Abstract:
Uncertainty analyses are important to evaluate the response of severe accident phenomena associated with using computer codes. MAAP Users’ Group (MUG) has been divided the accident phenomena into three categories, i.e. dominant, significant, and minor based on their importance to the accident sequence and progression. Ex-vessel debris cooling has been categorized as a dominant uncertainty with respect to severe accident phenomena. MAAP5 analyses have been performed to investigate the phenomena uncertainties of ex-vessel debris cooling.
Lungmen nuclear power plant (NPP), an advanced boiling water reactor (ABWR), and a station blackout accident, SRBC-PF-R-N sequence based on Final Safety Analysis Report (FSAR) of Lungmen NPP, are selected as a reference plant and a based case to investigate the uncertainties of ex-vessel debris cooling. This paper presented key parameters associated with ex-vessel debris cooling in MAAP5. For the MAAP5 uncertainty studies on ex-vessel debris cooling, heat transfer coefficients and critical heat flux Kutateladze number (FCHF) are investigated. Increase heat transfer coefficient will increase erosion distances. Maximizing the heat transfer coefficients can increase erosion distances in downward directions (about 0.3 m). But the erosion distances are less than the depth of lower drywell concrete floor (1.6 m). No sideward erosions predict for the different heat transfer coefficients. But corium can be cooled down after passive flooder opens. For FCHF is greater than 0.036, water can ingress into the debris, thus cool down the debris. Little erosions (below 0.6 m) happen in downward directions. No sideward erosions predict for FCHF greater than 0.036. However, corium can’t be cool down for the small value of FCHF. Small value of FCHF represents impermeable debris. When FCHF is smaller than 0.01, that results in higher corium temperature and continuous erosion on the lower drywell floor and pedestal. At the end time of calculation (250,000 s), erosion distances in both downward and sideward directions exceed the thickness of lower drywell floor and pedestal when FCHF is equal to 0.0036. Therefore, FCHF is a very important parameter to affect ex-vessel debris cooling. This paper successfully demonstrates the key parameters that effect ex-vessel debris cooling, and analysis results can provide useful information for the MAAP5 users, Level-2 probability risk assessment (PRA), and accident management.
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