Methodology for spent fuel integrity assessment in rewetting

An essential condition in nuclear safety is to preserve the fuel rod cladding integrity. This strongly depends on the thermo-mechanical performance of the fuel rod. Some processes related to the spent fuel management after the irradiation period implies conditions that could jeopardize the cladding integrity; particularly relevant are the drying phase previous to dry storage and the rewetting in case it becomes necessary to return the dry stored fuel to the spent fuel pool. The drying conditions could embrittle the cladding due to the radial reorientation of hydrides formed, which would notably reduce the resistance to failure by thermal shock during the rewetting.

CIEMAT has developed a methodology to analyse the cladding integrity of a spent fuel rod under rewetting conditions, including the analysis of the cladding mechanical state after the drying and storage phases. This methodology meets the existing thermo-mechanical needs after the thermohydraulic analysis carried out so far. The analytical tools used have been FRAPCON-x to characterize the cladding ductility after drying and storage, and FRAPTRAN to evaluate the material integrity under the rewetting transient.

The methodology derived has been applied to the drying and rewetting processes of a PWR fuel rod irradiated to high burnup (62 GWd/tU) with an enveloping power history. The best estimate results in the drying at the maximum temperature allowed (400ºC) have given rise to a cladding stress below the threshold established for hydrides radial reorientation (90 MPa). Therefore, it is determined that the cladding keeps the remaining ductility under the rewetting actions, so that the stress elastic limit is applied as a conservative failure criterion under the simulated conditions. The axial stress calculated by thermal shock under rewetting shows a wide safety margin with respect to that limit (minimum value of 550 MPa estimated by FRAPTRAN), so it is concluded that the conservative conditions simulated are safe for the fuel integrity.

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