원자로 압력용기 외벽 냉각 상황에서 산화된 표면이 수조비등열전달에 미치는 영향에 대한 실험적 연구

Abstract

During the postulated severe accidents, the final goal of the In-Vessel Retention through External Reactor Vessel Cooling (IVR-ERVC) strategies is to terminate the accident without release of radioactive molten-corium to the environment and the public. To successfully fulfill the goal, effective removing decay heat generated from molten-corium is a criterion associated with the implied IVR-ERVC strategy. It is emphasized that the degree of success is determined by cooling capability on the reactor pressure vessel (RPV) outer surface. If insufficient cooling is provided, the RPV will be ruptured by the occurrence of Critical Heat Flux (CHF). Thus, defining the CHF on the RPV outer surface is a key issue in terms of securing the integrity of the RPV. Generally, the CHF is highly dependent of the heating surface conditions. Since the RPV is made of carbon steel, it continuously undergoes the significant corrosion even under the dry containment condition. Thus, it is essential to define the realistic surface condition in contingent with the initiation of the ERVC strategy and the CHF can be measured more realistically given such aged conditions. If any treatments such as nanofluids or surface coating to enhance the CHF are not applied, the heating surface condition will be determined only by natural corrosion. Since the RPV is irreplaceable during plant life time and is made of low carbon steel alloy, whose chromium concentration is very low, the effect of natural corrosion will be dominant. Therefore, in this study, pool boiling heat transfer experiment under the condition of atmospheric saturated water was conducted to evaluate pre-oxidation effect on the CHF. Test heater materials adopted are stainless steel grade 316 (SS316) and low carbon steel alloy (SA508). By treating different oxidation periods in each specimen, distinctive oxidation states were prepared and characterized. After oxidation treatment, wettability and roughness of all oxidized specimen increased regardless of the oxidation periods. Oxidation state was classified as local- and complete-oxidation on the basis of microstructure of the heater surface and oxide layer thickness. Accordingly, with the local oxidation, the CHF maintains the similar value compared to the bare heater surface. On the contrary, with complete oxidation with thin oxide layer, the CHF showed increasing tendency. However, it is of particular interest to note that when the oxide layer becomes too thick so that the thickness exceeds certain oxide layer thickness, the CHF decreased, which was opposite to the general belief. This suggests that capillary wicking in porous media acts in the opposite way with respect to oxide layer thickness. In other words, capillary wicking is inactive and thus not promoting the CHF on the heater surface with local oxidation, whereas existence of the thin oxide layer contributes on the delay of the vapor merge and coalesce by increasing the laterally assisted capillary. With the heater surface of thick oxide layer, however, since the evaporation works rapidly, it traps the escaping vapor and thereby reducing the liquid permeability. Therefore it showed rather a decreasing trend for the CHF due to early formation of vapor film. As a consequence, it is expected that the CHF is enhanced in the thin