Sensitivity and uncertainty analysis of the group distribution header blocking at Ignalina NPP

Abstract

Ignalina Nuclear Power Plant is a twin-unit with two RBMK-1500 reactors. The primary circuit consists of two symmetrical loops. RBMK-1500 rector design is unique and extremely complex with respect to western type reactors. This paper presents analysis of the postulated blocking of coolant flow rate in Group Distribution Header (GDH) event at Ignalina NPP. Coolant flow rate blocking can lead to the sharp coolant flow rate decrease in 38 to 43 fuel channels. Thus, fuel cladding and pressure tube wall temperature sharply increases. Temperatures start to decrease after reactor shutdown, which is initiated due to protection against coolant flow rate decrease through GDH. Fuel cladding temperature peak in the maximum power channels is close to the acceptance criteria temperature (700ºC for fuel cladding). Thus, sensitivity and uncertainty analysis is important for this case. The best-estimate analysis consists of deterministic analysis by employing best-estimate code RELAP5/MOD3 and sensitivity and uncertainty analysis by employing GRS (Germany) developed System for Uncertainty and Sensitivity Analysis (SUSA). For that purpose RELAP5 Ignalina NPP model was developed and main contributors to the uncertainty of the results were identified. Since the selected uncertainty and sensitivity analysis methodology is a statistic-based methodology, a certain number of calculations to perform. The results show dependence on the initial and boundary conditions and code selected models. This modern approach applying these two analyses, which complement each other, is used in the Ignalina NPP Unit 2 Safety Analysis Report (SAR) production. The analysis showed that fuel cladding temperature curves in the maximum power channels do not exceed the temperature acceptance criterion of 700ºC.