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The paper focuses on the 14C, 36C1, and 60Co radionuclide distribution in graphite stacks and blocks, which play a crucial role in the decommission of uranium-graphite reactors. The shutdown uranium-graphite reactor ADE-5 provides an access to the graphite stack for the removal of graphite blocks. New interpretations are suggested for the radionuclide distribution in bulk graphite stacks based on testing results of graphite blocks removed from the uranium-graphite reactor ADE-5 in 2018. [ABSTRACT FROM AUTHOR]

The release of long-lived radionuclides 14C and 36Cl , which are key products for assessment of disposal variants (near-surface, buried, or deep) of irradiated graphite, was analyzed. Test material was irradiated graphite from IUGR and RBMK-1000 reactors. Unlike other types of radioactive waste (RAW), graphite has the developed pore structure and, consequently, its effective surface area in contact with the leaching medium is increased in comparison with non-porous materials. With due to regard for the graphite pore structure, the mechanism describing the leaching process is proposed and experimentally substantiated. The parameters numerically characterizing graphite RAW in terms of leaching resistance, independent of the geometric shape of individual fragments (samples) are also introduced. The proposed parameters and an algorithm for their determination are recommended by the authors for use in predictive calculations of the release of radionuclides from graphite under conditions of long-term storage/disposal. [ABSTRACT FROM AUTHOR]

Specific features of the porous structure of irradiated and virgin reactor graphite of Russian brands have been analyzed. The porosity parameters affecting the leaching of radionuclides from graphite radioactive waste (RAW) were determined: fraction of open porosity, distribution of pore throat cross-sections, surface of open pores. The volume of open pores accessible for leaching aqueous media in the irradiated graphite was estimated. A model describing the leaching of radionuclides from irradiated graphite accounting for the porosity contribution is proposed. The parameters that numerically characterize the resistance of graphite RAW against leaching and are independent of the geometric shapes of separate fragments (samples) were determined. [ABSTRACT FROM AUTHOR]