EXPERIMENTAL INVESTIGATION OF THE REWETTING PROCESS AT LOW STEAM PRESSURES

The rewetting of the overheated water cooled nuclear reactor fuel rods after a Loss-of-Coolant-Accident, which is defined as the re-establishment of a water film on the hot rod surfaces, is an important process for the safety of nuclear reactors. Several water experiments have been carried out to investigate the rewetting phenomena and to predict the liquid film behaviour either at atmospheric conditions or in a steam environment. Yet, there seems to be a scarcity of experimental data for the very important low pressure range from 2 to 7 bars with parameters the liquid film flowrate and the subcooling. An experimental facility to simulate two-phase low-pressure water flow phenomena has been designed, constructed and employed to investigate the rewetting process of hot surfaces in a water-steam environment at pressures in the above range, as well as at atmospheric conditions; the facility is on-line interfaced to computers which control its operation and undertake all the data acquisition tasks. The present paper describes the experimental work aimed at studying the wet front propagation along a stainless steel fuel rod in a top-flooding saturated water-steam environment in the pressure range 1 - 7 bars, under various initial wall temperatures up to 550 °C and at a liquid flowrate of 1Lmin??. Following the experimental results a correlation has been derived to predict the rewetting rate as a function of pressure and initial wall temperature. This correlation agrees well with formerly proposed correlations at higher pressures by several other investigators, thus permitting to extend their validity down to the pressure of 1 bar. Furthermore, a numerical method is introduced to experimentally evaluate the wet front position along the rod.