Influence of long-term aqueous leaching of irradiated graphite on surface properties and behavior of radionuclides
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.37
Influence of long-term aqueous leaching of irradiated graphite on surface properties and behavior of radionuclides Andrey A. Shiryaev1,2,3, Anna G. Volkova1, Stanislav Dvoryak2, Maximillian S. Nickolsky3,1, Elena V. Zakharova1 1 Institute of physical chemistry and electrochemistry RAS, Leninsky pr. 31, korp. 4, 119071, Moscow, Russia ([email protected])
2
Department of Chemistry, Lomonosov Moscow State University, Leninskie gory, 1 bld.3, Moscow, 119991, Russia
3
Institute of geology of ore deposits, petrography, mineralogy and geochemistry RAS, Staromonetny per. 35, 119017, Moscow, Russia
Abstract. Samples of real irradiated (i-)graphite from bushings of RBMK reactor after several types of decontamination treatments were put in contact with aqueous solutions modelling underground water of Nizhnekansky massif (Russia) equilibrated with bentonite for periods up to 1.5 years. Leach rates of radionuclides and evolution of graphite surface morphology and oxidation state were monitored using nuclear spectroscopy, SEM and XPS. After the experiment, less than one third of surface carbons remains unoxidised. Extensive precipitation of secondary phases (alumosilicates, carbonates) was observed on some samples and is possibly correlated with surface oxidation extent. The leach rates of dose-forming 14C and 36 Cl are comparable with the leach rates of radionuclides in glassy waste forms. According to the current study i-graphite could be regarded as a waste form, which is suitable for nearsurface disposal.
INTRODUCTION. Disposal of large quantities of irradiated graphite (i-graphite) remains controversial issue and several possibilities are considered. Extensive studies have shown that thorough decontamination from principal dose-forming radionuclides such as 14C 177
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and 36Cl by chemical or thermochemical methods is barely possible without complete destruction of graphite crystalline lattice (e.g., [1-2] and references therein). The principal reason for high retention of radionuclides lies in physical mechanisms of their production, since non-negligible fraction of 14C reside in graphite lattice is generated by neutron activation of 13C. This fraction depends on i-graphite production method, cooling gas and fluence-temperature history of a reactor, and varies widely, but in all cases this fraction accounts for several tens percent’s of total radiocarbon inventory. In Russian Federation, two principal approaches for disposal of i-graphite are considered: a) shallow burial of the reactor graphite pile on-site with creation of multiple barriers; and b) dismantling of the pile and storage of i-graphite (possibly after partial decontamination) in shallow or deep underground repositories [3]. The selection of the approach depen
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