Advanced methods development for equilibrium cycle calculations of the RBWR

The Resource-Renewable Boiling Water Reactor (RBWR) was designed to burn Transuranic (TRU) isotopes while maintaining a conversion ratio of one. The unique design of the core has presented several challenges for computer codes used to analyze the core since the axial fuel design consists of several axially alternating blanket and fissile fuel regions in order to increase breeding of plutonium in the blanket. This creates a severe double peaked power distribution and axial heterogeneities which are not typical of light water reactors (LWRs) and poses challenges to the conventional methods used for LWR design and analysis. In particular, it has led to a reconsideration of the typical 2D lattice calculations used to generate homogenized few group cross sections for the core simulator. As part of the methods development for the RBWR analysis, 3D cross sections and axial discontinuity factors have been generated with the Monte Carlo code SERPENT and implemented in the 3D core simulator PARCS. For the analysis of the core at full power and transient conditions, a complete set of 3D "branch" cross sections were generated at anticipated temperature, density, and burnup conditions. These XS libraries were then used in the coupled code system consisting of PARCS and the thermal-hydraulic drift flux code, PATHS, in order to simulate a full power equilibrium cycle for the RBWR. The equilibrium core converged to a near critical condition (k-eff=0.99698) after nine burnup cycles and showed little variation over the length of the equilibrium cycle. Additional isotopic studies of this simulation also confirmed the breeding capability of the core. The same core modeling using 2D cross sections required 25 cycles to converge and did not confirm the breeding capability of the core. These results indicate that 3D cross sections and axial discontinuity factors can be used to accurately determine the equilibrium cycle of axially heterogeneous cores.