Analysis of stress applied to a ruptured cladding tube under horizontal vibration

Abstract

ABSTRACT After the termination of a loss-of-coolant accident (LOCA), the reactor continues to be cooled for a long term until fuel assemblies are withdrawn from the reactor core. The fuel cladding tube degrades in strength due to high-temperature oxidation during a LOCA event. It is important to confirm that fuel rods exposed to LOCA conditions can withstand earthquakes during the long-term cooling in terms of preserving the coolable geometry of the reactor core. Finite element method analyses were performed to estimate the deformation of fuel rods in a fuel assembly under vibrations simulating an earthquake as well as the stress applied to the fuel cladding tube with a rupture opening. The localized stress at the rupture opening in the analyses was compared with the strength assessed through bending tests of the cladding tube samples that were ruptured and oxidized to less than 15% equivalent cladding reacted (ECR) in advance. As the result, the fuel rods are expected to be prevented from fracture due to bending at earthquakes during the post-LOCA cooling unless the oxidation of cladding tubes exceeds the limit defined in the current Japanese LOCA criteria, 15% ECR and a deflection of the fuel rodexceeds approximately 40 mm.