Measurement of transient critical heat flux in a heated tube at intermediate pressures in water

Predicting the CHF for the wide range of abnormal or accident conditions is an essential part of the design and analysis of nuclear power reactors. Most design basis accident predictions are based on transient computer codes that compare the cladding surface heat flux to the predicted CHF. The prediction of CHF is based on empirical data obtained under quasi-static conditions. In applying a steady-state experimental results to transient analyses these codes implicitly assume that dryout will occur for the same local conditions under transients. There is contradictory evidence in literature as to the differences observed in steady tests versus transient behaviour. This paper assess the prediction of transient critical heat flux using quasi-steady methods by performing new experiments with depresurization rates up to 1MPa/s and comparing prediction methods to the new data. Experiments were conducted at pressures of 2-6 MPa, mass fluxes from 1000-2500 kg m² s¹, and inlet flow quality of -0.148 in a 1.321 m vertical, uniformly heated 4.6 mm inside diameter tube. The experimental results support the application of existing prediction methodologies to fast transients starting from either preexisting PDO conditions or with small initial margins to dryout.