PREDICTION OF PRESSURE DIFFERENCE IN A DOWNCOMER PIPE DURING A DEPRESSURIZATION PROCESS.

In the event of the loss of integrity of a nuclear power plant, complex thermo-hydraulic problems may be encountered in both the primary and secondary circuits. The analysis of such complex transient problems requires both a rigorous mathematical model and reliable input information in terms of items such as friction factors, slip ratios and heat transfer coefficients to the equations describing it. This paper describes the mathematical formulation of the governing equations and some of the constitutive laws in the two-phase regions of a downcomer tube of a forced circulation Freon test rig. Two suitable numerical schemes are employed to solve the one-dimensional equations system describing the transient flow in the tube, caused by a depressurization in the main circuit of the rig. Comparison between the experiments and the predictions are made on the downcomer section of the test rig where the flash evaporation effects are most apparent. The results show reasonably good agreement when the effects of phase slip and thermal non-equilibrium are introduced.