Implementation of the exponential directional weighted SN differencing scheme in RAPTOR-M3G for LWR radiation transport and dosimetry applications

A high-accuracy spatial differencing scheme is implemented in RAPTOR-M3G, a Westinghouse-developed three-dimensional parallel discrete ordinates (SN) radiation transport code. The exponential directional weighted (EDW) scheme offers enhanced accuracy and superior convergence properties compared to conventional differencing schemes at a relatively modest additional computing cost. This method enables complete convergence to typical industrial convergence thresholds on large commercial reactor problems where other differencing schemes fail. This paper explores the theoretical foundations of the EDW differencing scheme and discusses its implementation in LWR reactor problems to calculate exposure quantities of interest to the commercial reactor integrity analysis community. Comparisons to existing differencing schemes are included for several important characteristics. The computing requirements of EDW are quantified relative to the requirements of more conventional theta-weighted (TW) formulations. The convergence behavior of both methods is investigated with consideration given to spatial areas of non-convergence. Finally, the practical accuracy of both methods is assessed through comparisons of reactor dosimetry measurements to calculations from the operating fleet of commercial reactors. The EDW method and RAPTOR-M3G are demonstrated to be particularly powerful for solving the challenging three-dimensional radiation transport problems faced by today’s radiation analysis community, specifically in the extended beltline region. The EDW method enables accurate characterization of the total radiation environment in large LWR models on timescales conducive to commercial applications.