PLANE STRAIN MODELING OF A SEVERE CRUSH ACCIDENT

Japan Nuclear Cycle Development Institute (JNC) is leading Japan’s efforts in fast breeder reactor research and development. As a part of its comprehensive safety assessments, JNC has been ensuring safe fuel transport by performing regulatory tests and analyses on its various Type B packages. In addition, the severe earthquake in Kobe Japan in 1995 motivated JNC to estimate package performance in such severe accident environments as one of the phenomena to consider for emergency preparedness, despite their ultra-low probabilities of occurrence. Although an experimental test would be difficult to implement, numerical simulation is an effective alternative, particularly with respect to cost and time. For that purpose, large deformation and highly nonlinear explicit integration analytical methods was investigated. The postulated accident environment was that of a 1500 metric ton concrete bridge section falling 10.4 meters onto a fresh fuel package laden truck traveling on an asphalt roadway below it. As a first stage of this severe accident assessments, JNC and the author from Sandia National Laboratories working as a JNC International Fellow, performed a series of plane strain large-deformation finite element analyses (FEA) of this hypothetical bridge crush (extra-regulatory) accident condition using the ABAQUS/Explicit finite element analysis code. A wide range of material constitutive models was required to accurately model the behavior of various ductile metals, brittle concrete, crushable woods and foams, and pressure-dependent yield materials such as soil and asphalt. Fuel assembly loading results obtained from an overall bridge/package/roadway crush analysis were subsequently applied to a highly detailed FEA of the fuel assembly’s individual fuel pins and their cladding. Analysis results showed that although deformations were relatively large compared with regulatory accidents, the two containment boundaries provided by the primary containment vessel and individual fuel pin claddings retained their structural integrity. Because of the simplified nature of the two-dimensional plane strain analyses (appropriate for such a long, slender package), these results should be verified by full three-dimensional analyses, which include out-of-plane bending and shear forces as well. These calculations are presented in another paper in these proceedings (Harding 2001). Preliminary conclusions about the ABAQUS/Explicit code’s ability to simulate package performance during even severe accidents can be made. These could be utilized for a comprehensive land transport risk assessment that evaluates additional postulated events and their probabilities of occurrence.