Structural impact stability of commercial nuclear fuel rod

Abstract

The United States Nuclear Regulatory Commission (USNRC) requires that all licensees evaluate the potential for handling accidents that may occur during the transfer of fuel from the reactor vessel to the spent fuel pool. In this process, a bottom end drop scenario resulting in the release of radiological fissions products is of primary concern. For this reason, the U.S. Code of Federal Regulations requires that each applicant provide an analysis and evaluation of accident conditions that present a risk to public health and safety at the facility of operation. Furthermore, as stated in the USNRC Regulatory Guide 1·183, the number of fuel rods damaged during a handling accident such as that described above should be based on a conservative analysis that considers the most limiting case pertaining to weight, drop height and the compression, torsion and shear stresses on the irradiated fuel rods with the potential damage of adjacent fuel assemblies being considered. It is further recommended by the USNRC that the limiting mode of failure associated with such a postulated handling accident is that of elastic buckling, which should be evaluated through application of Euler’s static load limit. However, as will be shown in this article, the lack of inertial terms and assumed axially continuous compressive load present in Euler’s solution result in an overly conservative design limit when applied to a transient impact analysis. In an effort to illustrate the conservative nature of this recommended design limit, the theories and limitations of closed form static buckling are introduced, followed by a more rigorous treatment of dynamic pulse buckling, and finally, a complete solution via the finite element method. It may be concluded from the results of this investigation that elastic instability during a transient impact event develops only at loads well beyond Euler’s critical load. Such results require that an alternative stress based limit be introduced, which establishes a more reasonable design limit for evaluating the structural integrity of a spent fuel assembly bottom end drop scenario.

Keywords:

• Pulse buckling
• Elastic instability
• Bottom end drop