Key Scientific Issues Related to the Evolution of Spent Nuclear Fuel in Long Term Dry Storage and Geological Disposal
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(YROXWLRQRIWKHFODGGLQJLQWHJULW\ After irradiation; cladding is submitted to a significant internal pressure (40-60 bar at 25°C) which could increase if any gas is released with time. Although it does not have any function in geological disposal, cladding may act as a confinement barrier in long term dry storage if its integrity can be demonstrated. Therefore, we are developing a combined experimental and modeling work in order to derive a long term creep law and a related breaching criterium for the irradiated cladding [6,9]. Current results are based on 6-8 months mechanical tests on irradiated Zircaloy-4 cladding at different temperature, pressure and stress. We expect to get some > 1y. experimental results in the next year. Beyond these developments, the current challenge is to develop a model based on the microscopic mechanisms responsible for this strain in order to support any extrapolation to longer time. (YROXWLRQRIWKHIXHOPLFURVWUXFWXUH From a synthetic review on irradiated fuels and natural analogues samples, we demonstrated that no microscopic swelling due to irradiation damage the spent fuel is expected in a closed system [4,5]. However, the possibility of gases bubbles formation, growing and migration and currently not assessed. Several experimental results enlightened that the grain boundaries appear to be the weakest mechanical points of the whole pellet: (i) they easily loss their cohesion in fast transient heating experiment, (ii) 10 to 50% of their surface has been shown by HRTEM to be covered by gas bubbles, which are shown to be highly over-pressurized, (iii) fractures are mainly inter-granular in the outer part of the rod … [10]. Similarly, 45y. old PuO2 sintered pellets were shown to be transformed into powder due to grain de-cohesion [11]. We therefore estimate not possible to currently demonstrate that grains boundaries will not slightly open to gases and/or water with time, hence increasing the accessible RN and surface area. Such process would obviously
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significantly enhance the RN source term. The evolution of the grain boundaries is on our mind the major scientific question to be currently assessed to better understand the evolution of the fuel pellet and consequently to have a better assessment of radionuclide source term. (YROXWLRQRIUDGLRQXFOLGHVORFDWLRQZLWKLQWKHSHOOHW RN distribution is not homogeneous after irradiation and could slowly evolve by diffusion. Thermal diffusion has been demonstrated to be not relevant for the low temperature range expected during dry storage and geological disposal [4,5]. However, an “athermal” diffusion process of U
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