INVESTIGATION OF INTERFACE-CURRENT SOLUTION TECHNIQUES FOR COUPLED HETEROGENEOUS CELLS

The coupled collision probability and interface-current equations in t he flat-flux, isotropic approximation are implemented in a light water reactor (LWR) lattice code that is under development. By using tradit ional Gaussian elimination of the subregion fluxes of the cells, the i nterface-current equations are turned into a nodal form. In the transf ormed flux and current equations, expressed in terms of response matri x theory, the coefficients become multiple-flight probabilities, which obey analogous conservation and reciprocity relations as the first-fl ight probabilities. The convergence of the iterative solution for the interface currents is accelerated by successive overrelaxation (SOR) a nd global rebalancing techniques. The improvement of the convergence r ate is investigated in a series of test calculations. The optimal stra tegy is found to be alternation between one SOR iteration and three to four free iterations while the rebalancing should be limited to two i nitial iterations in normal LWR cases.