Release of Radionuclides from Spent Fuel Under Repository Conditions: Mathematical Modelling and Preliminary Results
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Release of Radionuclides from Spent Fuel Under Repository Conditions: Mathematical Modelling and Preliminary Results Esther Cera, Juan Merino and Jordi Bruno EnvirosQuantiSci (Spain), Parc Tecnològic del Vallès, 08290 Cerdanyola (Spain) Email: [email protected] ABSTRACT In the framework of the Enresa 2000 PA exercise and as a continuation of the developments made during SR 97, we have developed a conceptual and numerical model to calculate the release of radionuclides from spent fuel under repository conditions. The model includes both thermodynamic and kinetic considerations. Hence, although certain radionuclides are solubility controlled, for other radionuclides their release is governed by kinetic processes such as radiolytically promoted oxidative dissolution of the matrix and the associated water turnover in the gap. The fluxes of selected radionuclides are calculated as an indication of the relative importance of the various processes considered to define source term concentrations in the performance assessment of the spent fuel repository. INTRODUCTION The disposal of high level radioactive waste in geological repositories relies on the long term stability of the fuel matrix, which must be assured for thousands of years. The development of kinetic models based on mass balance [1, 2, 3] and recent studies of fuel matrix oxidation induced by alpha radiolysis [4, 5] have increased our confidence in predicting the evolution of oxidant generation, its impact on the stability of the UO2 matrix and the ensuing release of the radionuclides embedded in the matrix. The work presented here is based on the implementation of the model [6] and shows the results of the model in terms of concentrations of selected elements in the gap water and fluxes of some radionuclides out of the system. MODEL DESCRIPTION Geometry of the system The system consists of an irradiated UO2 pellet from a PWR reference fuel with 40 GWd/tU of burnup [7]. The pellet, with a mass of 7.38 g, is encapsulated in a Zircaloy sheath and groundwater is allowed to saturate the gap between the pellet and the sheath. The physical dimensions of the system are given in Figure 1. The volume of water in the system, calculated from the above parameters, is 3.9·10-5 dm3.
Figure 1. Geometry of the system. Conceptual Model The main factors that affect the release of the different elements associated to the fuel matrix are: -
The generation of oxidants due to the alpha radiolysis of water and subsequent matrix alteration due to the contact of groundwater. Release by dissolution of the elements embedded in the matrix. Possible precipitation of secondary phases and eventual equilibrium conditions by means of their saturation.
Matrix alteration as a result of radiolytic processes is not specifically treated here, but it has been studied by Quiñones et al. [8]. Their data have been included in the model. Only the effects of alpha radiolysis are taken into account, as it is considered that the radiation field will consist mainly of alpha particles (the time when gro
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