Yoshitaka Nishino

Initial oxidation of zirconium and Zircaloy-2 with oxygen and water vapor at room temperature

The initial oxidation of zirconium and Zircaloy-2 with oxygen and water vapor has been investigated at room temperature with Auger electron and X-ray photoelectron spectroscopies. Three suboxides of Zr2O, ZrO and Zr2O3 accompanied by the oxide of ZrO2 are formed on surfaces in both oxygen and water vapor atmospheres. Formation of ZrO2 starts at about 0.2–0.4 L (1 L ≡ 10−6 Torr s) oxygen exposure. The oxidation rate of zirconium with oxygen is slightly larger than that of Zircaloy-2. In the case of low pressure water vapor exposure (< 10 L), the amounts of suboxides on the zirconium are larger than on the Zircaloy-2, since dissociation of H2O molecules proceeds more easily on the former. The ZrO2 starts to form after a 1.0 L water vapor exposure, and its formation rate in water vapor ambience is much smaller than that in oxygen. An average composition of ZrO1.50–1.53 and film thickness of 2.3 nm are obtained for surfaces exposed to 30 L oxygen, and a 1.3 run thick film with composition Zr1.06 is obtained for 30 L of water vapor exposure.

Formation and dissolution of oxide film on zirconium alloys in 288°C pure water under γ-ray irradiation

Laboratory corrosion tests for zirconium alloys which were based on Zircaloy-2 were performed in 288°C oxygenated pure water for 100 days both with and without60Co γ-ray irradiation. No nodular oxide was observed. Corrosion weight gains of the alloy which had the lowest nodular corrosion resistance were lower for the irradiated condition than the non-irradiated condition. On the other hand, the alloys which had higher nodular corrosion resistances showed almost the same weight gains for both conditions. Differences of weight gain with and without irradiation were attributed to dissolution of the oxide film in the high temperature water. Dissolution tests of single crystal yttria-doped ZrO2 indicated that 30–40% larger amounts dissolved into water under the γ-ray irradiation. Low angle incident X-ray diffraction analysis showed that the tetragonal-ZrO2 fraction in the oxide film was lower with irradiation than without it, especially for the near surface area. The water radiolysis species accelerated the dissolution of the oxide film, especially film on the alloy with lower nodular corrosion resistance. This dissolution led to the lower tetragonal-ZrO2 fraction and was considered to be one of the factors causing a localized breakdown of the barrier oxide film to make the nodular oxide.

Hydrazine and Hydrogen Co-injection to Mitigate Stress Corrosion Cracking of Structural Materials in Boiling Water Reactors, (III): Effects of Adding Hydrazine on Zircaloy-2 Corrosion

The effects of hydrazine on the corrosion of Zircaloy-2 were examined in supercritical water. Hydrazine could be used as a reducing agent to control the corrosive environment for the coolant of boiling water reactors (BWRs). Before the corrosion test, the applicability of supercritical water for corrosion testing of zirconium alloys was studied. Supercritical water was found to be a useful solvent for testing corrosion based on the following facts: (1) the weight gain of Zircaloy-2 in supercritical water followed the same cubic law with the activation energy of 133 kJ/mol as that in water and steam did, and (2) the weight gain in supercritical water at 723 K and 24.5 MPa was more than 8 times greater than that in water at 561 K and 7.8 MPa depending on immersion time. The corrosion tests in supercritical water at 723 K and 24.5 MPa under γ-irradiation for 1,000 h were conducted to study the effects of adding nitrogen and ammonia on the corrosion of Zircaloy-2. Nitrogen and ammonia are decomposed products of hydrazine. The measured weight gain, oxide film thickness, and amount of hydrogen pick-up had slight differences between cases with and without the additives. Based on these data, it was concluded adding hydrazine to the coolant has little influence on the corrosion of Zircaloy-2 used in BWR cores.