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Under the auspices of the DOE LWRS Program RISMC Industry Application ECCS/LOCA, INL has engaged staff from both South Texas Project (STP) and the Texas A&M University (TAMU) to produce a generic pressurized water reactor (PWR) model including reactor core, clad/fuel design and systems thermal hydraulics based on the South Texas Project (STP) nuclear power plant, a 4-Loop Westinghouse PWR. A RISMC toolkit, named LOCA Toolkit for the U.S. (LOTUS), has been developed for use in this generic PWR plant model to assess safety margins for the proposed NRC 10 CFR 50.46c rule, Emergency Core Cooling System (ECCS) performance during LOCA. This demonstration includes coupled analysis of core design, fuel design, thermalhydraulics and systems analysis, using advanced risk analysis tools and methods to investigate a wide range of results. Within this context, a multi-physics best estimate plus uncertainty (MP-BEPU) methodology framework is proposed. The set of modeled results shows that peak clad temperature (PCT) and fuel cladding oxidation responses (measured by equivalent cladding reacted (ECR)) are well characterized by performance based modeling under large break LOCA conditions. Both PCT and ECR comply with a proposed acceptance criteria with sufficient margins available. Furthermore, these demonstration calculations indicate the importance of safety margin management and planning for future operating cycles. Since nuclear fuel stays in a reactor for multiple operating cycles, planning of loading and operating strategies needs to be well thought of.

Step 2 Analysis performed: 1) 11 th cycle to identify 8 points in time to initiate the maneuver (load-following); 2) Load following maneuver at each pre-selected points Goals: skewed power shapes for initiating LOCA analyses Used tools: PHISICS/RELAP5-3D in multi-deck modeStep 3Analysis performed: Steady state calculation to migrate from 1 core channel/assembly to 6 channel model used in LOCA analysis Goals: initial conditions in the lumped TH model (6 channels) Used tools: RELAP5-3DStep 4Analysis performed: LOCA transients at each preselected point in time (after the maneuver) Goals: identify the limiting conditions Used tools:

Center for Advanced Energy Studies (CAES) caesenergy.org Gateway for Accelerated Innovation in Nuclear (GAIN) gain.inl.gov Idaho National Laboratory (INL) www.inl.gov