oxide layer, whereas the CHF decreases when the oxide layer thickness exceeds certain oxide layer thickness. Considering operation time of Korean Optimized Power Reactor 1000 (OPR1000) ranges 40 to 60 years, the RPV outer surface is expected significantly oxidized with time and thus the CHF on the RPV outer surface could be a concern depending on the oxide layer thickness. In conclusion, this study shows that natural corrosion can deteriorate the CHF limit in conducting IVR-ERVC depending on the oxide layer thickness. As a future work, large-scale oxidation experiments may be useful in understanding the size effect coupled with natural corrosion. In addition, to ultimately prevent the deterioration of CHF by natural corrosion, the engineered surface with corrosion-inhibitive coating by chromium will be considered. |중대사고 시, 노 내 사고 완화의 일환으로 사용되는 압력용기외벽냉각 전략은 노심용융물의 외부 방출 없이 사고를 종결시키는 것을 목적으로 한다. 즉, 노심용융물에서 발생하는 붕괴열을 압력용기의 파손 없이 지속적으로 제거해주는 것이 전략 달성의 기준이 된다. 이는 곧 외벽냉각능력에 의해 결정되며, 만약 충분한 냉각이 수행되지 않을 시 압력용기는 외벽에서의 임계열유속 발생에 의해 파손될 수 있다. 따라서 압력용기 외벽에서의 임계열유속을 정의하는 것은 압력용기의 건전성을 유지하기 위한 관점에서 핵심 이슈이다. 일반적으로 임계열유속이 가열표면조건에 의해 지배적인 영향을 받는다는 것을 고려하면, 압력용기외벽냉각 수행 시의 가열표면조건을 정의하는 것은 임계열유속 발생 여부를 판가름하는데 있어 필수적인 요소이다. 이 때, 만약 압력용기외벽에 임계열유속 증진을 목적으로 나노유체나 표면코팅 등 특수한 처리를 하지 않는다면, 가열표면조건은 오직 자연산화에 의해 결정된다. 압력용기는 원자로 운영 기간 동안 교체되지 않고, 크롬함량이 매우 낮은 저탄소합금강(SA508)을 재료로 하기 때문에 자연산화에 의한 표면변화는 불가피하므로 자연산화에 의한 영향을 지배적일 것이다. 또한, 자연적으로 생성되는 산화막은 원자로 운전 시간에 따라 점점 두꺼워지므로 원자로 전 수명에 걸친 압력용기외벽에서의 임계열유속은 다르게 정의될 수 있다. 따라서 본 연구에서는 금속표면에 자연적으로 생성된 산화막이 임계열유속에 미치는 영향에 대해 평가하기 위해 대기압 조건에서 물을 작동유체로 하는 수조비등열전달실험을 수행하였다. 가열시편으로는 SS316과 SA508을 사용하였으며, 각 시편에 다양한 산화기간을 부여하여 각기 다른 산화상태를 구현하였다. 산화처리 후, 모든 산화시편은 산화기간에 무관하게 젖음성과 거칠기가 증가하였다. 산화기간은 산화표면 구조 및 산화막 두께에만 영향을 주었다. 실험결과로써 임계열유속은 산화막 두께에 지배적인 영향을 받았다. 즉, 산화막이 특정 두께를 형성하지 않고 국부적으로 형성되어 있을 경우, 임계열유속은 변하지 않았고, 산화막이 얇은 두께를 형성할 경우, 임계열유속은 지속적으로 증가됨을 보였다. 하지만 특정 두께 이상으로 산화막이 두꺼워질 경우, 임계열유속은 감소하는 경향을 보였다. 이는 산화막 내부에 존재하는 다공성 구조에 의한 모세관 현상이 두께에 따라 다르게 작용한다는 것을 의미한다. 즉, 산화막이 국부적으로 형성되어 있을 시에는 모세관 현상이 발생하지 않고, 얇은 산화막을 형성할 시에는 가열표면에서의 기화 현상을 억제하는데 기여를 할 수 있지만, 두꺼운 산화막이 형성되어 있을 때에는 다공성 구조 내부에서 기화 현상이 강하게 발현되기 때문에 더 이상 모세관 현상에 의한 냉각수 주입이 이루어지지 않았다. 오히려 과도한 기화로 인해 기포막이 조기에 형성되어 임계열유속이 낮아지는 경향을 보여주었다. 결과적으로, 산화초기에는 얇은 산화막이 형성에 따른 임계열유속 증진을 기대할 수 있지만 특정산화시점 이 후에는 산화에 따른 임계열유속이 오히려 감소하였다. 즉, OPR1000의 운전기간이 40년에서 60년까지 확장되고 있는 점을 감안하면 대부분의 운전기간 동안 압력용기외벽에서의 임계열유속은 깨끗한 표면상태보다 낮게 유지될 수 있다. 본 연구 결과는 자연산화가 압력용기외벽냉각에 악영향을 미칠 수 있음을 보여준다. 향 후에는 산화가 히터 크기에 미치는 영향을 검증하기 위해 압력용기 하반구에 대면적 산화를 수행하여 실증실험을 수행 할 필요가 있다. 또한 궁극적으로 자연산화에 의한 임계열유속 저하를 억제 또는 방지하기 위해 크롬코팅과 같이 부식억제성이 확보되는 표면코팅에 심도 있는 고려가 요구된다.; During the postulated severe accidents, the final goal of the In-Vessel Retention through External Reactor Vessel Cooling (IVR-ERVC) strategies is to terminate the accident without release of radioactive molten-corium to the environment and the public. To successfully fulfill the goal, effective removing decay heat generated from molten-corium is a criterion associated with the implied IVR-ERVC strategy. It is emphasized that the degree of success is determined by cooling capability on the reactor pressure vessel (RPV) outer surface. If insufficient cooling is provided, the RPV will be ruptured by the occurrence of Critical Heat Flux (CHF). Thus, defining the CHF on the RPV outer surface is a key issue in terms of securing the integrity of the