RPV Irradiation Simulation Using the Contributon Flux Solution

An important aspect of PWR lifetime monitoring is supporting radiation shielding analyses which can quantify various in-core and out-core effects induced in reactor materials by varying neutron-gamma fields. A good understanding of such radiation environment during normal and accidental operating conditions is required by plant regulators to ensure proper shielding of equipment and working personnel. The complex design of a typical PWR is posing a deep penetration shielding problem for which elaborate simulation model is needed, not only in geometrical aspects but also in efficient computational algorithms for solving particle transport. This paper presents such hybrid shielding approach of FW-CADIS for characterization of the RPV irradiation using SCALE6.2.4 code package. A fairly detailed Monte Carlo (MC) model of typical reactor internals was developed to capture all important streaming paths of fast neutrons which will backscatter of biological shield and thus enhance RPV irradiation through cavity region. Several spatial differencing and angular segmentation options of the discrete ordinates (SN) flux solution were compared in connection to a SN mesh size and were inspected by VisIt code. To optimize MC neutron transport toward the upper RPV head, a particularly problematic region, a deterministic solution of discrete ordinates (SN) in forward/adjoint mode was convoluted in so called contributon flux, which proved to be useful for subsequent SN mesh refinement and variance reduction (VR) parameters preparation. The pseudo-particle flux of contributons is coming from spatial channel theory which can locate spatial regions important for contributing to a shielding response.