BACO (Barra Combustible), a computer code for simulating a reactor fuel rod performance

The basic philosophy and mathematical structure of the fuel performance simulation code BACO is described. This code is based on a central finite-difference quasi-bidimensional approximation. Within that approximation, the thermoelastic-plastic behaviour of a in-service fuel rod is calculated by a set of equations which are linearized and solved for each time step by a sparse matrix inversion subroutine. The numerical method is shown to be stable and to converge rapidly to physically sound results for the stresses and strains. Changes in the fuel shape due to cracking and restructuring are included in the calculation within a self-consistent mathematical frame. Code convergence and accuracy are discussed by comparing some predictions against thermoelastic and plastic analytic solutions. An example of the code predictions for the rod state during a reactor shutdown is presented and discussed.