Modelling Experimental Results on Radiolytic Processes at the Spent Fuel Water Interface. I. Radiolysis Products and U R
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Modelling Experimental Results on Radiolytic Processes at the Spent Fuel Water Interface. I. Radiolysis Products and U Release J. Bruno1, E. Cera1, T.E. Eriksen2, M. Grivé1, S. Ripoll1 1 Enviros Spain SL, Pg. de Rubà 29-31, 08197 Valldoreix (Spain) 2 Dept. Nuclear Chemistry, KTH, 100 44 Stockholm (Sweden) ABSTRACT Experimental and modelling efforts in the last decade in the frame of nuclear waste management field have been focused on studying the role of the UO2 surfaces in poising the redox state of solid/water systems. For this purpose, an experimental programme was developed consisting on dissolution experiments with PWR spent fuel fragments in an anoxic environment and by using different solution compositions. The collected data so far, indicate that production and fate of radiolysis products follow the same trends independently on the solution composition used in the tests. Hydrogen and oxygen concentrations show an initial increase with time until reaching a constant concentration. The trend observed for hydrogen peroxide is a decrease at short contact times to reach again a constant concentration with time. These steady-states indicate an overall balance of the generated radiolytic species. Modelling work indicates that uranium dissolution is controlled by the oxidation of the spent fuel matrix in 10mM bicarbonate solutions while in the tests carried out at lower or without carbonate concentrations uranium in the aqueous phase is governed by the precipitation of schoepite. These results are determinant to highlight that reducing conditions are restored in the aqueous phase in relatively short periods of time and at short distances away from the dynamic redox spent fuel/ water interface. INTRODUCTION The spent fuel matrix is the first barrier within the repository design given the high stability of this material in an anoxic media. One of the critical parameters to define its stability is the Eh and it will depend on the oxidant species able to oxidise the UO2 matrix. In this context, it is important to highlight that the spent fuel matrix is a dynamic redox system by itself given the generation of oxidants and reductants at the fuel/water interface due to water radiolysis. The role of mineral surfaces in poising the redox state of a solid/water system is a well established phenomenon. In this sense, there are evidences on the active redox role of UO2 surfaces both in natural systems [1,2] and laboratory experiments [3]. At laboratory scale, well-controlled dissolution experiments of PWR Ringhals spent fuel fragments in an initially anoxic closed system and by using different solution compositions have been carried out in the last decade to understand the processes that control the radiolytic generation of oxidants and reductants at the spent fuel water interface and their consequences for spent fuel matrix stability and radionuclide release. This paper focus on the results obtained on the radiolysis products, hydrogen, oxygen and hydrogen peroxide as well as on the role of the spent fuel matrix acting as redo
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