Verification of the IRIS Numerical Model for the Severe Accident Calculation

The interest in the IRIS (International Reactor Innovative and Secure) reactor is revived by today's popularity of small modular reactors. The IRIS reactor project, led by Westinghouse Electric Corporation, was active in the first decade of the 21st century. Different groups of institutions such as nuclear manufacturers, academic institutions, national laboratories, etc. from 10 countries around the world participated in the IRIS team. IRIS is an integral, modular, medium sized (1000 MWt) pressurized water reactor. The IRIS reactor pressure vessel houses, beside the reactor core, also other major reactor coolant system components such as the pressurizer, reactor coolant pumps and steam generators. The lack of large pipes ensures high safety of the IRIS power plant and eliminates many causes of major accidents. This principle is known as “safety-by-design” approach. Polytechnic of Milan and the University of Zagreb were leading institutions in performing safety analyses for the IRIS reactor. The explicit coupling of RELAP5 and GOTHIC codes has been set up to cover the sequence of most probable LOCA transient events. This was necessary because the reactor vessel and the containment, once when the LOCA is initiated, become one hydraulic system with strong interaction. They exchange mass and energy which affects both systems in short time period and therefore cannot be treated separately as in a conservative analysis of a classic PWR nuclear power plant. In addition, the ASYST code model was recently developed to cover possible severe accident sequences. The core heat structures were replaced with SCDAP components to simulate core degradation. A couple of different GOTHIC models were developed to represent various arrangements of passive safety systems. A steady state analysis was performed to confirm the applicability of the IRIS numerical model in the safety analyses.