This report, along with report INL/EXT-19-53556 published in August 2019, documents the activities performed by Idaho National Laboratory (INL) during the fiscal year (FY) 2019 for the Department of Energy (DOE) Light Water Reactor Sustainability (LWRS) Program, Risk-Informed System Analysis (RISA) Pathway, Enhanced Resilient Plant (ERP) Systems research. The purpose of the RISA Pathway research and development is to support plant owner-operator decisions with the aim to improve the economics,

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... eliability, and maintain the high levels of safety of current nuclear power plants over periods of extended plant operations. The concept of ERP refers to the combinations of Accident Tolerant Fuel (ATF), optimal use of Diverse and Flexible Coping Strategy (FLEX), enhancements to plant components and systems, and the incorporation of augmented or new passive cooling systems, as well as improved fuel cycle efficiency. The objective of the ERP research effort is to use the RISA methods and toolkit in industry applications, including methods development and early demonstration of technologies, in order to enhance existing reactors' safety features (both active and passive) and to substantially reduce operating costs through risk-informed approaches to plant design modifications to the plant and their characterization. Compared with the analysis documented in INL/EXT-19-53556, this report includes additional accident scenarios for risk-informed ATF analysis, and examines the risk impacts and benefits of FLEX and passive cooling system on nuclear power plants (NPPs) from other perspectives. The same analysis process, risk analysis approaches, and analysis tools as in FY 2018 were used for near-term ATF cladding (i.e., Iron-Chromium-Aluminum [FeCrAl] cladding and Chromium [Cr]-coated cladding) designs under the small-break loss-of-coolant accident (SBLOCA or SLOCA) including SBLOCA with anticipated transient without scram (ATWS) scenarios, two types of general transients (locked rotor and turbine trip) including locked rotor with ATWS scenarios, and main steam line break (MSLB) scenarios. For benchmarking purpose, University of Wisconsin compared the ATF station blackout (SBO) analysis results using RELAP5-3D as in the FY 2018 report INL/EXT-18-51436 with the results using MELCOR for the same SBO scenarios. In addition, based on the generic probabilistic risk assessment (PRA) model, the risk impacts and benefits of FLEX and the dynamic natural convection (DNC) system on NPPs are evaluated through a case study with the risk-informed decision-making process, i.e., significance determination process (SDP). In the ATF SBLOCA analysis, eight SBLOCA scenarios were developed based on the generic SAPHIRE PRA model for a Westinghouse three-loop pressurized water reactor (PWR) used in previous ERP analysis, and analyzed using RELAP5-3D for thermal hydraulic analysis with traditional fuel design and near-term ATF designs. The RELAP5-3D simulation results, as presented in Tables ES-1 and ES-2, show that the gain in coping time for FeCrAl varies between 2 and 36 minutes, except for Scenario SBLOCA-2.0 where the gain in coping time is more than 3 hours. The average increase in coping time with the Cr-coated cladding, which is also referred to as Chromite or chromium, is about 3 minutes. A slightly better estimate of the gain in coping time can be obtained by subtracting the time to core uncovery from the time to core damage and averaging. Using this metric, the gain in coping time is about 16 minutes for FeCrAl and 4 minutes for Chromite when Scenario SBLOCA-2.0 is excluded. With these relatively small increases of the time to core damage, the risk benefit to the core damage frequency (CDF) brought by the ATF designs would be very small and was therefore not conducted in this analysis. However, the RELAP5-3D simulation results show the clear benefit in adopting ATFs with much less hydrogen produced at the time of core damage. The SBLOCA with ATWS scenarios are exceptions in that the hydrogen production with the ATF claddings can exceed that with Zircaloy because of longer heatup times. Excluding the ATWS scenarios, the average hydrogen production with FeCrAl was 6.4% of that with Zircaloy. The average hydrogen production with Chromite was 36.9% of that with Zircaloy. iv Table ES-1. Time to Core Damage Comparison for SBLOCA Scenarios with ATF Designs. Scenario Description Time to core damage (hr:min) Zircaloy FeCrAl Δt (FeCrAl) Chromite Δt (Chromite)