DEVELOPMENT AND PERFORMANCE OF Mo FUEL CLADDING UNDER LWR AND SEVERE LOSS OF COOLANT ACCIDENT CONDITIONS

摘要

Molybdenum alloy with an external coating has been proposed as a candidate fuel cladding to provide enhanced tolerance to severe loss of coolant accidents. Mo-alloy has been known to possess good tensile and creep strength at temperatures exceeding 1200°C, and some Mo-alloys also have adequate ductility at room temperature suitable for fabrication of thin-wall tubes. The proposed Mo alloy cladding for accident tolerant fuel (ATF) includes an external coating of either Zr-alloy or Al-containing stainless steel; both are anticipated to provide good corrosion resistance at light water reactor operation temperatures and form an in-situ oxide of ZrO_2 and Al_2O_3 to protect the Mo-alloy in a loss of coolant environments. The corrosion resistance of bare and coated Mo tubes was evaluated in high temperature water, simulating three different LWR water chemistry conditions; (1) BWR normal water chemistry (NWC) containing lppm O_2 at 290°C, (2) BWR hydrogen water chemistry (HWC) containing 0.3ppm H_2 at 290°C, and (3) PWR primary coolant containing 3.57ppm H_2 at 330°C. It is shown that Mo is more stable in high temperature water containing hydrogen, while a high susceptibility of pure Mo to corrosion is observed in the presence of oxygen due to the formation of soluble MO_3. Coated specimens show excellent corrosion resistance. The high temperature oxidation resistance of bare and coated Mo-alloy was also tested in 1000°C steam. Mo specimens coated with Zircaloy or Al-containing stainless steel (FeCrAl) have shown stability in 1000°C steam up to 4 days with the formation of protective ZrO_2 or Al_2O_3.