CHF experiments with a plate-type large size of carbon steel heater considering long-term oxidation process

During a severe accident in nuclear power plants (NPPs), the reactor core begins to melt and the melted core accumulates at the bottom of reactor pressure vessel in a shape of corium. As a mitigation strategy of NPP severe accident, In-vessel retention external vessel cooling (IVR-ERVC) is operated to remove a huge amount of thermal load from the corium. Numerous researchers have evaluated the thermal load induced on RPV surface during a severe accident and have studied critical heat flux (CHF) using a carbon steel (RPV material) as a function of orientation (Downward-facing to vertical-facing) to evaluate a thermal margin of RPV. Rempe et al. [1] studied the thermal load generated on RPV inner surface by the corium in several accident scenarios, and the highest heat flux was reported in vertical surface region. Kam et al. [2] conducted CHF experiments using a large size of heater and pool, and showed the CHF trends as a function of heater dimensions, orientations (0~90), materials (Stainless steel, Carbon steel), and pressure (1~10bar). They observed that CHF increases with an increase of orientation and pressure. During a normal operation of a NPP, RPV surface is exposed to dry and high temperature environment. Since hot leg and cold leg temperature of primary side are around 568.98K and 600.48K in OPR1000, the RPV surface temperature is expected to be as 573.15K (300oC). The hot and dry environment outside of RPV has caused the oxidation on carbon steel surface to be accelerated, and the surface oxidation has a significant influence on a heat transfer performance, especially CHF. Kim et al. [3] considered this normal operation condition into carbon steel and evaluated the CHF with oxidized upwardfacing heater surface. They asserted that the CHF decreased despite the surface morphology modification. For the CHF evaluation under the IVR-ERVC condition, the experiments are required with a relatively large and inclined heater surface considering bubble dynamics generated by the vapor flow along with heater length and its effects on CHF. In this study, a large size of carbon steel heater was oxidized by a box furnace. Using the oxidized heater, CHF experiments were conducted under the vertical orientation condition (90o), and the surface morphology was analyzed by SEM, XRD, and contact angle measurement. As a result, magnetite and hematite oxidation layer were observed on heater surface, and the enhancement of wettability and CHF was confirmed after all.