Potential Recovery Actions from a Severe Accident in a PWR: MELCOR Analysis of a Station Blackout Scenario

Abstract

Accident-tolerant fuel (ATF) cladding materials have been a focus of recent work to provide a greater resistance to fuel degradation, oxidation, and melting in light water reactors for beyond-design accident scenarios such as a station blackout (SBO). In a previous study, researchers at The University of Wisconsin–Madison used the Surry Nuclear Plant as the pilot plant to examine the effect of ATF substitute clad materials with the short-term SBO as the postulated accident, examining the effect of a loss of auxiliary feedwater (AFW) with the MELCOR systems code. In this work, we examine the effect of recovery actions for an SBO in Surry as a follow-on topic. Specifically, we selected two kinds of core cladding materials (Zircaloy and FeCrAl), and then conducted comparative analysis of the effect of water injection; first with a delay in water injection start times into the reactor pressure vessel (RPV) and then with steam generator (SG) steam-side AFW end times. We find that alternative cladding materials (FeCrAl) can effectively delay fuel degradation and system failures for both water injection strategies. One finds that RPV water injection can prevent such severe accident effects if restored in a few hours into the SBO. Conversely, SG steam-side AFW flow with alternative cladding materials (FeCrAl) can delay the fuel degradation and system failure processes by hours. We mainly focus on analyzing the severe accident progression by different quantitative signals, such as the onset of rapid hydrogen production, hot-leg creep rupture failure, and core slump. Analyses are now underway to consider the effects of proposed coating materials on Zircaloy cladding and if such coatings can afford similar benefits.