Multilayer coatings to enhance the performance of Zry cladding as advanced fuel cladding

Pham, V. H.   ; Nemoto, Yoshiyuki  ; Yamashita, Shinichiro   

Various coatings ranging from ceramics to metals and alloys have been proposed to enhance the performance of Zry as accident tolerant fuel cladding. Ceramic coatings that formed AlO or SiO tend to show better steam oxidation resistance, but exhibit poor performance in long-term corrosion resistance due to their dissolutions of cations in aqueous environments. In the meantime, metals and alloys coatings seemed to have improvements in steam oxidation resistance as well as corrosion resistance in water. Nevertheless, the interdiffusion and eutectic reactions of the metallic coatings and Zry substrate at high temperatures may become a serious concern in the event of severe accident conditions. Multilayer coatings on Zry could be a good solution to achieve maximum benefits of each individual coating layer. In attempt to search for alternative coating solutions to improve the performance of Zry4 cladding, in this study, multi-coating of TiN and TiAlN layers on Zry4 was proposed and fabricated by physical vapor deposition coating technique. After the coating works, oxidation tests of coated Zry4 and uncoated-Zry4 in steam via a thermogravimetric system were conducted at 800-1000 C to evaluate the protection effectiveness of the coating layers. Post-test analysis of the oxidized samples was carried out with X-ray diffraction and scanning electron microscope. The results of this investigation showed that oxidation kinetics of TiN/TiAlN coated Zry4 followed linear laws. The coating layers only showed some positive effects on oxidation resistance in steam at 800C. At higher temperatures, the samples with coating layers even exhibited faster oxidation rate than that of the uncoated-Zry4. Horizontal cracking at ZrO zone due to the formation of ZrN is attributed to the main cause of this degradation. Multilayer coating on Zry4 remained under discussion. However, nitrides coatings on Zry4 therefore should be avoid for this application.