Modeling Enhancements and Benchmarking of Pebble Bed Reactors in the Shift Monte Carlo Code

The Shift Monte Carlo (MC) code is part of the Oak Ridge National Laboratory (ORNL)-developed SCALE code system.This paper describes recent enhancements to Titan, a new easy-to-use reactor physics interface/front-end for Shift and summarizes the geometry acceleration methods implemented into Oak Ridge Adaptable Nested Geometry Engine (ORANGE), the constructive solid geometry (CSG) package used by Shift and other SCALE codes for pebble bed reactor modeling.One key feature of pebble-bed reactor modeling is the automated calculation of the particle or pebble locations in a parent geometry unit.This paper outlines the random sphere packing algorithm implementation which can achieve packing fractions up to approximately 55% and currently supports four parent domains: regular and annular cylindrical and spherical geometries.Memory and run-time results for geometry construction and eigenvalue calculations are provided for three pebble-bed reactor benchmarks.The eigenvalue calculations include the fine-group flux tally necessary for ORIGEN coupling for depletion and is a major focus of ongoing development.The selected benchmarks include the HTR-10 core benchmark, the Idaho National Laboratory (INL)-developed general pebble bed reactor (GPBR) benchmark, and the Kairos-developed generic fluoride salt-cooled high-temperature reactor (FHR) benchmark.Shift results show excellent eigenvalue agreement with Serpent across all benchmarks, very fast sphere packing for the gFHR benchmark, and comparable run-time and memory requirements with Serpent across all benchmarks.