Transient response of LWR fuel using the FREY code

The development of FREY, Rashid et al. (1983), by the Electric Power Research Institute was motivated by the need for an industry code to support fuel reload licensing. The code was developed as a general purpose Fuel Rod Evaluation System for LWR fuel transient and steady state analysis in a fully two-dimensional continuum framework. The numerical solutions for the heat transfer part of the problem uses the Crank-Nicholson central differencing technique, and the deformation part of the problem uses a forward differencing algorithm with a Newton iteration technique. The principles of continuum thermomechanics are rigorously applied in the formulation of the governing equations for the heat transfer and deformation solutions. Thus theoretically consistent and continuous behavior is maintained for the fuel rod as it undergoes various thermal and deformation regimes from the low temperature, small deformation response to high temperature, large deformation response. Using time-dependent input and previous step information (or initial conditions information for the first time step), the thermal solution is performed first, and the nodal temperatures are calculated using an efficient frontal solver. The incremental displacements are obtained next, using as input the nodal temperatures, the internal and the external pressure distributions and the boundary constraints. As in the case of the thermal problem, the same frontal solver is used in the deformation solution except for the modification from a scalar to a vector field. The element strains and stresses are calculated at several integration (interior) points of the fuel and clad elements and similarly for the gap elements. (orig./GL)