Sensitivity Analysis of 200 mm LOCA at Armenian NPP Unit 2

Abstract

This paper presents the results of sensitivity analyses of a loss-of-coolant accident (LOCA) with equivalent diameter of 200 mm on a cold leg at Armenian Nuclear Power Plant (ANPP) Unit 2. ANPP is a six-loop WWER-440/270 model of Russian design. Analyses performed for the Technical Assignment for WWER-440/270 reactors showed that this reactor had high safety margins with 100 mm LOCA, which may allow increasing the safety level for the maximum design base LOCA. To demonstrate the adequacy of the safety margin with equivalent diameter of 200 mm using the existing high-pressure injection (HPI) system, analyses of LOCAs with break sizes of the equivalent diameter of 200mm in the primary side were previously performed with highly conservative assumptions using the RELAP5 code (Mod3.2.2β). Results of this study were reported at ICONE 11. The results of LOCA analyses with highly conservative assumptions showed that the acceptance criteria were not exceeded in the cases of a surge line break and equivalent break on a hot leg. However, calculations for a break with equivalent diameter of 200 mm on a cold leg showed that the cladding temperature reached 1235 °C, narrowly exceeding the safety limit mentioned in acceptance criteria (1204 °C), and prompted to conduct additional sensitivity studies with slightly relaxed conservative assumptions. At ANPP, there are two trains of HPI systems with two diesel generators on each train and one additional generator in the reserve state. Each train of diesel generators supplies power to two HPI pumps. However, in the current ANPP configuration, only one HPI pump in each train is automatically switched on to diesel generators in case of a station blackout. Therefore, the previous analysis of LOCA with highly conservative assumptions took credit for only two HPI pumps available after station blackout (i.e., two of diesel generators and reserve diesel generator are not available). The current ANPP modernization plan includes automatic starts of four HPI pumps. For this sensitivity analysis we took credit for one additional HPI pump (namely, three HPI pumps all together). As in the previous calculations, a loss of off-site power was assumed at the moment of accident beginning and simultaneous reactor scram: diesel generators were assumed to be available 20 seconds after the off-site power loss. The results of this sensitivity calculation of LOCA with equivalent diameter of 200 mm on a cold leg show that the cladding temperature stayed far below the safety limit. The peak cladding temperature was 850°C, well below that of the original study (1235°C) and the safety limit in the acceptance criteria (1204°C).