Corrosion products are released from the materials of primary coolant systems in a nuclear power plant (NPP). As corrosion products are transported, activated and redeposited on the surface of the coolant systems, they have been a concern of safe operation in NPPs. This is related to axial offset anomaly, power reduction, acceleration of fuel cladding corrosion and radiation source term increase.
The objectives of this study are to simulate the corrosion products that deposit on the surface of the fuel cladding in operating NPPs and to investigate the effect of dissolved hydrogen on the deposition behavior of the corrosion products in high temperature primary water conditions. A high temperature and high pressure loop system was constructed to simulate the operating conditions around the fuel cladding including temperature (325℃), pressure (120.5 bar) and water chemistry. Deposition tests were divided into three different categories. First, deposit formation on the Zirlo cladding surface was carried out to evaluate the effect of the types of Ni and Fe ion source. Second, deposition tests were conducted to investigate the effect of the concentration ratios of Ni and Fe ions. Finally, the effect of dissolved hydrogen concentration on the deposition behavior was investigated in the hydrogen content ranges of 5~70cc(STP)/kg. Scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) were used for analysis on the morphology and chemical composition of the oxide deposits. The deposits formed on the Zirlo surface were removed by high power ultra-sonication in distilled water. The Fe and Ni concentration were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES), and then they were converted to the amount of oxide deposits per unit surface area.
Porous particulate deposits and needle-like deposits were formed in the condition of acetate form of Fe and Ni ions, which were analyzed to be Ni-rich oxides. Deposits with a polygonal structure were predominantly formed in the condition of EDTA form of Fe and Ni ions, which were identified to be nickel ferrite.
NiO deposits with a needle shape were formed in the condition of high Ni to Fe ion ratio (39:1), whereas iron oxides with a polygonal type were formed in the condition of low Ni to Fe ion ratio (1:39). In the case of the same ratio of Fe and Ni ions (20:20), nickel ferrite with a polygonal shape was formed. The amount of deposits increased with decreasing the Ni/Fe ion ratio. This means that Fe rich oxides stimulated Ni oxide deposition.
The deposition tests for the effect of dissolved hydrogen concentration were conducted in the condition of high Ni to Fe ion ratio (39:1), this is because needle shape deposits were formed in this condition, which were observed in the plants suffered the axial offset anomaly. Needle shaped NiO deposits were formed in the range of dissolved hydrogen contents of 5~25cc/kg, whereas polygonal nickel ferrite deposits were observed in the range of higher dissolved hydrogen contents of 35~70cc/kg. Therefore, to prevent formation of needle structures on the fuel cladding and mitigate the AOA, it is suggested that NPPs be operated at a hydrogen concentration above 25cc/kg. The effect of dissolved hydrogen concentration on the amount of deposits was minute in the range of dissolved hydrogen contents of 5~70cc/kg. This result means that operation strategies for mitigation of primary water stress corrosion cracking could be chosen at lower or higher hydrogen contents without a concern of increase in fuel crud deposition.