Two-phase flow water hammer transients and induced loads on materials and structures of nuclear power plant (WAHALOADS)

Abstract

In view of developing and validating a new code aimed at predicting fast transient two-phase flows in NPPs, extensive experimental data sets are being collected by means of three test facilities in the WAHALoads project. The code will enable to calculate the forces acting on the pipes and structures when these fast transients take place. The project began in 1st Pctober 2001 and extends to 1st April 2003. It started with a benchmark exercise which enabled to measure some deficiencies of the system codes when these codes have to simulate fast transients like those occurring in water hammers induced by the rapid closure of a valve or by the release of the fluid through a pipe breach. The benchmark exercise has also confirmed the need for a specific code using advanced numerical methods. In this project a new and original data base is obtained by means of experiments made on three test facilities which differ from each other by the range of pressures, temperatures and geometries, as well as by the mechanisms of water hammers they can produce (condensation water hammer or fast flow transient). Advanced types of the Wire Mesh Sensor to measure the void fraction instantaneous profiles are used in the three test facilities. The code, called WAHA, is based on a two fluid model with special closure relationships adapted to the transient character of the flow and to the severe non-equilibriums of interphase mass, momentum and energy transfers. The numerical scheme minimizes the numerical diffusion: it consists of a second-order accurate characteristic upwind numerical scheme, and operator splitting for relaxation source term. Validation of the code is achieved by using it for the simulation of the experimental data.