Conditioning of high level reactor core graphite waste using self-propagating high temperature synthesis
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.
- Bulanenko V.I., Frolov V.V. Radiation characteristics of fuel residuals in stacking of the shut-down uranium-graphite reactors. Atomnaya energiya. 1995, v.78, no. 6, pp.396-400 (in Russian).
- Bagayev V.D., Baranov I.I., Kabanov Yu.I. et at. Decommissioning of commercial reactors at the Siberian Chemical Combine. Atomnaya energiya. 1996, v.80, no. 2, pp.71-73 (in Russian).
- Bushuyev A.V., Verzilov Yu.M., Zubarev V.N. et al. 60Co contents in spent stacking graphite in commercial reactors at the Siberian Chemical Combine. Atomnaya energiya. 1999, v. 86, no. 3, pp.183-188 (in Russian).
- Bushuyev A.V., Verzilov Yu.M., Zubarev V.N. et al.Experimental studies of graphite stacking radioactive contamination in commercial reactors at the Siberian Chemical Combine. Atomnaya Energiya. 2002, v. 92, no. 6, pp.477-485 (in Russian).
- Korenyov S.V., Levunin S.L., Pryanichnikov A.G.et al. Research in radiation characteristics of graphite samples from the commercial uranium-graphite reactors under decommissioning. Transactions of the IV Russian Conference on Radiochemistry. Ozyorsk. 2003, pp.124-125 (in Russian).
- Bushuyev A.V, Aleeva T.B., Petrova E.V. et al. Possible disposal of spent graphite sleeves from the Siberian Chemical Combine reactors by their incineration. Atomnaya Energiya. 2003, v. 94, no. 2, pp.130-138 (in Russian).
- Shidlovsky V.V., Mescheryakov V.N., Tsyganov A.A. et al. The problems of reactor graphite management at the shut-down commercial uranium-graphite reactors of the Siberian Chemical Combine (SCC). Transactions of the VII International Conference on Nuclear Technology Safety. Radioactive Waste Management. St.Petersburg, September 27 – October 1, 2004, pp. 480-485 (in Russian).
- Bushuyev A.V., Petrova E.V., Kozhin A.F et al. Reseach in radioactive contamination of AM reactor graphite samples. Atomnaya energiya. 2006, v. 101, no. 5, pp. 356-364 (in Russian).
- Girke N.A., Bushuyev A.V., Kozhin A.F 14С in spent graphite of the commercial uranium-graphite reactors at the Siberian Chemical Combine. Atomnaya energiya. 2012, v. 112, no. 1, pp. 51-53 (in Russian).
- Vasilenko I.Ya., Osipov V.A., Rublevsky V.P. Radioactive carbon. Priroda, 1992, no. 12, pp.59-65 (in Russian).
- Merzhanov A.G., Borovinskaya I.P., Manokhin N.S., Zakorzhevsky V.V., Konovalov E.Е., Lisitsa F.D.., Starkov O.V., Myshkovsky M.P. The way to reprocess solid high-level graphite-containing waste. RF Patent №2065220. Registered in the State Register of Inventions on 10.08.96 (in Russian).
- Konovalov E.Е., Starkov O.V., Myshkovsky M.P. et al. Reprocessing of reactor high-level graphite into stable carbide-oxide materials by self-propagating high-temperature synthesis. Atomnaya energiya. 1998, v. 84, no. 3, pp. 239-242 (in Russian).
- Romenkov A.A., Tuktarov M.A., Pyshkin V.P. Flameless incineration of radioactive waste in molten salt. Bezopasnost’ okruzhayuschey sredy. 2008, no. 3, pp. 44-47 (in Russian).