Small break LOCA studies for different layouts of passive safety systems in the IRIS reactor

Abstract

IRIS (International Reactor Innovative and Secure) is an integral, medium power, light water reactor with advanced safety features. In the first decade of the 21^st century, 22 institutions under the leadership of Westinghouse Electric Corporation were involved in its development. The University of Zagreb, along with the Polytechnic of Milan, was in charge of performing safety analyses. A detailed plant model is developed using the RELAP5 code for the analyses of thermal–hydraulic processes in the reactor vessel, the GOTHIC code for the analysis of the processes in the containment and, in addition, the ASYST code for the calculation of a severe accident. Some of the previous small break loss-of-coolant accident analyzes at the existing pipelines are repeated to test the improved plant model. However, the focus of the paper is on the new set of analyzes of hypothetical breaks along the reactor vessel with the aim of determining whether the passive safety systems can ensure successful core cooling. For this purpose, two models are developed with different configurations of the emergency heat removal system and the safety systems inside the containment that inject water into the reactor vessel. The results show the complex and rather ambiguous dependence of the reactor coolant system thermal–hydraulic behaviour on the selected boundary conditions. The scenarios analyzed vary from design basis events to severe accidents. The capabilities of specific safety systems in mitigating the consequences of an accident are determined, depending on the position and size of the break on the reactor vessel wall.