KOTO special report: Effect of creep on fuel behaviour in RIA

Creep is a high temperature deformation mechanism that becomes significant when the temperature exceeds about half of the material absolute melting point. An earlier study at VTT shows that creep is a significant deformation mechanism in certain reactivity initiated accidents (RIA). So, to be able to realistically simulate RIA creep has to be included in the simulation. VTT Energy is using the SCANAIR computer code to analyse the behaviour of nuclear fuel in accident conditions. The code is meant to simulate a reactivity-initiated accident in a light water reactor. SCANAIR was designed to handle the closely interconnected thermal-mechanical calculations that are needed to simulate the accident behaviour of a fuel rod. The code simulates the thermal-mechanical behaviour of a single nuclear fuel rod with numerical models. A finite element model is used in the mechanical module of SCANAIR to simulate the mechanical behaviour of the rod. The mechanical module is designed to describe the highly nonlinear mechanical behaviour of the fuel rod taking into account elasticity, plasticity, thermal expansion, cracking, and fission gas swelling. Also a creep model has been implemented in SCANAIR module at VTT Energy. This paper briefly describes the implemented creep model in the SCANAIR code. A simple example calculation is also made with SCANAIR to show the effect of the creep on RIA calculation.