가압경수로 1차계통 환경에서 pHT가 크러드 부착 피복관의 부식가속화에 미치는 영향 = The Effect of pHT on Corrosion Acceleration of Crud-deposited Fuel Cladding in Primary Coolant Condition of Pressurized Water Reactor

다국어 초록 (Multilingual Abstract)

Fuel cladding materials in pressurized water reactor(PWR) are mostly made of zirconium alloys. To ensure the fuel integrity, the oxide thickness of fuel cladding tube should be generally no thicker than 100 ㎛ during three-cycle operations. Many PWR nuclear power plants have been recently employing the economic operating strategies such as power uprate, high burnup and long-term operation. These can lead to accelerating the oxidation of fuel cladding and the deposit of corrosion product called as ‘crud’. Crud is known to be mainly composed of oxides such as nickel ferrite (NixFe3-xO4), nickel oxide (NiO), zirconium oxide (ZrO2), bonaccordite (Ni2FeBO5), and magnetite (Fe3O4), etc. The morphology of crud has been found to be polyhedral and needle-like shapes, and mostly porous structure due to the transport of sub-cooled nucleate boiling (SNB) steam. Meanwhile, heavy deposition of crud has often caused various problems by threatening the fuel integrity such as crud-induced power shift (CIPS) or crud-induced localized corrosion (CILC). If the crud is thick enough when heat flux level is high, it leads to a degradation in heat transfer of fuel rod surface and dryout. As the cladding temperature increases above saturation temperature of coolant due to crud dryout, cladding corrosion at local region (CILC) is accelerated. However, the investigation of fuel cladding corrosion has not been actively reported due to high radioactivity and experimental difficulties. In this work, the thick porous crud was produced using chemistry-controlled recirculation loop and the effect of pHT on corrosion acceleration of fuel clad was investigated through the corrosion test under heat flux condition of nuclear fuel. The oxidation layer of cladding and fuel crud were analyzed using scanning electron microscope and X-ray diffraction at different pHT conditions. The effect of pHT on corrosion rate of crud-deposited cladding was found to be not significant.