Reliability methods in the design point of free-standing spent fuel racks under seismic conditions

摘要

Spent fuel racks are used in the first step of the nuclear waste management process, during the wet storage of the irradiated fuel assemblies. Their maximum sliding displacement and vertical force on support due to seismic load governs the design and layout of the units within the spent fuel pool. Their particular free-standing and submerged conditions lead to a highly nonlinear transient motion with decisive fluid-structure interactions. An analysis methodology based on the hydrodynamic mass concept was developed ad-hoc, but some dispersion of results still remains. Numerical outputs fluctuate as a consequence of the randomness of input data, model properties and solution controls. This uncertainty has traditionally been approached through deterministic analysis with conservative safety factors following guidelines based on practice. However, continuously increasing computer capacities open the door for a more accurate safety assessments based on reliability analysis. This paper carries out a reliability analysis for a basic 2-rack finite element model built in ANSYS Mechanical. The output uncertainty is estimated through the statistical analysis of a 500 deterministic simulation sample: transient envelopes, coefficient of variation, Wilks' confidence intervals, etc. The final probability of failure is subsequently assessed through direct simulation, statistical inference and subrogate modelling of the response surface.