Analysis of Onset of Oxidation during Hypothesized Severe Accidents of Korean Optimized Power Reactor 1000

Abstract

Recent Probabilistic Safety Analysis (PSA) Level 1 reported that possible severe accident initiators for the Korean Optimized Power Reactor 1000 (OPR1000) were Small-Break Loss-of-Coolant Accident (SBLOCA) without safety injection, Station Blackout (SBO), and Total Loss of Feedwater (TLOFW). Thus in this study severe accident analysis was conducted using MELCOR code to investigate the detailed accident progression upon hypothesized accident scenarios. Especially, the most reliable severe accident monitoring variables of Core Exit Temperature (CET) and Reactor Coolant System (RCS) pressure were utilized to derive a meaningful representative indicator to determine the onset of oxidation departing from the saturation of the core water inventory. The representative indicator developed is Jakob number (Ja), which addresses the ratio of sensible heat (superheat due to core heatup) to latent heat (heat of vaporization at the saturation state). In this study, Ja was redefined as Ja-CET to evaluate the reactor coolant coolability and eventually to find the onset of oxidation. Three base cases without mitigation strategy and twelve cases with application of mitigation strategy and various severe accident entry conditions were simulated for the aforementioned initiating events. In the base cases, severe accidents are initiated at relatively high pressure and therefore the Ja-CET at the onset of oxidation ranged 1.77-3.32. On the other hand, for the mitigated cases effective depressurization lowered the pressure and the resulting Ja-CET at the onset of oxidation ranged 0.56-1.39, which are apparently lower than those in the base cases. Monitoring Ja-CET is expected useful for diagnosing the thermal-hydraulic state of the reactor core. Especially, Ja-CET was useful to find the onset of oxidation. As the onset of oxidation defers the core degradation, proper diagnosis could help understanding the plant status.