New Perspectives for the Spent Nuclear Fuel Radionuclides Release Model in a Deep Geological Repository
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0985-NN02-06
New Perspectives for the Spent Nuclear Fuel Radionuclides Release Model in a Deep Geological Repository Christophe POINSSOT, Cécile FERRY, and Arnaud POULESQUEN Department of Physics & Chemistry, Service for the Studies of the Radionuclides Behaviour, CEA, Nuclear Energy Division, CEA SACLAY, BP.11, GIF SUR YVETTE, F-91650, France
ABSTRACT Spent Nuclear Fuel (SNF) source terms are used to define the release rate of radionuclides (RN) in a direct disposal and to assess the performance of this waste form. They classically distinguish between two contributions: (i) the Instant Release Fraction (IRF) of RN which are directly leached when water contacts the fuel, (ii) the slow and long term release of RN which are embedded within the fuel matrix. Recent experimental results bring significant input in our understanding and assessment of both contributions. However, they have not yet been integrated in the definition of the SNF source term. This paper will present the impact on the RN source term of the latest results on the SNF long term evolution and the key remaining scientific issues. INTRODUCTION Direct disposal in deep geological environment is being studied in many countries as a possible solution to ultimately manage Spent Nuclear Fuel (SNF). In France, the new 2006 French waste management Act clearly states that the French reference scenario is reprocessing and direct disposal of nuclear glass canister. However, as a precaution, R&D on direct disposal is still pursued to provide a reliable scientific knowledge to the stake-holders on the different possible options to manage SNF. In a deep repository, the main function of the waste form is to restrict the radionuclide release as a function of time, and therefore to delay the release towards the biosphere to take benefit from the natural radioactive decay. Gaining a reliable source term for SNF is therefore of prime importance for assessing the long term performance and safety of any SNF repository. The SNF source term is normally described as the combination of two contributions: (i) An instantaneous release of radionuclides, often referred to as the Instant Release Fraction (IRF), which corresponds to the radionuclides (RN) which are directly accessible to water [1]. Recent models have proposed to allocate to this fraction any radionuclide located within the fuel microstructures which are supposed to have relatively low confinement properties, or additionally, the performance of which is not easily demonstrable [2;3;4]. (ii) A slow long-term contribution which corresponds to the dissolution of the uranium oxide matrix. By difference to the Yucca Mountain site, European repositories are anticipated to be located deep underground in reducing environment. In these conditions, uranium solubility is much lower than in oxidising environment, in the range of 10-9.5 mol.L-1. From the end of the 90’s, radiolytic oxidising dissolution is assumed to be in such an environment the dominant process and many studies have dealt with this phenomenon [e.g. 5]. Howeve
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