Conditioning of high level reactor core graphite waste using self-propagating high temperature synthesis

12/29/2014 2014 - #04 Fuel cycle and nuclear waste management

Konovalov E.E. Naumov V.S. Lastov A.I.

https://doi.org/10.26583/npe.2014.4.09

The decommissioning of uranium-graphite commercial reactor is associated with formation of considerable quantities of radioactive graphite wastes, part of them being of high level class and requiring special safety measures in their long-term isolation from the biosphere. The radioactivity of graphite stacking of the reactor under decommissioning is determined by the products of graphite and its impurities activation, as well as the nuclear fuel elements and fission products penetrating the graphite stacking (“spill”) as a result of abnormal situations and events associated with a broken tightness of fuel element and destruction of fuel assembly. The principal radionuclides resulting in graphite activation are 14С, 3Н, and 60Со uniformly distributed over the entire graphite stack. Their specific activity in terms of graphite amounts to 108– 109 Bq/kg. Among the fission products, the largest contribution to the radioactivity is from such radionuclides as 90Sr, 137Cs, 147Pm, 151Sm, and 154Eu and actinides such as 238-240Pu, 241,243Am, 244Cm. The contribution of the latter to the total activity of transuranium elements in the first years after the reactor is shut down achieves the values exceeding 90%. The specific activity of 137Cs in graphite varies in a wide range from 105 to 109 Bq/kg depending on the duration of fuel irradiation.

The method of conditioning the HLW of graphite using self-propagating high temperature synthesis proposed in this work (SHT) is an alternative to the methods suggested earlier. The method for conditioning the graphite HLW discussed is a one-stage method, it excludes the formation of secondary wastes, which would also need conditioning. The process does not need energy supply because it develops as out-furnace, owing to the chemical energy release in an exoenergic self-sustained solid state process.

Actinides of the spill present in the HLW in oxide form are immobilized into stable matrix materials, for example, into the Y-Al garnet, which is considered to be one of the promising materials for immobilization of minor actinides.

X-ray diffraction analysis of the SHT product of processing the model HLW of graphite has shown that the composition of carbide-oxide composite (matrix) predominantly includes phases of garnet, corundum, and titanium carbide. No independent phases of the spill simulators-oxides are formed because they participate in the structure of Y-Al garnet.

Thus, technological operation of conditioning of the high level wastes of graphite from uranium - graphite reactors with immobilization of transuranium elements, fission products of 14С radionuclide in a stable carbide-oxide composite has been studied in this work. Oxides of actinides and elements – fission products are isomorphically fixed in the garnet structure, and 14С is fixed in the structure of titanium carbide. The composite synthesized (matrix) reliably isolates the spill from the biosphere, this is proved by low values of leaching 238,235U and 137Cs from the matrix.

It is recommended to use the results of laboratory studies carried out for the development and designing the experimental industrial equipment for processing and rendering graphite HLW harmless. The technology proposed offers high technical and economic indexes.

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decommissioning graphite stacking activation products fission products transuranium elements one-stage process Y-Al-garnet corundum titanium carbide radionuclide leaching velocity

Link for citing the article: Konovalov E.E., Naumov V.S., Lastov A.I. Conditioning of high level reactor core graphite waste using self-propagating high temperature synthesis. Izvestiya vuzov. Yadernaya Energetika. 2014, no. 4, pp. 82-91; DOI: https://doi.org/10.26583/npe.2014.4.09 (in Russian).