Worst-Case Accident Analysis of Accident Tolerant Fuel in NuScale Using RELAP5/MOD3-Based Code

Abstract

Research in nuclear engineering focusses on improving the safety of light water reactors (LWRs), driven by accidents like Fukushima in 2011. The severity of this accident was a result of active cooling system failures and cladding material oxidation resulting in hydrogen explosions. To this end, efforts concentrate on the improvement of passive safety in LWRs and the development of accident tolerant fuel (ATF). Natural circulation small modular reactors (SMRs) are new reactor concepts designed around passive safety. One such SMR called the NuScale SMR was chosen for this study. This study aimed to evaluate the combination of one selected passively safe reactor system with two selected ATF cladding materials, namely FeCrAl alloy and a silicon-carbide (SiC) composite, to determine if this combination would greatly increase the safety of LWRs. The use of ATF cladding materials in the NuScale for possible mitigation of a potential worst-case scenario accident has not been evaluated. A model of the SMR was developed using ASYST 3.4 thermal–hydraulics code and validated for steady state operation and one worst-case scenario accident transient using data taken from the NuScale final safety analysis report (FSAR) and literature. Results indicated minimal impact on steady state operation when employing ATF cladding materials. FeCrAl and SiC caused minor changes in peak cladding temperature (PCT) and peak fuel centerline temperature (PFCT). The worst-case scenario accident transient involved the inadvertent opening of one reactor vent valve (RVV) with subsequent failure of the emergency core cooling system (ECCS) and decay heat removal system (DHRS). In the current model, this led to failure of the Zr-alloy cladding after 6.42 h. This coping time was increased with the use of ATF cladding, with FeCrAl and SiC providing an increase of 0.5 and 4.35 h, respectively.