Irradiation tests of a BWR advanced Zr alloy (HiFi alloy) and Zircaloy-2 (Zry-2) were carried out in a Japanese commercial reactor and the irradiation performances of the materials were investigated. HiFi alloy and Zry-2 showed excellent resistance to corrosion up to 70 GWd/t, and furthermore, HiFi kept lower hydrogen pickup compared with Zry-2. TEM observation showed that the Fe/(Fe+Cr) ratio of Zr(Fe,Cr) 2 type second phase particles (SPPs) for HiFi alloy and Zry-2 tended to decrease as fast neutron fluence increased and to saturate at high fluence. Zr-Fe-Cr SPPs did not completely disappear even for 6 cycles for the irradiated HiFi alloy and Zry-2. In order to clarify the mechanism of hydrogen absorption, an electrochemical technique was used for the oxide film of both materials as part of the out-ofpile test. The relation between the oxide surface potential and the hydrogen pickup fraction was estimated suggesting that the potential difference over the oxide film suppressed hydrogen (proton) diffusion in the oxide film.

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Irradiation tests of a BWR advanced Zr alloy (HiFi alloy) and Zircaloy-2 (Zry-2) were carried out in a Japanese commercial reactor and the irradiation performances of the materials were investigated. HiFi alloy and Zry-2 showed excellent resistance to corrosion up to 70 GWd/t, and furthermore, HiFi kept lower hydrogen pickup compared with Zry-2. TEM observation showed that the Fe/(Fe+Cr) ratio of Zr(Fe,Cr) 2 type second phase particles (SPPs) for HiFi alloy and Zry-2 tended to decrease as fast neutron fluence increased and to saturate at high fluence. Zr-Fe-Cr SPPs did not completely disappear even for 6 cycles for the irradiated HiFi alloy and Zry-2. In order to clarify the mechanism of hydrogen absorption, an electrochemical technique was used for the oxide film of both materials as part of the out-ofpile test. The relation between the oxide surface potential and the hydrogen pickup fraction was estimated suggesting that the potential difference over the oxide film suppressed hydrogen (proton) diffusion in the oxide film.

The latest PIE results of Zry-2 and HiFi alloy (0.4 %Fe-Zry2) showed that iron addition reduces the hydrogen pick-up ratio. In order to clarify this lower hydrogen absorption mechanism, three types of experiments were carried out for both alloys: (1) Measurements of the hydrogen pick-up ratio in the pre-transition region using an autoclave. (2) CPD (Contact Potential Difference) and FBP (Flat-band Potential) measurements of oxide film using a High Temperature Kelvin system and Photocurrent system. (3) Investigation of hydrogen absorption properties by Sieverts system and corrosion properties by autoclave for intermetallic compounds simulating SPP's Fe/Cr and Fe/Ni ratio in Zry-2 and HiFi. Autoclave tests showed that the corrosion behaviors of both alloys were similar. However, the hydrogen pick-up ratio depended on not only iron addition but also surface treatment (with and without pickling). Iron addition and non-pickling (as-received) reduced the hydrogen pick-up ratio. ΔCPD and FBP shifted to the positive side as the result of iron addition, and FBP changed with and without pickling. This difference in surface potential might be caused by the existence of anion vacancy concentration in the oxide film and furthermore may affect the electrochemical potential gradient over the oxide film. This potential gradient might control the proton diffusion in the oxide film. In the results of the hydrogen absorption tests on intermetallic compounds, Zr(Fe,Cr)2 with a higher Fe/Cr ratio showed lower hydrogen absorption, and Zr2(Fe,Ni) showed higher hydrogen absorption than Zr(Fe,Cr)2. In the results of corrosion tests on intermetallic compounds, Zr2(Fe,Ni) showed a much larger corrosion rate than Zr(Fe,Cr)2. It is considered that Zr2(Fe,Ni)-type precipitate might be oxidized easily in the oxide film, thus it might not contribute to the window for hydrogen absorption, and Zr(Fe,Cr)2 with a higher Fe/Cr ratio would reduce the amount of hydrogen penetrating through SPP at the oxide/metal interface. Based on these results, a tentative hydrogen absorption mechanism is proposed, explaining both roles of the electrochemical potential gradient over oxide film and the SPP window for the hydrogen absorption.