SCALE inventory and reactivity analysis as part of the Hermes 2021 PSAR review

The readiness of SCALE for comprehensive studies of pebble-bed reactors has been demonstrated through detailed analysis of a fluoride salt–cooled, high-temperature pebble-bed reactor (PB-FHR). The methods developed for pebble-bed reactor modeling in SCALE, particularly for inventory generation, have proven effective in gaining insights into the reactor physics of this advanced reactor. Excellent agreement with another code package has been observed, further highlighting SCALE’s strong performance. The SCALE results supported the US Nuclear Regulatory Commission’s construction permit application review of the Hermes low-power PB-FHR demonstration reactor. A SCALE model of the Hermes reactor was developed at Oak Ridge National Laboratory using information from the Preliminary Safety Analysis Report (PSAR) and supplemented with publicly available data. SCALE reactivity coefficient simulations reproduced PSAR results within 1 σ statistical uncertainties. Sensitivity studies emphasized the importance of graphite specifications for accurate k eff predictions. • Analysis demonstrated SCALE’s strong performance for pebble-bed reactor modeling. • Excellent agreement for reactivity coefficients between SCALE and published results. • Significant sensitivity of k eff results to graphite specifications found. • SCALE analyses supported US NRC’s CPA review of the Hermes demonstration reactor.