Simulation of a SBLOCA in a hot leg. Scaling considerations and application to a nuclear power plant

The main goal of this work is to study the physical phenomena observed during a Small Break Loss-Of-Coolant Accident transient performed in a small-scale Integral Test Facility and to determine the capability of the thermal hydraulic code TRACE5 to reproduce these phenomena in a scale-up model. The accident scenario analyzed is based on Test 1.2 in the frame of the OECD/NEA ROSA Project, which simulates a 1% hot leg Small Break Loss-Of-Coolant Accident in the Large Scale Test Facility of the Japan Atomic Energy Agency. During this test, natural circulation in primary loops occurs, cooling the core during some minutes. This is an important phenomenon, which needs to be checked by means of different TRACE5 models. With this aim, Test 1-2 has been simulated using a TRACE5 model reproducing the geometrical and thermal hydraulic features of Large Scale Test Facility. In order to determine if this phenomenon can be reliably extrapolated to a scale-up plant, a new TRACE5 model has been developed. The geometrical features of this scale-up model are determined using a fixed scaling ratio respect to the original LSTF features. On the other hand, 4 and 3-loop standard Westinghouse PWR models are used in order to simulate the same transient and compare the behaviour of the main thermal hydraulic variables with those obtained in the Large Scale Test Facility model and in the Large Scale Test Facility scale-up model. Results show that both Large Scale Test Facility and the scale-up models present the same behaviour during the whole transient. Important discrepancies are found in the results corresponding to 4 and 3-loop PWR TRACE5 models. In both models, natural circulation is not properly reproduced. Trying to improve the simulation results, the nodalizations of U-tubes and pressure vessel were tested. Results state that the nodalization of U-tubes clearly affects the natural circulation simulation. However, the vessel nodalization effect is not as important.