Analyses of an Integral Pressurized Water Reactor During Postulated Accident Conditions Using the System Code Package AC²

Abstract

In the frame of this work, the model of a generic integral Pressurized Water Reactor (iPWR) with 160 MWth is analyzed regarding the possibility of severe fuel damage during a postulated accident. This is relevant due to a growing interest on building iPWR and consequently intensified research on this reactor type. The simulation model is developed using the system code package AC² developed by GRS gGmbH focusing on the thermohydraulic and core degradation modules. The accident tolerance during the postulated failure of selected components is analyzed. Overall, the code is able to calculate plausible results for the generic model of the iPWR. Under design conditions, the passive core cooling mechanisms are efficient enough to prevent further degradation. Postulating the failure of additional selected components results in endangered core integrity. The heat up and steam oxidation at the top part of the core starts after the core level lowered, as expected. In case of core melting and relocation, the late-phase module AIDA is assessed with regard to possible melt retention and the influence of the necessary heat transfer coefficient in the lower plenum for a successful external cooling. Depending on the progression of the postulated accident, one important factor might be the fuel cladding, which is usually a Zr-based alloy. This cladding material possibly contributes to a temperature escalation due to the exothermic steam reaction. As a possibility, the accident tolerance could be further increased using alternative materials. Therefore, the application of slower oxidation kinetics of near-term candidates of Advanced Technology / Accident Tolerant Fuel (ATF) is investigated.