Spatially Dependent Resonance Self-Shielding in CASMO5

The Spatially Dependent Self-Shielding (SDSS) method has been implemented into CASMO5 within the framework of Equivalence Theory. The Optimal Two-Term Rational (OTTR) approximation is extended in the SDSS method to the Stoker-Weiss treatment of concentric annular fuel subdivisions. Reference fuel-to-fuel probabilities are required by the OTTR and obtained by performing a series of fixed-source, two-dimensional transport calculations for individual pin cell types using the method of characteristics. Several algorithms used in searching for the OTTR coefficients are evaluated with the goal of obtaining the best practical accuracy at minimal computational cost. Numerical results are presented that provide a comparison of various choices of search algorithms and show improved accuracy obtained by increasing the degrees of freedom in the rational approximation. Spatial profiles of the ²³⁸U microscopic absorption cross sections in the resonance range obtained using the Distributed Resonance Integral (DRI) and SDSS methods are compared to reference results from Monte Carlo calculations. The comparison highlights the inherent advantages of SDSS over the previous DRI method.