APPLICATION OF STEADY STATE VERSUS TRANSIENT DNB PREDICTION APPROACHES TO A PWR ROD EJECTION ACCIDENT

摘要

Accurate prediction of departure from nucleate boiling (DNB) is very important in pressurized water reactor (PWR) safety analysis. DNB correlations are generally available in steady state conditions, so the quasi-steady state approach is typically used to simulate reactor transients. This introduces an error that is negligible for slow transients, but more significant for faster ones. Here the challenge of assessing this error is addressed. A transient DNB mechanistic model based on the prediction of the liquid layer thickness in contact with the heated surface is adapted and modified from existing studies into a generalized formulation, in order to allow implementation into reactor simulation codes. This formulation is tested against experimental data, which are available only at low pressure. The formulation is next applied to a PWR-specific transient, the rod ejection accident, to perform a preliminary assessment of the error in using the quasi-steady state DNB approach. The SIMULATE-3K and VIPRE codes are used to calculate local thermal-hydraulic properties in the hot subchannel during the transient. The formulation has shown to be versatile and suitable for use in reactor simulation codes. A significant time lag has been observed in simulation results between DNB predicted by the quasi-steady state and by the transient approach. In some cases, DNB has been predicted only by the quasi-steady state approach, not by the transient one. However, confirmation of this conclusion awaits the availability of high-pressure transient DNB data to validate the applicability of the investigated formulation to PWR conditions.