Abstract
Interest in evaluation of severe accidents induced by extended station blackout (ESBO) has significantly increased after Fukushima. In this paper, the severe accident process under the high and low pressure induced by an ESBO for a small integrated pressurized water reactor (IPWR)-IP200 is simulated with the SCDAP/RELAP5 code. For both types of selected scenarios, the IP200 thermal hydraulic behavior and core meltdown are analyzed without operator actions. Core degradation studies firstly focus on the changes in the core water level and temperature. Then, the inhibition of natural circulation in the reactor pressure vessel (RPV) on core temperature rise is studied. In addition, the phenomena of core oxidation and hydrogen generation and the reaction mechanism of zirconium with the water and steam during core degradation are analyzed. The temperature distribution and time point of the core melting process are obtained. And the IP200 severe accident management guideline (SAMG) entry condition is determined. Finally, it is compared with other core degradation studies of large distributed reactors to discuss the influence of the inherent design characteristics of IP200. Furthermore, through the comparison of four sets of scenarios, the effects of the passive safety system (PSS) on the mitigation of severe accidents are evaluated. Detailed results show that, for the quantitative conclusions, the low coolant storage of IP200 makes the core degradation very fast. The duration from core oxidation to corium relocation in the lower-pressure scenario is 53% faster than that of in the high-pressure scenario. The maximum temperature of liquid corium in the lower-pressure scenario is 134 K higher than that of the high-pressure scenario. Besides, the core forms a molten pool 2.8 h earlier in the lower-pressure scenario. The hydrogen generated in the high-pressure scenario is higher when compared to the low-pressure scenario due to the slower degradation of the core. After the reactor reaches the SAMG entry conditions, the PSS input can effectively alleviate the accident and prevent the core from being damaged and melted. There is more time to alleviate the accident. This study is aimed at providing a reference to improve the existing IPWR SAMGs.
Highlights
After the Fukushima accident, the severe accident due to extended SBO caused widespread concern
Considering conservative assumptions, 10 s after the accident, the reactor is emergency shutdown. e decay heat continues to heat up the core, assuming all safety systems are not available. is section compares the core damage process of two scenarios. is section is limited to the in-pile stage of the accident’s progress, that is to say the heating and melting of the core
It is observed that the core degradation is very fast. e duration of core degradation is faster, and the core damage is earlier in the low-pressure scenario. e reactor core will melt in both types of selected scenarios
Summary
After the Fukushima accident, the severe accident due to extended SBO caused widespread concern. Erefore, the core degradation mechanism in a small IPWR needs to be further studied due to its special design characteristics and safety characteristics. It could provide reference for severe accident management of small IPWRs. SCDAP/RELAP5 is characterized by its detailed, mechanistic models of severe accident phenomena; the calculations can be rather time-consuming. Diverse early thermal hydraulic behavior and core degradation process of IP200 in integrated and low power design is compared with a large distributed reactor. As IPWR differs greatly from the large distributed reactor in terms of the system structure and parameter design, it is necessary to evaluate the SAMG entry condition for IP200 severe accident mitigation. The effect of the passive safety system on severe accident mitigation is quantitatively evaluated. is study can provide a reference for practical engineering and design of SAMG for IPWR
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