EVALUATION OF EFFECT OF DEPRESSURIZATION TIMING ON THE THERMAL LOAD TO PRIMARY CONTAINMENT VESSEL DURING SEVERE ACCIDENT AT BWR

According to accident of a Boiling Water Reactor (BWR), the operators generally do not depressurize the Reactor Pressure Vessel (RPV) until low-pressure core injection systems are recovered. However, when the core water level decreases below a certain limit, the RPV is depressurized to prevent the RPV failure at high pressure, which may lead to failure of the Primary Containment Vessel (PCV) by direct heating of the ejected melt. In such a case, if recovery of the core cooling cannot be expected, earlier depressurization may be preferable from the viewpoint of reducing thermal load on PCV, because it may effectively reduce the amount of fission product (FP) aerosol deposition on the RPV dome and reduce the risk of PCV top head heating via convective and radiative heat transfers from the RPV dome. However, significance of such risk is not evident as the steam dryer and separator may hinder direct aerosol deposition on the RPV dome and the RPV dome is thermally insulated. Moreover, the narrow space between the RPV dome and the PCV top head may hinder convective heat transfer. In this study, sensitivity analyses have been carried out using MELCOR-2.2 to support such understanding.