This manuscript presents the study of a large-break loss-of-coolant accident (LB-LOCA) sequence with the thermal-hydraulic system code ATHLET applied to a four-loop German pressurized water reactor (PWR) model. The investigation focuses predominantly on two-phase critical flow (TPCF) phenomena emerging under reactor blowdown. The safety implications of occurrence of critical flow conditions in nozzle-like constrictions along the discharge path, namely the main coolant pump (MCP) impeller, are evaluated based on discharge flow partition, primary system depressurization, global coolant circulation pattern, and ultimately the key safety parameter of peak cladding temperature (PCT). Critical flow is represented with the 1D thermal non-equilibrium model CDR1D incorporated in ATHLET. The simulation results show the development of a depressurization cascade along the affected loop during reactor blowdown, which is triggered by the MCP impeller constriction, with consecutive formation of multiple critical flow conditions after the two-phase front propagates through the coolant circuit. The flow limiting condition shifts from the break towards the MCP impeller casing due to its smaller cross-section area (CSA). Critical flow conditions persist furthermore at both break stubs simultaneously. The negative core spatial pressure gradient imposed by the shorter discharge path towards the vessel-side break stub favors top-bottom cooling mode during blowdown. Choking is likewise attained at the pressurizer surge line inlet nozzle and hot leg bypass in the affected loop. A set of parametric runs shows that variation of the MCP impeller constriction leads to re-partitions of mass flow rates between both discharge paths. Enlargement of the constriction diameter increases overall discharged mass causing a faster primary system depressurization and affects the spatial pressure gradient along the core channels, thereby shifting the stagnation point downwards. This behavior enhances core cooling resulting in a lower PCT during reactor blowdown compared to the as-built case. An additional parametric study with the empirical equilibrium model Moody shows re-partition of discharge flows based on the upstream thermodynamic conditions, which alter coolant circulation through the primary circuit and therefore the cladding temperature. The effects are stronger during vessel refill leading to a significantly lower re-flood peak and earlier termination of the temperature excursion.

This manuscript presents the study of a large-break loss-of-coolant accident (LB-LOCA) sequence with the thermal-hydraulic system code ATHLET applied to a four-loop German pressurized water reactor (PWR) model. The investigation focuses predominantly on two-phase critical flow (TPCF) phenomena emerging under reactor blowdown. The safety implications of occurrence of critical flow conditions in nozzle-like constrictions along the discharge path, namely the main coolant pump (MCP) impeller, are evaluated based on discharge flow partition, primary system depressurization, global coolant circulation pattern, and ultimately the key safety parameter of peak cladding temperature (PCT). Critical flow is represented with the 1D thermal non-equilibrium model CDR1D incorporated in ATHLET. The simulation results show the development of a depressurization cascade along the affected loop during reactor blowdown, which is triggered by the MCP impeller constriction, with consecutive formation of multiple critical flow conditions after the two-phase front propagates through the coolant circuit. The flow limiting condition shifts from the break towards the MCP impeller casing due to its smaller cross-section area (CSA). Critical flow conditions persist furthermore at both break stubs simultaneously. The negative core spatial pressure gradient imposed by the shorter discharge path towards the vessel-side break stub favors top-bottom cooling mode during blowdown. Choking is likewise attained at the pressurizer surge line inlet nozzle and hot leg bypass in the affected loop. A set of parametric runs shows that variation of the MCP impeller constriction leads to re-partitions of mass flow rates between both discharge paths. Enlargement of the constriction diameter increases overall discharged mass causing a faster primary system depressurization and affects the spatial pressure gradient along the core channels, thereby shifting the stagnation point downwards. This behavior enhances core cooling resulting in a lower PCT during reactor blowdown compared to the as-built case. An additional parametric study with the empirical equilibrium model Moody shows re-partition of discharge flows based on the upstream thermodynamic conditions, which alter coolant circulation through the primary circuit and therefore the cladding temperature. The effects are stronger during vessel refill leading to a significantly lower re-flood peak and earlier termination of the temperature excursion.