Changes in spent nuclear fuel due to dry interim storage
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Changes in spent nuclear fuel due to dry interim storage L. Duro1, O. Riba1, A. Martínez-Esparza2 and J. Bruno1 1 Amphos 21 Consulting, S.L., P. Garcia Faria 49-51, 1-1, Barcelona, E-08019, Spain 2 EMPRESA NACIONAL DE RESIDUOS RADIACTIVOS S.A C/ Emilio Vargas, 7 Madrid, E-28043 Spain. ABSTRACT The assessment of the main changes expected for spent nuclear fuel from its discharge to its deposition in a deep geological repository is of the outmost relevance to establish the initial conditions of the disposal. In this work, a literature review and a critical discussion of the main processes that will affect the structure and the inventory of the spent nuclear fuel during its interim dry storage is presented. Once the irradiation period is finished, the following changes are observed: i) the fuel pellet is fragmented due to the temperature gradient established during the irradiation stage. On average between 10-15 fragments are observed per pellet. ii) the initial gap existing between the pellet and the cladding decreases or disappears depending on the burnup. iii) a radial zonation is observed in the microstructure of the pellet. For burnup over 40MWd/KgU, the rim develops a porosity increase due to the high local burnup and the low temperature in the periphery. The rim also presents small bubbles of fission gases. This high burnup structure implies a degradation of the thermic conductivity in the pellet, that leads to a temperature increase in the center of the pellet with a subsequent migration of the fission gases and other impurities to the grain boundaries. The implications that all these changes may have on the spent fuel behaviour is presented and discussed. INTRODUCTION Important efforts have been focused on investigating the spent fuel behaviour in deep geological repository (DGR) conditions although the implementation of this approach for high level radioactive waste (HLRW) management faces important limitations mainly due to public perception issues. Most countries policies in waste management adopt intermediate storage concepts in the form of individual interim storages (ATI) placed in nuclear plants or a central interim storage (ATC). Interim storages has a design life for HLRW of 60-100 years after this it has been cooling in containment pools for at least 2-6 years and before a more definite approach such as DGR are engineered and built . Interim storage has been implemented in United States, Canada, Germany, Belgium, France, Holland, United Kingdom, Japan and Sweden. Similarly, the construction of an interim central storage was reported by the Spanish nuclear waste management agency (ENRESA) in the Official State Gazette, BOE (Boletín Oficial del Estado), 29th of December, 2009. There are two individual interim storages in the Trillo nuclear power plant in operation based in metal casks and another in the Zorita power plan based in concrete casks. In terms of safety regulations it is required that spent fuel after its period in interim storage is safely and readily retrievable from storage systems for fu
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