlor inhibit diffusion on the surfacc of the decontaminated article by infiltration of at least one protective substance, e.g. pyrocarbon, suitable for this purpose.

A proteetive substance of this type is deposited, in particular, into the pores of the decontaminated article and prevents, in particular, an unwanted penetration of substances with which the residues of the toxicant remaining in the article could be expelled chemically or by dissolution during the temporary or permanent storage. Certain protective substances, such as pyrocarbon, furthesmore, offer an effective protection against combustion and against the diffusion of the toxic substance to the surface of the article.

The infiltration of the pyrocarbon can take place, for example, at temperatures above 1000Q by exposing the article to a hydrocarbon atmosphere.

The process according to the invention can also be carried out in such a manner that, to protect against the release of the residual contamination, at least one closed layer is formed to seal andlor prevent diffusion at the surface of the decontaminated article by chemical reaction with a substance suitable therefor.

By the chemical treatment, in particular, rendering inert to further chemical attack, can be achieved. In the case of articles having a carbon surface layer, a closed layer can, for example, be reacted with silicon compounds and, by further treatment steps, converted to silicon carbide which is chemically resistant even at high temperatures, i e. is also highly resistant to combustion. A surface conditioned in this manner, furthermore, offers protection against penetration of substances and diffusion of the toxicant out of the article.

0 The process according to the invention can also be carried out in such a manner that, to 0 protect against the release of the residual contamination, the surface of the decontaminated article is coated.

In particular, CVD processes, slurry processes and plasma spraying processes can be employed therefor, where CV1) stands for ChemicaI Vapour Deposition..

Finally, it can be advantageous to carry out the process according to the 0 invention in such a manner that, before, after andlor during the decontamination of the article, the concentration of the toxicant in the article andlor the concentration of the collected part of the toxicant is monitored.

The concentration can be monitored in the case of radiotoxic substances, for example, by (-spectrometry or using a Geiger counter.

The parameters to be used in the process according to the invention, such as temperature, residence times, pressure, coating thickness, are each dependent on the type and original concentration of the toxicant and on the sought-after properties of the decontaminated 6 article and of the collected toxicant and may be established individually on the basis of the concentration monitoring.

A preferTed embodiment of the process according to the invention is desciibed below, by way of example.

As an example of an article to be disposed of, a carbon moderator element used in a nuclear power station will be considered below. Moderators of this type have a ceramic structure composed of graphite and carbon brick. Usually, a moderator of this type is contaminated by various toxic substances. In order to remove these toxic substances as completely as possible, the moderator element is heated in a high-temperature furnace by direct passage of current. Alternatively, it can be heated inductively or by separate heating elements- The heating is performed in vacuo or under a protective gas to prevent, during heating, reaction with the atmospheric oxygen resulting in the formation of carbon monoxide and carbon dioxide, which contain radioactive 14C generally present in the irradiated carbon and therefore must not escape in an uncontrolled manner.

The temperature level depends on the type of toxic substances and the decontamination factors sought after. In this case, recourse can be made essentially to the procedure known in the graphite industry for graphite purification. Readily volatile toxic substances, such as tritium or caesium, are outgassed even at relatively low temperatures. Toxic substances chemically bound to carbon must be detached from the ceramic structure by pyrolysis at higher temperatures. If individual toxic substance cannot be mmoved by this means from the moderator element, e.g. carbides which are difficult to decompose, these can be converted into volatile halides by addition and infiltration of gaseous halogen compounds.

0 Ile toxic substances removed from the moderator element are deposited on condensation plates or collected by means of traps or by means of filters (e.g. for tritium), where they are then present at a considerably higher concentration than in the moderator element- The decontaminated moderator element then still contains, at most, toxic substances which could not be removed by thermal andlor thermochemical treatment. It follows from this that these toxic substances would not diffuse out or be leached out even over extremely long periods. TOXic substances bound so tightly in the cerarnic struccurc, however, permit 7 the decontaminated moderator element to be stored in a simplified container or without a surroundina container. In this case, articles made of combustible materials, such as the moderator element considered here, should be made inert on the surface and in the surface-near regions, e.g. by closing pores by i=ducing impermeable substances or applying protective coatings. Furthermore, the article should. be effem.vely protected against long-term chemical attack and leaching. The infiltrations of, or coatings with, suitable substances required for this can be carried out if appropriate immediately subsequently to the purification process, that is at the high temperatures then achieved.

The decontaminated moderator element which is protected from combustion and chemical attack can thereafter directly be permanently stored with d=eased expenditure. On account of the then, at most, very low residual concentration of toxic substances, decontaminated moderator elements can be stored at relatively high density without exceeding the toxic substance concentrations, critical for permanent storage, in the waste package.

0 The toxic substances which were trapped during heating of the moderator element and c collected at high concentration can be disposed of using known methods, as are carried out, c > for example, for filters from nuclear plants, since their disposal does not represent a quantity problem.

8 CLA" Process for the disposal of an article contaminated by a toxicant, in particular a radlotoxicant, in which a) for the decontamination of the article, at least some of the toxicant is removed from the article by heating this, b) the removed portion of the toxicant is collected and the decontaminated article and the collected toxicant are fed separately to further disposal steps.

Claims

2. Process according to Claim 1, characterized in that the toxicant is

converted by chemical reaction with a suitable substance, e.g. a halogen, into a chemical compound, e.c. a halide, which can be removed thermally from the article.

0 Process according to Claim 1 or 2, characterized in that, if the toxicant is present at an elevated concent-ation at the surface andlor in a surface-near region in comparison to the interior of the article, this surface or the surfacenear region is decontaminated by selecting for the heating a temperature at which a further pan of 0 the toxicant can diffuse from the surface or from the surface-near recion to the interior of the aiticle.

Process according to one of Claims 1 to 3, characterized in that, to protect against the release of the residual contamination, at least one closed layer is formed to seal and/or inhibit diffusion at the surface of the decontaminated article by infiltration of at least one protective substance suitable therefor, e.g. pyrocarbon.

Process according to one of Claims 1 to 4, characterized in that, to proteci against the release of the residual contamination, at least one closed layer is formed to seal 9 andlor inhibit difflision at the surface of the decontaminated article by chemical reaction with a substance suitable therefor.

6. Process according to one of Claims 1 to 5, chara terized in that, to protect against CR the release of the residual contamination, the surface of the decontaminated article is coated.

7. Process accordin to one of Claims 1 to 6, characterized in that, before, after andlor 0 durinc the decontamination of the article, the concentration of the toxicant in the tp article andlor the concentration of the collected part of the toxicant is monitored- 8. A process for the disposal of an article contarninated, by a toxicant substantially as hereinbefore described by way of example.