(A) study on design enhancement of automatic depressurization system in a passive PWR

Abstract

In a Passive PWR, the successful actuation of the Automatic Depressurization System is essentially required so that no core damage is occurred following small LOCA. But it has been shown in the previous studies that Core Damage Frequency form small LOCA is significantly caused by unavailability of the ADS. In this study, the design vulnerabilities impacting the ADS unavailability are identified through the reliability assessment using the fault tree methodology and then the design enhancements towards improving the system reliability are developed. A series of small LOCA analyses using RELAP5 code are performed to validate the system requirements for the successful depressurization and to study the thermal-hydraulic feasibility of the proposed design enhancements. The impact on CDF according to the change of system unavailability is also analyzed. In addition, aqualitative analysis is performed to reduce the inadvertent opening of the ADS valves. From the results of the analyses, the ADS is understood to have less incentive on the reliability improvement through system simplification. It is found that based on system characteristics, the major contributor to the system unavailability is the first stage. A series-parallel configuration with two trains of eight valves, which shows a higher reliability compared to the base ADS design, is recommended as an alternative first stage of the ADS. In addition, establishment of the appropriate ADS operation strategy is proposed such as allowing manual operation of the first stage and allowing the forced depressurization using the normal residual heat removal system connected to the RCS following the successful depressurization up to the 3rd stage and the failure of the 4th stage.