Effectiveness and adverse effects of reactor coolant system depressurization strategy with various severe accident management guidance entry conditions for OPR1000

Severe accident analysis for Korean OPR1000 with MELCOR 1.8.6 was performed by adapting a mitigation strategy under different entry conditions of Severe Accident Management Guidance (SAMG). The analysis was focused on the effectiveness of the mitigation strategy and its adverse effects. Four core exit temperatures (CETs) were selected as SAMG entry conditions, and Small Break Loss of Coolant Accident (SBLOCA), Station Blackout (SBO), and Total Loss of Feed Water (TLOFW) were selected as postulated scenarios that may propagate into severe accidents. In order to delay reactor pressure vessel (RPV) failure, entering the SAMG when the CET reached 923 K, 923 K, and 753 K resulted in the best results for SBLOCA, SBO, and TLOFW scenarios, respectively. This implies that using event-based diagnosis for severe accidents may be more beneficial than using symptom-based diagnosis. There is no significant difference among selected SAMG entry conditions in light of the operator's available action time before the RPV failure. Potential vulnerability of the RPV due to hydrogen generation was analyzed to investigate the foreseeable adverse effects that act against the accident mitigation strategies. For the SBLOCA cases, mitigation cases generated more hydrogen than the base case. However, the amount of hydrogen generated was similar between the base and mitigation cases for SBO and TLOFW. Hydrogen concentrations of containment were less than 5% before RPV failure for most cases.     

Efficacy assessment of independent severe accident mitigation measures in OPR1000 using MELCOR code

In-vessel and ex-vessel mitigation strategies have been revisited to improve the severe accident management (SAM) for operating nuclear power plants. Because independent mitigation measures tend to produce positive and adverse effects simultaneously, it is necessary to investigate the efficacy of individual measures by means of proper quantification. Thus, in the present study we investigated the overall efficacy of existing SA mitigation strategies prepared for the Optimized Power Reactor 1000 MWe (OPR1000) by means of MELCOR 1.8.6 code. The numerical evaluation showed that the Mitigation-01, feeding water into the steam generators, is the most effective among the other mitigations. In addition, Mitigation-02, reactor coolant system depressurization, could not mitigate the SA sufficiently when applied individually. Among the four ex-vessel mitigation strategies, execution of containment spray was effective in removing most of the aerosol fission product but also intensified hydrogen combustion by increasing the partial hydrogen pressure owing to steam condensation. Mitigation-07, operation of passive autocatalytic recombiners (PARs), could reduce the hydrogen concentration, though the catalytic reaction was predicted to increase the containment pressure. In conclusion, this study suggests that mitigation measures should be carefully selected, and that counteracting measures should be prepared to minimize potential adverse effects.     

Analysis of B4C Influences on Thermodynamic Properties and Phase Separation of Molten Corium with Ionic Liquid U-Zr-Fe-O-B-C-FPs Database

Boron carbide influences on thermodynamic properties and phase separation of molten corium such as liquidus temperature were estimated with our U-Zr-Fe-O-B-C-FPs thermodynamic database. The liquidus temperature of the oxide for the typical corium was estimated to increase by a hundred degrees with B₄C addition when the corium included up to 10 wt% Fe. On the other hand, the liquidus temperature was hardly changed when the corium included 50wt% Fe. The interaction temperature between the steel and the corium with B₄C was estimated at 1130K. We define the interaction temperature as the lowest temperature where the solid Fe and the liquid phase of a corium are in equilibrium, at which interactions such as microstructure change of the vessel were observed in test studies. Although it is 180K lower than that without B₄C, the estimated temperature is still over 200K higher than the criterion temperature where the vessel loses its structural strength, which has been used in the feasibility evaluation of the in-vessel retention. Other thermodynamic influences of B₄C were also estimated as not having a negative impact on the in-vessel retention.