Progress in the simulation of transient events observed during the blowdown of a medium-scale pressurized-water loop

The computer code RAMA has been developed to analyze postulated loss-of-coolant accident (LOCA) scenarios for a CANDU-PHW/sup +/ reactor circuit. The reactor circuit consists of a branched pipe network, with components such as fuel channels, steam generators, headers and pumps at discrete locations. The horizontal fuel channels distinguish this reactor from all other commercial reactors. As part of the code validation process, comparisons of predictions from the model are made with experiments conducted in the RD-12 loop, a facility that duplicates the important geometrical features of the reactor system. This paper focuses on two experiments conducted in RD-12 that illustrate the modelling requirements for different circuit flow conditions. Where high flow rates prevail, predictions of a homogeneous equilibrium model are in good agreement with experiment. At lower flow rates, the steam and water separate and may have different velocities and temperatures. Greatly improved predictions can be obtained for such conditions by using an unequal velocity, unequal-temperature (UVUT) model.