The TRANSURANUS mechanical model for large strain analysis

The TRANSURANUS fuel performance code was extended in order to take into account large creep strains for the simulation of clad ballooning occurring during loss-of-coolant accidents (LOCA). Additionally, the application range has been assessed. While the first order theory gives very good results for small strains below approximately 5%, simulating large strains implies numerical instabilities evidenced by the violation of the volume balances. To this purpose, an extension of the algorithm used in TRANSURANUS was proposed and verified adopting different solution methods: an analytic solution and finite element calculations by means of COMSOL. In particular, creep calculations were carried out applying a Norton law that defines cladding creep strains under varying stress conditions. The verification against the numerical solution given by COMSOL and the analytic solution shows an excellent agreement of the radial profiles of stresses and strains. w code was applied to the simulation of cladding ballooning tests, and to an integral LOCA test of the FUMEX-III project. The impact of the new algorithm on the predicted time-to-burst and clad strain was assessed pointing out the effect of different failure criteria.