Study on fracture behavior of Cr-coated Zr-based alloy claddings under high-temperature LOCA conditions

Luu, V. N. ; 谷口 良徳 ; Nguyen, T. H.; 宇田川 豊  ; 田崎 雄大 ; 三原 武 ; 勝山 仁哉  ; 垣内 一雄 

Luu, V. N.; Taniguchi, Yoshinori; Nguyen, T. H.; Udagawa, Yutaka; Tasaki, Yudai; Mihara, Takeshi; Katsuyama, Jinya; Kakiuchi, Kazuo

To obtain the knowledge and data necessary for the regulatory assessment of Cr coated Zr-based alloy claddings, which is expected to be installed to Japanese plants in the near future, integrated LOCA tests were conducted using 190mm-long uncoated and Cr-coated Zr-based alloy claddings filled with zirconia pellets to simulate the thermal behavior of UO pellets. The test rod was initially pressurized to 5 MPa and heated in 100% steam atmosphere at a heating rate of 5 K/s until burst. This was followed by isothermal oxidation at temperatures between 1200 and 1350C and quenching at 700C under an axial constraint of 500N. The test temperature covered conditions both below and above the Zr/Cr eutectic temperature (1320C) to establish fracture limits under LOCA conditions. Post-test observations showed that fractures into two segments occurred only at the burst locations due to circumferential cracking. Below the eutectic temperature, the fracture limit exceeded 28% BJ ECR (Equivalent Cladding Reacted calculated using the Baker-Just correlation) for both uncoated and Cr-coated claddings. Above the eutectic temperature at 1350C, the fracture limit remained unchanged for Cr-coated claddings but decreased to 20% BJ ECR for uncoated claddings. This reduction is attributed to increased hydrogen uptake (up to 500 wtppm), as confirmed by hydrogen analysis at the burst locations. The similar fracture limits of Cr-coated claddings below and above the eutectic temperature, further supported by metallographic observations and Vickers hardness measurements, suggest that the eutectic reaction did not appear to impact the fracture limit of Cr-coated claddings.