RPV. Generally, the CHF is highly dependent of the heating surface conditions. Since the RPV is made of carbon steel, it continuously undergoes the significant corrosion even under the dry containment condition. Thus, it is essential to define the realistic surface condition in contingent with the initiation of the ERVC strategy and the CHF can be measured more realistically given such aged conditions. If any treatments such as nanofluids or surface coating to enhance the CHF are not applied, the heating surface condition will be determined only by natural corrosion. Since the RPV is irreplaceable during plant life time and is made of low carbon steel alloy, whose chromium concentration is very low, the effect of natural corrosion will be dominant. Therefore, in this study, pool boiling heat transfer experiment under the condition of atmospheric saturated water was conducted to evaluate pre-oxidation effect on the CHF. Test heater materials adopted are stainless steel grade 316 (SS316) and low carbon steel alloy (SA508). By treating different oxidation periods in each specimen, distinctive oxidation states were prepared and characterized. After oxidation treatment, wettability and roughness of all oxidized specimen increased regardless of the oxidation periods. Oxidation state was classified as local- and complete-oxidation on the basis of microstructure of the heater surface and oxide layer thickness. Accordingly, with the local oxidation, the CHF maintains the similar value compared to the bare heater surface. On the contrary, with complete oxidation with thin oxide layer, the CHF showed increasing tendency. However, it is of particular interest to note that when the oxide layer becomes too thick so that the thickness exceeds certain oxide layer thickness, the CHF decreased, which was opposite to the general belief. This suggests that capillary wicking in porous media acts in the opposite way with respect to oxide layer thickness. In other words, capillary wicking is inactive and thus not promoting the CHF on the heater surface with local oxidation, whereas existence of the thin oxide layer contributes on the delay of the vapor merge and coalesce by increasing the laterally assisted capillary. With the heater surface of thick oxide layer, however, since the evaporation works rapidly, it traps the escaping vapor and thereby reducing the liquid permeability. Therefore it showed rather a decreasing trend for the CHF due to early formation of vapor film. As a consequence, it is expected that the CHF is enhanced in the thin oxide layer, whereas the CHF decreases when the oxide layer thickness exceeds certain oxide layer thickness. Considering operation time of Korean Optimized Power Reactor 1000 (OPR1000) ranges 40 to 60 years, the RPV outer surface is expected significantly oxidized with time and thus the CHF on the RPV outer surface could be a concern depending on the oxide layer thickness. In conclusion, this study shows that natural corrosion can deteriorate the CHF limit in conducting IVR-ERVC depending on the oxide layer thickness. As a future work, large-scale oxidation experiments may be useful in understanding the size effect coupled with natural corrosion. In addition, to ultimately prevent the deterioration of CHF by natural corrosion, the engineered surface with corrosion-inhibitive coating by chromium will be considered.