Simplified approach to study fuel rod rupture risks by bending or Euler buckling

Abstract

TN International and International Nuclear Services started commonly in early 2000s the Fuel Integrity Project (FIP) to develop a methodology to assess the response of fresh and used light water reactor (LWR) fuel assemblies (F/A) during 9 m regulatory drops. The analysis of experimental knowledge acquired from fresh F/A behaviour during drop tests and various mechanical tests on fresh and used fuel rod (F/R) samples has led to the elaboration of FIP methodology, allowing the assessment of F/A damage during lateral and axial accident drops of a packaging. In particular, the developed pessimistic mechanical evaluations based on comparison with results of reference bending tests permit to assess, using elastic beam formulas, F/R rupture risks in intergrid bending or Euler buckling configurations. These simple, but interesting, static mechanical approaches can be used for several possible loadings and boundary conditions. They take into account the following: the F/R maximum possible deflection in cask lodgement determined from a conservative hypothesis of array packing down; the F/R deflection at rupture transposed from reference bending tests to determine the bending deflection at rupture of a given F/R in a lateral drop configuration, using appropriate mechanical characteristics and boundary conditions; consideration of similarities between F/R bending under lateral loading and buckling under axial loading: in both cases, F/R deformation results from the action of moments so that it is possible to determine the relation between their deflections at rupture. Some numerical applications, developed for typical LWR F/A geometries, show variable margins that depend mainly on F/R boundary conditions.

Keywords:

• Nuclear fuel
• Package drop
• Fuel integrity
• Bending and buckling deflection
• Simplified approach