Seismic Qualification of Spent Fuel Storage Stacks

Abstract

CANDU reactors use short (0.5 meter) fuel bundles to facilitate on-power refueling. The spent (irradiated) fuel bundles are removed from the reactor during the re-fueling process and are transferred to the spent fuel (SF) port by fueling machines. From there, the S/F bundles are loaded onto trays for storage in the SF storage bay, where they are stacked on support structures. The leveled stack supports rest on the epoxy- lined concrete floor of the S/F storage bay. The S/F bay is filled with water designed for adequate shielding and cooling of S/F bundles. The S/F storage stacks are required to maintain their structural stability and integrity under a DBE (Design Basis Earthquake). The purpose of this paper is to present the seismic analysis techniques and the stability evaluations developed for the seismic qualification of the SF storage stacks. In the seismic qualification of the S/F storage stacks, models are developed in stages with two main objectives: (i) to accurately represent the structural behavior by a finite element method, and (ii) to simplify and reduce the model size by sub- structuring techniques (two level condensation). The reduced and simplified global models are of a spring-and-lumped-mass type. Also, in the three-directional time- history seismic analysis, off-set beams are implemented in the seismic model in order to capture the true combination of the overturning moments due to simultaneously applied horizontal motion and vertical motion. The total applied forces produced by the earthquake consist of the seismic inertia effects and the hydrodynamic pressure effects. Stability and structure integrity are assessed for various design configurations of the S/F storage stacks. The stability evaluation involves checks against toppling and sliding based on defined stability criteria. Structural integrity is ensured by the stress analysis to have stresses within the acceptable limits. Both the stability evaluation and the stress analysis confirm that the design of the S/F storage stacks satisfy the design requirements.