Spent fuel alteration model integrating processes of different time-scales
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MRS Advances © 2020 Materials Research Society DOI: 10.1557/adv.2020.51
Spent fuel alteration model integrating processes of different time-scales O. Riba, E. Coene, O. Silva and L. Duro Amphos 21 Consulting, S.L., C/ Veneçuela, 103, 2ª Planta, Barcelona, E-08019, Spain
ABSTRACT
A 1D reactive transport model has been implemented in iCP (interface COMSOL Multiphysics and PhreeqC) to assess the corrosion of Spent Fuel (SF), considered as homogeneous UO2(am,hyd) doped with Pd. The model couples: i) generation of water radiolysis species by alpha and beta radiation considering the complete water radiolysis system with the kinetic reactions involving: H+, OH-, O2, H2O2, H2, HO2-, HO2·, O·, O-, O2-, H·, ·OH and e- ii) processes occurring in the spent fuel surface: oxidative dissolution reactions of UO2(am,hyd) and subsequent reduction of oxidized fuel, considering H2 activation by Pd, and iii) corrosion of Fe(s) in oxic and anoxic conditions. Process i) has been implemented in COMSOL and processes ii) and iii) have been implemented in PHREEQC with their kinetic constants being calibrated with different sets of experimental data published in the open literature. The model yields a UO2(am,hyd) dissolution rates similar to the values selected in safety assessments.
INTRODUCTION The safety assessment of deep geological disposal of used nuclear fuel requires a fundamental understanding of the processes controlling fuel corrosion and the release of radionuclides into the geosphere. In the last years, source-term models describing spent fuel (SF) dissolution have been improved by incorporating new processes that were not considered in classical quantitative approaches. The late advances include the work of Wu et al. (2012, 2014a,b) [1-3], Jerden et al. (2015) [4] and Odorowski et al. (2015) [5], who developed reactive transport models based on the calculation of the corrosion potential at the surface of the fuel. Improved models implemented in software such as COMSOL [1-3], MATLAB [4], and HYTEC [5] have allowed to include the most relevant processes of the system by introducing some simplifications. These simplifications were mainly applied to the
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chemistry of the system and to the radiolytic scheme, for instance, considering only the production of H2O2 and H2 instead of the complete radiolysis scheme. Radiolysis and chemical complexation/dissolution reactions occur at very different time scales, often with rates differing by more than 6 orders of magnitude. Uranium (the main element of the fuel) and the iron of the steel-container and metallic insert have a complex chemistry. This complexity and the difference in time scales represent an important modelling challenge and these are the main reasons for adopting simplifications. The focus of the current study is to assess the behaviour o
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