DOE OSTI2017
It is a great challenge to ensure the safety of nuclear power plant during a severe accident. One of the major concerns is to overheat the reactor core which potentially may create a nuclear crisis if the core cooling fails. The concept of In-vessel retention (IVR) by passive external reactor vessel cooling (ERVC) in a flooded cavity of pressurized water reactor (PWR) is a viable and effective approach to retain radioactive core melt within the reactor pressure vessel (RPV). As long as the heat flux at the lower head of the RPV is lower than the critical heat flux (CHF) limit at the corresponding point, nucleate boiling will be the prevailing regime. In that case, the temperature of outer surface can be maintained near the saturated temperature of water such that the reactor vessel is able to be sufficiently cooled to sustain its thermal and structural integrity. Owing to the practical significance of this IVR-ERVC concept, there were many experimental and theoretical studies to investigate the two-phase flow natural circulation phenomena including the effect of inclination angle on the CHF, based on which many different empirical or semiempirical correlations were developed for CHF by fitting the experimental results. For the engineering applications, reactor-scale facilities such as ULPU series, KAIST facility, CYBL and FIRM were built to experimentally verify and validate the feasibility of ERVC for different design of reactors. Besides, for the research on physical mechanism of downward facing pool boiling, laboratory-scale experiments such as DELTA, CASA and SBLB were performed to visually and theoretically simulate the process of IVR-ERVC. All of the aforementioned test facilities and experiments provided a simulation of the downward facing boiling process and experimental study on the CHF limits during the ERVC conditions of different types of reactors. There were also some visualized investigations on the two-phase flow behavior in both quenching and steady state experiments. In this study, a scale-down three-dimensional downward facing pool boiling facility (iSBLB) was built to investigate the two phase natural circulation flow characteristics during the ERVC process. The general morphologies of vapor mass at different locations were visually recorded using the high-speed camera. The observation results showed that vapor behavior along the hemispherical curved surface was periodical. The boiling phenomenon was quite different from those on all other surface orientations.
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