Thermal-hydraulic (T/H) analyses are used to support the level 1 Probabilistic Risk Assessment (PRA) success criteria and the manual operation time. To address the multiple-failure accident scenarios that are considered in the PRA, usually numerous T/H analyses were performed. So it is meaningful to develop a relative simple T/H model with acceptable accuracy for level 1 PRA T/H analyses. To achieve this object, the core modeling effects on the core damage progression were studied according to ASME/ANS RA-Sa-2009. Two types of core modeling methods were studied, including single channel core modeling and multi-channel core modeling. For the single channel core modeling, the study was focused on the axial nodes number effect. For the multi-channel core modeling, the cross-flow effects were studied. Several cases were calculated on a 3-loop PWR medium size break LOCA core damage scenario with Relap5/MOD3.2. Some key parameters related to the core state, such as peak cladding temperature (PCT), core water level and coolant inventory, were compared and analyzed. A kind of core modeling for level 1 PRA T/H analyses was suggested at the end.

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