Analysis of IBLOCA Scenarios in ATLAS: Simulation Approaches with the TRACE Code

Abstract

In 2005, the USNRC proposed the Intermediate Break causing Loss of Coolant Accidents (IBLOCA) as a design basis event for assessing the effectiveness of emergency core cooling systems. Since then, the number of experiments in integral test facilities related to these scenarios, which were very limited up to that date, has increased. Korea Atomic Energy Research Institute (KAERI) has included IBLOCA scenarios in the ATLAS facility experimental programs. Specifically, two tests were performed under the frame of OECD-ATLAS2 project in 2017. Test B3.1 represents a pressurizer surge line IBLOCA, and Test B3.2 is a direct vessel injection (DVI) line IBLOCA. The size of these breaks corresponds to 11.4 % and 8 % of the cold leg flow area, respectively. Both tests were performed to evaluate the effectiveness of available injection systems, including the safety injection pumps (SIP) and the safety injection tanks (SIT). Parallel to experimental studies, simulations can contribute to understanding thermal-hydraulic phenomenology when reproducing experimental and hypothetical scenarios. In this framework, this work aims to analyze both scenarios with the TRACE thermal-hydraulic code. First, the experiments are simulated with TRACE to verify its capability to reproduce the sequence of major events and relevant phenomena. During the transient simulations, thermal-hydraulic parameters such as the system pressures, temperature, collapsed water levels, and natural circulation flows in the system were studied. Subsequently, sensitivity analyses on the break size spectrum of an IBLOCA and its location are conducted to evaluate the effects and identify “cliff-edge effects”.