A study of strain distributions in stainless steel fuel rod cladding

A general method is described for calculating stress and strain distributions that may occur in stainless steel cladding during a given power transient. The method is based on a generalized plane strain finite element model of a fuel rod segment used in conjunction with a finite element code developed specifically to account for the dominant material effects of creep and plasticity in both the fuel and cladding. The basic physical model is based upon that originally suggested by Gittus: strain and stress concentrations arise in the cladding because of the combined effects of the presence of radial cracks in the oxide fuel and frictional forces developed at the cladding-fuel interface after the cladding and fuel come into contact. In this study, ten cracks were imagined to have been introduced in the fuel by earlier thermal gradients. Thus the finite element model consisted of a segment of 18 degrees of arc, one edge of which has boundary conditions forcing symmetry is displacements about the radial line defining that edge, and the other edge of which has boundary conditions simulating a radial crack of an assumed depth in the fuel. The frictional forces arising at the fuel-cladding interface after contact is made during a power transient are modeled by an artificial orthotropic material layer whose radial and tangential moduli are chosen so as to simulate a desired friction coefficient. By applying to this model a given power transient, the stress and strain states in the cladding in the vicinity of the fuel crack can be calculated for a variety of interface conditions such as low fuel cladding friction, high fuel cladding friction, and bonding between fuel and cladding. Typical cladding stress and strain contours are presented for these various interface conditions