Study on secondary hydriding effects during simulated LOCA tests on the embrittlement and fracture behaviors of Cr-coated Zr-based alloy cladding

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

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

The proposed use of chromium (Cr)-coated zirconium (Zr)-based alloy cladding as an accident-tolerant fuel (ATF) highlights the importance of understanding its mechanical response under loss-of-coolant-accident (LOCA) conditions. In anticipation of the deployment of Cr-coated cladding, it is necessary to establish appropriate LOCA safety criteria and verify existing fuel performance codes. Under LOCA scenarios, fracture may occur either at the locations with oxidation or secondary hydriding. Nagase and Fuketa evaluated fracture maps at the rupture site as a function of the initial hydrogen content, however, no correlation was reported between fracture limits and post-LOCA hydrogen content. Alakiozidis at al. reported a hydrogen threshold of approximately 2700 ppm for out of burst failure in both coated and uncoated claddings; however, experimental data for coated claddings remains limited. This study aims to evaluate the effects of hydrogen on cladding embrittlement and fracture characteristics of Cr-coated cladding. To this aim, oxidation tests were conducted on Cr-coated claddings with controlled pre-hydrogen absorption to simulate low hydrogen concentrations, and integral LOCA tests were performed on both Cr-coated and uncoated claddings to evaluate secondary hydriding effects. For both tests, cladding tube samples were two sided steam oxidized to achieve equivalent cladding reacted (ECR) values ranging from 10 to 40%, corresponding to oxidation temperatures of 1200 and 1350C for various exposure times in a vertical furnace, followed by water quenching at 700C. Post-test examination includes the hydrogen measurement, microstructure observation by optical microscopy (OM) and scanning electro-microscopy (SEM), Vickers hardness test and nanoindentation. According to the results obtained, hydrogen promotes embrittlement of prior -Zr after exposure at 1200C, while having no significant effect after exposure at 1350C. The eutectic layer forms after exposure of Cr-coated cladding to 1350C. This region consists of two distinct phases: a Cr-rich and a prior -Zr phase. Cr-rich phase exhibits higher nano-hardness than that of prior -Zr phase and hydrogen absorption enhances the hardness of both phases. The fracture threshold of Cr-coated cladding is determined by different controlling mechanisms at burst and out-of-burst positions. At the burst, fracture limits were found to be decreased with increasing hydrogen content (up to approximately 500 ppm), and the Cr-coated cladding fractured at longer oxidation time than uncoated cladding. At out-of-burst positions, no fracture observed up to 2700 ppm hydrogen, in agreement with Alakiozidis et al. and fracture limit was determined based on the remaining prior -Zr thickness at oxygen concentrations 0.9 wt% by Chung [3]. These findings indicate that the Cr-coated cladding exhibits hydrogen-related embrittlement behavior similar to conventional Zr alloys, with the additional advantage of reduced oxidation kinetics, which mitigates oxidation-driven embrittlement.