Effect of oxide layer thickness on the pool boiling critical heat flux of pre-oxidized RPV material

Abstract

For successful execution of the In-Vessel Retention (IVR) and External Reactor Vessel Cooling (ERVC) strategy, defining the Critical Heat Flux (CHF) on the outer surface of the Reactor Pressure Vessel (RPV) is a key objective to secure the thermal integrity of the RPV. Since the CHF is strongly affected by surface conditions of the heating material, if any surface treatments are not applied to the RPV outer surface, corrosion can govern the effects on the CHF. Note that natural corrosion occurs continuously for long time on the RPV. In this regard, pool boiling heat transfer experiment under atmospheric pressure with deionized water was conducted with the pre-oxidized RPV material, whose grade is SA508. The CHF of the bare SA508 increased by 1.5 times, as the heating surface is gradually oxidized approaching the CHF. As a result, it became hydrophilic and nano-porous with the formation of magnetite nanoparticles during nucleate boiling. When the SA508 was pre-oxidized at 300 °C in dry air, however, the CHF was deteriorated even though the oxidized surface was still hydrophilic. Considering that the thermal properties of the pre- oxidized surface experiences some changes as the crystallized oxide layer forms with the specific thickness, the thermal effusivity of magnetite layer is lowered by 0.28 times than that of the bare SA508. Based on the thermal activity-based theory, it may disseminate the effect of lateral heat dissipation, subsequently promotes local dry spot. Consequently, the oxidation behavior of the RPV shows some potential in generating adverse effect on the CHF.