| dc.description.abstract | The Fukushima Daiichi reactor accident showed that Zircaloy-4 fuel cladding tubes under accident conditions involving loss of cooling can lead to strong hydrogen generation and explosion arising from severe surface oxidation of the cladding with water steam. Among various approaches advanced to improve the accident tolerance of fuel pins, coating of the Zircaloy cladding surface is promising, especially if a thin layer of the chosen coating can be minimized to avoid significantly perturbing the neutron economy, thereby allowing the low neutron absorption benefit of Zircaloy-4 cladding to be retained.
One coating candidate, chromium, has good high temperature strength, hardness and wear resistance. Most importantly, chromium can form a thin layer of tenacious chromium oxide on the cladding surface for superior corrosion resistance, which makes it ideal as additional protection on Zircaloy cladding for enhanced accident tolerance under reactor off-normal conditions. Compared to other candidates, chromium has a relatively low neutron absorption cross section but its impact on the neutron economy cannot be completely ignored.
Radiation resistance of any coating deposited on fuel cladding must be as high as zirconium alloy underlayer. Neutron and heavy ion irradiation data as well as void swelling behavior analysis on pure chromium in a form of bulk material or coatings is very limited and still requires additional research efforts.
In the present study, pure chromium was irradiated with 5 MeV Fe2+ ions to 50 peak dpa at temperatures of 450, 500, 550, 600 and 650°C. Then irradiation at the peak swelling temperature of 550°C was conducted to 50, 100, 150 peak dpa. The maximum swelling temperature identified from the present study is much lower than the earlier studies. Swelling of pure chromium at 50 dpa was observed over the entire 450 and 650°C temperature range studied but appeared to be decreasing strongly at the temperature boundaries of the experiment. Chromium swells at a rate of ~0.03-0.04%/dpa, which is much lower than pure Fe. This low swelling rate was found to be relatively insensitive to dpa rate, which varied a factor of ~2 over the depth of data collection. Swelling begins quickly with essentially zero incubation dose but exhibits a much higher transient swelling rate before settling onto the post-transient value. Self-organization in the form of void ordering into was observed to be developing at 50 dpa, becoming better defined with increasing dose. The void alignment direction is determined to be [111] axial direction. | en |
