Oxidation effect on the pool boiling critical heat flux of the carbon steel substrates

Abstract

For successful execution of the In-Vessel Retention (IVR) and External Reactor Vessel Cooling (ERVC) strategy as an accident management of a nuclear power plant, defining the critical heat flux (CHF) on the outer surface of the Reactor Pressure Vessel (RPV) is a key objective to secure thermal and structural integrity of the RPV. Since the CHF is strongly affected by surface conditions of the heating material, unless appropriate surface treatments are applied to the RPV outer surface, natural corrosion may strongly influence the effects on the CHF for the lifetime of the RPV. In this regard, pool boiling heat transfer experiment under atmospheric pressure with deionized water was conducted with the oxidized RPV material, which is SA508 Grade 3 Class 1. Various oxidation times of 3, 10, 20, 30, and 40days were explored to investigate the oxidation effect on the pool boiling CHF. As a reference test, the CHF of the bare SA508 was measured approximately 140% Zuber’s famous pool boiling CHF prediction because the bare SA508 surface became hydrophilic and nano-porous with the formation of magnetite nanoparticles during the nucleate boiling before reaching the CHF. When the SA508 was oxidized in 300°C air, however, the CHF decreased to approximately 65% of the Zuber’s prediction even though the oxidized surface was hydrophilic and non-porous. Judging that the crystallized oxide layer formed up to approximately 1μm estimated with mass gain, the thermal effusivity of the magnetite layer could decrease to approximately 28% that of the bare SA508. Based on a theory related to thermal activity, presence of a magnetite layer may deteriorate lateral heat dissipation near the CHF and subsequently promote local dry spots. Consequently oxidation behavior of the RPV material shows some potential to generate an adverse effect on the CHF.