Effectiveness of Modified Jakob Number as a SAMG Entry Condition

Abstract

To mitigate the consequences of transients and accidents that have caused plant parameters to exceed reactor protection system set points or engineered safety feature set point, or other established limits, operators' action is guided by plant procedures of emergency operating procedures (EOPs). If the operators' action with EOP fails in bringing the plant to a safe state, significant core degradation may result in and the nuclear power plant system may experience the severe accident condition. Once severe accident starts EOP action is terminated and accident management is handed into the technical support center (TSC). Since the severe accident, a new level of accident mitigation is invoked based on severe accident management guidance (SAMG). For a reasonable SAMG entry condition, core exit temperature (CET) has been accepted as the most realistic and practical parameter because of its reliability of detecting and diagnosing the severe accident condition. For the OPR1000, CET=923 K was set as the SAMG entry condition. In fact, monitoring and diagnosing the cladding temperature under severe accident condition is of importance. However, it is impossible to read the cladding temperature and as an alternative CET could be the best measurement in inferring the cladding temperature. Because of its importance in mitigating the severe accidents, the effect of CET has been studied by many researchers. Among many, recently Seo et al. investigated effect of the four SAMG entry conditions using MELCOR code. Their results showed that applying different CETs as a SAMG entry condition would result in different mitigation result in terms of delayed failure time of the reactor pressure vessel (RPV). However, no direct evidence was drawn for the optimized CET value to define the best SAMG entry conditions. On the other hand, Lee et al. suggested a new parameter for SAMG entry condition. Their results showed that modified Jakob number can be used to predict onset of oxidation and further can be applicable as an alternative SAMG entry condition in replacement of CET. Therefore, in this study, in extension of Lee et al.'s study, various modified Jakob numbers were used for SAMG entry condition in the severe accident simulation using MELCOR 1.8.6. The objective of this study is to investigate the effect of accident mitigation in view of delaying RPV failure. Three accidents of the SBLOCA without SI, TLOFW, and SBO were simulated using MELCOR code 1.8.6. Using Ja'_C_E_T, various SAMG entry conditions for the OPR1000 were applied for each accident. Total 21 cases of accidents were performed. Three of them were base cases without mitigation and 18 cases were simulated accidents with mitigation action. It was found that Ja'_C_E_T was useful to predict onset of oxidation. Thus, if mitigation action is performed before oxidation, delayed RPV failure can result in. Although delay of oxidation by mitigation action was significant in SBLOCA without SI and equivalent results was not achieved for the TLOFW and SBO. However, by adopting various Ja'_C_E_T rather than a single CET=923 K, more comprehensive implementation of the core thermal-hydraulic state is expected for more effective accident management. Finally, use of Ja'_C_E_T may bear in a more practical measure of the core thermal state and possible range of Ja'_C_E_T=1-2 could be applied. With Ja'CET=1.8, the mitigation effect was the best for the SBLOCA. With Ja'CET=1.0, the mitigation effect was greatest for the SBO and TLOFW. To confirm the general relationship between the Ja'CET and mitigation effect, a more detailed study is needed. Adoption of Ja'CET lower than 1 may be investigated in the future. Also more detailed analysis is needed to confirm that Ja'_C_E_T indeed can be used for another standard of suitable SAMG entry condition