Boiling heat transfer experiments under power transients for LWRs: A review study

The mechanistic understanding of the intrinsic thermal safety is of significance to enhance the sustainability of Light Water Reactors (LWRs) especially during power transients. Many boiling heat transfer experiments have been performed to characterize the thermal-hydraulic responses of the fuel-clad entity to various power transients. This study summarizes power transient boiling heat transfer experiments to illustrate the thermal hydraulics characteristics of power transients. Four different power transient modes that were reported in the literature are discussed for their applicability in the thermal safety margin evaluations of LWRs including exponentially escalating, ramp/quadraticly increasing, Nordheim-Fuchs reactivity-initiated accidents, and surface temperature surging. In the experimental studies, a wide variety of liquids including water, organic liquids (FC-72, ethanol and Freon liquids), and cryogenic liquids (nitrogen and helium) were used as working fluids of power transient boiling heat transfer experiments. In the flow boiling experiments, Critical Heat Flux (CHF) and heat transfer coefficients under power transients are higher than those of steady states. However, this thermal superiority of power transient becomes less significant over the progressive increasing of mass flux and/or liquid subcooling, which implies that the heat convection mechanism dominated by mass flux and liquid subcooling competes with the transient heat conduction mechanism. Different from power transient flow boiling experiments, CHF and heat transfer coefficients reported in some power transient pool boiling experiments could be less than those of steady states. This could not be sufficiently rationalized by any of power transient mechanisms. Moreover, the implications of the transient boiling experimental results lead to a conservative thermal safety evaluation of LWRs using the present knowledge of the steady-state boiling heat transfer experiments.