Fast Neutron Flux and DPA Rates in NuScale SMR Core Components and Vessels

Abstract

NuScale Power LLC (NuScale) is developing safer, smaller, scalable pressurized water reactor technology under the U.S. Department of Energy's cost-sharing program. The determination of vessel damage due to irradiation requires the spatial determination of energy-dependent neutron flux for each location of interest by combining calculations and measurements. The effect of the fast neutron (E > 1 MeV) flux level and displacements per atom (DPA) are significantly important parameters for predicting the core component and vessel embrittlement per U.S. Nuclear Regulatory Commission (NRC) Regulatory Guide 1.99, Revision 2 and Revision 3 (draft) in commercial reactors. The Monte Carlo transport method is acceptable to NRC for determining the best-estimate neutron fluence in vessels per NRC Regulatory Guide 1.190. NuScale uses the three-dimensional Monte Carlo N-Particle code, Version 1.0, to perform neutron-gamma transport calculations with the point-wise cross-section Evaluated Nuclear Data Files/B-VII.1 data library and standard iron DPA cross-section data from ASTM E693, Standard Practice for Characterizing Neutron Exposures in Iron and Low Alloy Steels in Terms of Displacements Per Atom (DPA). A three-dimensional Monte Carlo N-Particle model of the NuScale reactor module including the core, reflector, core barrel, reactor pressure vessel, and containment vessel has been developed. This paper presents a general discussion of NuScale calculational neutron fluence methodology for evaluating neutron flux (neutrons/cm2 · s−1) and the DPA rate (displacements/atom · s−1) at the lower core plate, upper core plate, core reflector, core barrel, pressure vessel and containment vessel at 0.25T, 0.5T, 0.75T, and 4/4T, respectively. The results of the best-estimate fluence analysis to support the Design Certification Application and technical details are included in the licensing technical report that had been submitted to the NRC for review.