A simplified method of computing clad and fuel, stress and strain during an overpower transient

A new transient model, which is simpler than that contained in the LIFE and DEFORM computer codes, but still considers all the effects mentioned above, has been developed by extending the KRASS steady state stress/strain code. The model is based on a calculation of the fuel to clad mechanical interaction pressure which is used to determine a cladding hoop strain. Failure at a particular axial location along the fuel rod is defined when a pre-specified cladding permanent hoop strain is calculated. The primary simplification of KRASS and of this model is the calculation of present time step strains using previous time step stresses. Thus, the time step length is limited by specifying an upper bound on the ratio of deviatoric stress change over one time step to the total deviatoric stress. The fuel/clad mechanical interaction pressure is approximately represented by a free body diagram of the fuel rod and the plane strain approximation is used. The fuel is considered to have a hot, ductile, strengthless inner region and a cooler, brittle outer region. The transition from elastic to plastic deformation in the cladding is determined by the TRESCA yield criterion with a temperature dependent yield stress. Strains from fuel swelling, hot pressing and creep are calculated from empirical correlations. Pellet cracking at any axial location is assumed when the peak fuel tensile stress exceeds the fracture strength of the fuel. Upon pellet fracture, the fuel is assumed to immediately contact the (orig.)