Investigation of interface-current solution techniques for coupled heterogeneous cells

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