NEUTRONICS AND THERMAL-HYDRAULICS ANALYSIS OF RIA TRANSIENTS FOR HIGH-BURNUP FUELS.

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A Reactivity-Initiated Accident (RIA) accident occurs when, due to some mechanical failure, we have a control rod drop (in BWRs) or ejection (in PWRs). A reactivity insertion will follow such an event, increasing the fission rate of the nearby fuel assemblies in tens of milliseconds. Such power excursion will increase the fuel temperature, leading to a rapid fuel thermal expansion against the cladding. The mechanical loading, cladding mechanical response, and temperature response determine cladding failure during RIA. The present work discusses the neutronics and thermal-hydraulics modeling of an RIA transient for high-burnup fuels and the associated mechanical interactions focusing on low-temperature failures. We based our computational model on the REP-Na 3 test. We use a BISON and THM coupled model to solve the fuel’s rod temperature distribution and the thermal-hydraulic behavior over the coolant channel. We verified the reliability of the BISON native neutronic solver against an OpenMC and a Serpent model. We also performed a parametric study on OpenMC to determine the key factors that will impact our neutronic model. To have more freedom to determine which correlation we will use to assess the occurrence of the Critical Heat flux (CHF) on our model, we replaced BISON’s native coolant channel object with a THM model, where Thom’s correlation for subcooled boiling flow and Groeneveld lookup table for CHF were implemented.