Sliding and Rocking of Free Standing Dry Storage Casks Under Earthquake Excitation

Abstract eng:
Nuclear power components must maintain a stringent safety margin during normal operation, accident scenarios, and seismic events. The spent nuclear fuel (SNF) at nuclear power plants is initially stored in pools to control the temperature of the fuel assemblies, and prevent melting of their cladding. Thereafter, SNF is transferred to Dry Storage Casks (DSCs), which can be designed as free-standing structures resting on a reinforced concrete foundation. DSCs have been considered as a temporary storage solution, and are usually licensed for 20 years, although they can be relicensed for operating periods up to 60 years. In order to extend relicensing periods, the increased seismic hazard on DSCs needs to be re-examined. Thus, the main goal of this study is to evaluate the long-term seismic performance of DSCs. To evaluate DSC seismic performance, experiments were conducted using a six degree-of-freedom (6DOF) shake table in the Earthquake Engineering Laboratory at the University of Nevada, Reno (UNR). During the earthquake excitation, the response of a free-standing cask was manifested as rocking, sliding, precession, nutation, or combination between any of these responses. In addition to the input motion, the response mainly depended on cask slenderness and the friction coefficient between the cask and the foundation pad. Friction is a critical parameter, but it can change significantly with time and is difficult to estimate. Slenderness of the cask affected its dynamic response, and it was calculated as the cask outer radius-to-centroidal height ratio (r/h cg ). Five scaled specimens were chosen to cover a range of commercially available DSCs with five different r/h cg ratios of 0.39, 0.43. 0.55, 0.56, and 0.62. Eight experiments were conducted using these aspect ratios; five of them were free-standing specimens, while three experiments were anchored. Three freestanding specimens with aspect ratios of 0.39, 0.43, and 0.55 are the focus of this paper. The DSC specimens were free-standing structures on the shake table, instrumented to capture the response. Several ground motions were chosen as evaluation earthquakes. The seismic hazard for the evaluation earthquakes was developed for 1,000-, 10,000-, and 30,000-year return periods. Experimental results were not consistent during motion repetition in some cases, however, matching results were obtained in other cases. Factors affecting experimental result consistency are investigated and discussed. LS-DYNA was used to develop a Finite Element Model (FEM) for the cask pad system, and model sensitivity was investigated in this study. The resulting FEM of the DSC showed acceptable correlation with experimental results. Several factors affecting the FEM response were considered, including: small changes in the coefficient of friction, specimen alignment relative to the center of the concrete pad, specimen centroid, damping, mesh size and type of contact. This sensitivity study was presented to provide more confidence in the developed model.

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