Finite Element Analysis of Calandria Tube Failure during a CANDU Core Collapse

Abstract

Each fuel channel in a CANDU®¹ reactor includes a calandria tube (CT), surrounding the pressure tube (PT), which contains the fuel bundles. The CTs are joined at both ends to the calandria tubesheets by a rolled joint. During a postulated severe accident, the moderator level may drop, with decay heat leading to heat-up, sagging, oxidizing, and the collapse of exposed fuel channels to form debris suspended onto the cooler, still submerged channels below. The resulting load may exceed the strength of the supporting CT, resulting in a cascade of CT failures and collapse of the suspended debris, and underlying fuel channels and their contents to the bottom of the calandria vessel. Finite element analyses were conducted to assess the effects of temperature, transverse and axial loads on the CT failure under a bending moment. The results were compared with those obtained from the experimental tests conducted at Canadian Nuclear Laboratories (CNL). The model predictions compared favorably with the experiments, with both having the same trends and agreeing within 15% for most scenarios. While the experiments were conducted using unirradiated material, the finite element model can be used to predict CT irradiation effects. Results from this study provide insight into the factors that would affect the progression of a core collapse event during a severe accident in a CANDU reactor.