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

Abstract

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 capabilityof the thermal hydraulic code TRACE5 to reproduce these phenomena in a scale-up model. The accidentscenario analyzed is based on Test 1.2 in the frame of the OECD/NEA ROSA Project, which simulatesa 1% hot leg Small Break Loss-Of-Coolant Accident in the Large Scale Test Facility of the Japan AtomicEnergy Agency. During this test, natural circulation in primary loops occurs, cooling the core during someminutes. This is an important phenomenon, which needs to be checked by means of different TRACE5models. With this aim, Test 1 2 has been simulated using a TRACE5 model reproducing the geometricaland thermal hydraulic features of Large Scale Test Facility. In order to determine if this phenomenon canbe reliably extrapolated to a scale-up plant, a new TRACE5 model has been developed. The geometricalfeatures of this scale-up model are determined using a fixed scaling ratio respect to the original LSTFfeatures. On the other hand, 4 and 3-loop standard Westinghouse PWR models are used in order tosimulate the same transient and compare the behaviour of the main thermal hydraulic variables withthose 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 behaviourduring the whole transient. Important discrepancies are found in the results corresponding to 4 and3-loop PWR TRACE5 models. In both models, natural circulation is not properly reproduced. Trying toimprove the simulation results, the nodalizations of U-tubes and pressure vessel were tested. Resultsstate that the nodalization of U-tubes clearly affects the natural circulation simulation. However, thevessel nodalization effect is not as important.