In-situ measurement of the effect of LiOH on the stability of fuel cladding oxide film in simulated PWR primary water environment, Paper no. 400

The trends towards higher burnups, increasing lithium concentration and higher coolant temperature in the pressurized water reactors (PWR) impose a demand for better fuel cladding corrosion and hydriding properties. Development of new improved cladding materials is a long process. There is also a lack of reliable and fast in-situ techniques to investigate zirconium alloys in high temperature water environments. This paper describes the results of the Contact Electric Resistance (CER) technique which was used to measure the electric resistances of the oxides growing on zirconium based fuel cladding materials. Different types of zirconium alloys were studied in simulated PWR water. It was found that LiOH decreases the electric resistance of the oxides when LiOH is in excess of about 70 ppm in PWR water at 300 C. The electric resistance of the oxides is dependent on LiOH concentration and is shown to correlate inversely with the effect of LiOH on the weight gain. The kinetics of the decrease of the electric resistance indicate that the mechanism of degradation is a phase transformation rather than a diffusion limited process. The increase of the electric resistance of the oxide in the early stage of oxide formation is shown to correlate well with the in-reactor performance of similar alloys. The results of the in-situ monitoring of the electric resistances of the oxides give the same ranking for the corrosion resistances as the long term in-reactor weight gain tests of the similar alloys. The major benefit is that the results of the CER-measurements can be generated within a fraction of the testing time needed for weight gain tests.