TRACG benchmark study: 2015 Fermi thermal-hydraulic instability event

On March 19, 2015, the Boiling Water Reactor (BWR) Fermi 2 (referred to hereafter as Fermi), located in Michigan, U.S., experienced a scram initiated by the Oscillation Power Range Monitor (OPRM) system. The plant had been operating in Single Loop Operation (SLO) for approximately 10 minutes before the scram occurred. During this time, the plant operating conditions reached a region within the power/flow (P/F) map susceptible to coupled neutronic/thermal-hydraulic (T-H) instabilities. Limited code benchmarking is available in literature for T-H Instability (THI) transients occurring in plants operating at SLO conditions due to scarce plant data for this type of operational condition. This study aims to model and benchmark the THI event at SLO conditions using TRACG, the GE Hitachi (GEH) proprietary best-estimate transient system code. The TRACG model was benchmarked by comparing the results with measured plant data, such as the total reactor power and core flow, from the event. Additionally, to evaluate the inherent stability of the plant at different P/F points, Decay Ratio (DR) calculations were performed. The capability of TRACG for calculating DRs is well documented, and it has also been approved by the U.S. Nuclear Regulatory Commission (NRC) as part of ESBWR TRACG stability licensing topical reports. The results of TRACG DRs were compared to the GEH best-estimate frequency domain code ODYSY, which is the current Fermi licensing basis code for reload analysis DR calculations