Material corrosion tests in the coolant with zinc addition; Confirmation of material integrity for zinc addition at ATR "Fugen"

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Zinc addition to the coolant of the reactor core of ATR "Fugen" is being planned for the purpose of reducing the radiation exposures. In order to confirm the integrity of the materials used in the reactor coolant system, a matrerial corrosion test with zinc addition (10 ppb) and that without zinc addition were performed for 2,000 h under the condition simulating the reactor core of "Fugen". Main results obtained from these tests are as follows : (1)For Zr-2.5%Nb alloy used for the pressure tubes, weight gain during the corrosion test tended to be large by zinc addition. It was concluded that this weight gain would not affect the integrity the pressure tube, because the reduction of the thickness of the base metal was estimated around 1 m during the corrosion test (extrapolated value for 100,000 h was about 10m). The hydrogen absorption during the test was not observed. (2)Some specimens of stainless steels showed weight loss caused by metal release from corroded surface without zinc addition. This weight loss is clearly observed for less corrosion resistance steels. Since all of the specimens tested with zinc addition showed slight gain of the weight, it is confirmed that zinc addition suppressed the corrosion of stainless steels. (3)From surface analyses after the corrosion test, it was clarified that zinc was concentrated in the surface oxide film, and that the morphology was changed. It was considered that the difference of weight changes between the specimens corroded with zinc addition and that without zinc addition was attributed to the difference of the composition and the morphology of the surface oxide films.