| dc.description.abstract | During the initial stage of a Loss of Coolant Accident (LOCA), known as the blowdown phase, the high-temperature and pressure break flow can impinge on thermal insulation and generate a substantial amount of debris in containment. This debris can accumulate in the sump compartment and become a major safety concern by potentially impacting the capabilities of the Emergency Core Cooling System (ECCS). Debris can accumulate in the sump and could cause ECCS pump head loss and/or pass through the filtering systems (debris bed and sump strainer) into the reactor primary system during the long term cooling phase. This scenario and its possible downstream effects are of primary concern under the US Nuclear Regulatory Commission (NRC) Generic Safety Issue 191 (GSI-191).
If the debris was to bypass the filtering and accumulate at the core inlet, core flow could theoretically decrease, affecting the core coolability (decay heat removal). If the debris accumulation at the lower core plate was high enough, it could potentially block the flow through the base of the core, the primary coolant flow path. In an even more severe scenario, debris could block all flow from the bottom of the core by blocking both the core inlet and core baffle/barrel bypass.
Under such conditions, core coolability is dependent on coolant reaching the core through alternative flow paths. One of these key flow paths is the core bypass (baffle/barrel). Additionally, the effectiveness of bypass flow in reaching and cooling the core is heavily impacted by certain plant specific features. One such plant specific feature, which the presence or lack of can have a major impact on core coolability, are the pressure relief (LOCA) holes. When the core inlet and bypass are unavailable, coolant must reach the core from the top by passing through the upper head region or steam generators.
The primary response during each phase of a double-end guillotine (DEG) Loss-of-Coolant Accident (LOCA) was analyzed for the model of a typical 4-loop Pressurized Water Reactor (PWR) using RELAP5-3D. The effectiveness of core cooling under the hypothesized cold leg break with full core inlet blockage was analyzed for models with and without pressure relief holes. Core cooling was also evaluated under a hypothesized hot leg DEG break with full core inlet and core bypass blockages, with particular emphasis on comparing simplified upper head geometry to a more realistic and more conservative model.
The results of the cold leg break with inlet blockage simulation showed that the presence of alternative flow paths from the bypass into the core may significantly increase core coolability and prevent cladding temperatures from reaching safety limits, while the lack of LOCA holes may lead to a conservative over-prediction of the cladding temperature. The simulation results also showed the impact of LOCA holes on total liquid level maintained in the core and driving flow through the bypass. The hot leg full inlet/bypass blockage showed that the upper head path was able to provide sufficient coolant, even under conservative models. These simulation results help inform safety impacts of some severe accidents under GSI-191 and serve as a reminder of the importance of modeling plant-specific features when performing best-estimate safety calculations. | en |
