Development of Fission Gas Release Modeling in CTF for High Burnup Fuel Transient Analysis

Abstract

As part of the high energy fuel modeling and simulation, existing computational tools must be adapted to maintain applicability and accuracy at high burnups and increased enrichments. For high burnups in pressurized water reactors (PWR), there are ongoing studies on steady state fission gas release (FGR) and its effect on the fuel rod steady state operation. As burnup increases, fission gases trapped in the fuel matrix migrate to the fuel rod edge, increasing fuel brittleness, and if released present a possibility of rod overpressurization and clad ballooning. These fission gases also present a non-negligible effect on heat transfer through the fuel rod gap region. Increases in gas inventory in the gap following release from steady state and transient operation changes the timing and likelihood of cladding liftoff during transient conditions. This change in rod gap conductance significantly changes the timing of cladding quench following DNB in transient conditions. An improvement to the CTF reactor core thermal hydraulic analysis code on its capability of modeling FGR for high burnup fuel is proposed. The FRAPGR FGR model is added as a new code feature in steady state modeling, moving CTF towards applicability at high burnup above a fuel rod average burnup of 62 GWD/MTU for PWRs. The improvement in calculation of rod internal pressure and gap heat transfer is quantified and the updated code is verified against available benchmark cases. Current work is presented, and future work is proposed for additional improvements to the CTF code for high burnup fuel transient analysis.