REACTIVITY INITIATED ACCIDENT ANALYSIS METHOD USING MULTI-PHYSICS COUPLED CODE SYSTEM BASED ON RAST-K v2.0 H.C. SHIN Korea Hydro & Nuclear Power Corporation, Yuseoung-daero 1312-70, 34101 Daejeon - Republic of Korea H. KIM, J. PARK, J. YU, D. LEE Department of Nuclear Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, 44919 Ulsan - Republic of Korea ABSTRACT A Reactivity Initiated Accident (RIA) analysis method is demonstrated using the multi- physics coupled code system of RAST-K. Four computational codes based on three different physics models are coupled in one direction without feedback. RAST-K, CTF, and FRAPCON/FRAPTRAN represent neutronics, sub-channel thermal hydraulics (T/H), and fuel performance, respectively. The neutronics code calculates fuel pin power distribution, the sub-channel T/H code calculates coolant properties, and the fuel performance codes calculate fuel behaviours. Steady-state calculation is performed by RAST-K, CTF, and FRAPCON to simulate the long-term behaviour of the reactor during burnup. Considering normal operating conditions as an initial point, a transient calculation is performed with RAST-K and FRAPTRAN to simulate a Rod Ejection Accident (REA) for the peak pin. Cycle 1 of a typical OPR1000 LWR reactor is analysed by the coupled code system at the beginning of a cycle. A relative core power level of 270%, a fuel centreline temperature of 860 °C, and a 25 cal/g fuel enthalpy increase were achieved for the peak pin during the REA simulation. 1. Introduction Ulsan National Institute of Science and Technology (UNIST) has developed a nuclear reactor core analysis code system called STREAM/RAST-K [1], [2], sponsored by Korea Hydro & Nuclear Power Central Research Institute (KHNP-CRI). The neutron transport code (STREAM) performs a lattice calculation to provide nuclear data to the nodal diffusion code RAST-K, adopting a two-step approach for nuclear reactor analysis. To construct a high fidelity, multi-physics, and multi-scale reactor core analysis code system, RAST-K has implemented multi-physics capability by employing the sub- channel thermal hydraulics (TH) code CTF [3] and fuel performance codes FRAPCON [4] and FRAPTRAN [5]. The coupled calculations are performed in one direction. A Reactivity Initiated Accident (RIA) is a nuclear reactor accident which involves an unwanted increase in reactor power [6]. The reactor power increase could damage the reactor core, and could lead to disruption of the normal performance of the reactor. In Pressurised Water Reactors (PWR), control Rod Ejection Accidents (REAs) can occur due to mechanical failure of the control rod drive mechanism or its housing, such that the reactor coolant system pressure would cause the ejection of a partially or fully inserted control rod, and drive the shaft to its fully withdrawn position. If the reactor is operating at or close to the critical position, the consequences of this mechanical failure include