Flow analyses using RELAP5/MOD3.3 code for the KSNP under the external reactor vessel cooling

Full text of publication follows: External vessel cooling via cavity flooding is adopted as one of the major severe accident management strategies in the APR1400 (Advanced Power Reactor 1400 MWe) and the current operating nuclear power plant, KSNP (Korea Standard Nuclear Power Plant). The coolability through the external reactor vessel cooling is highly affected by the thermal load from the molten pool and the accessibility and cooling capacity of water inside the RPV (Reactor Pressure Vessel) insulation. If steam generated via boiling at the RPV outer surface could not ventilate through the insulation, the pressure inside the annulus between the RPV and the insulation increased abruptly and consequently water swept out and steam dominated the flow path inside the annulus. In this case, effective heat removal through the external reactor vessel cooling cannot be achieved. Therefore, sufficient water ingression and steam venting through the insulation can be a key factor determining the success of the external reactor vessel cooling in the operating nuclear power plant, KSNP. In this study, flow analyses using RELAP5/MOD3.3 code have been performed to investigate the occurrence and the effects of steam binding for the KSNP under the external reactor vessel cooling. And separate effect test has been performed to examine the permeability through the typical RPV insulation mock-up. Flow area and energy loss coefficient, K through the insulation mock-up were evaluated from the separate effect test. And these values were directly used in the RELAP5 input. It could be found from the preliminary analyses for the flow path in the annulus of insulation that steam binding could occur in case of the limited steam venting capacity. In case of sufficient flow path for the steam venting, the vessel experienced effective cooling by nucleate boiling heat transfer. Additional sensitivity analyses are in progress to synthetically evaluate the probability of steam binding and its effects on the coolability through the external reactor vessel cooling for the KSNP. (authors)