Samples of irradiated graphite from a uranium-graphite reactor contaminated with fuel debris from loss-of-flow accidents, are investigated. Peculiarities of spatial distribution, speciation, and kinetics of leaching by aqueous solution of actinides and fission products are studied at scales from the whole reactor stack down to sub-mm graphite fragments. The main fraction of fuel debris is associated with aluminosilicate-based phases decorating cracks and other defects of damaged graphite details. Elemental composition of actinides and fission products depends on both irradiation and leaching conditions. Significant differences are observed between leach kinetics of various radionuclides; integral leached fraction varies between ~5% for 244Сm to ~90% for 134,137Cs. For some radionuclides sudden massive release due to uneven dissolution of the carrier phases is observed. Disposal of irradiated graphite with significant fraction of fuel debris requires dedicated treatment aimed either at decontamination or at conversion of the radionuclides to insoluble well-fixed form.
The paper discusses engineering, economic and legislative aspects influencing the ways of addressing the problem associated with the final stage of irradiated graphite management from water-graphite NPP reactor units in Russia. It presents some calculated and experimental data demonstrating the feasibility of introducing some corrective changes to the legislative and regulatory framework existing in the Russian Federation to enable near-surface disposal of irradiated graphite from power reactors.
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