Study on the relationship between fuel fragmentation during a LOCA and pellet microstructure

ABSTRACT

To systematically examine the relationship between the fuel fragmentation during a loss-of-coolant accident (LOCA) and the pellet microstructure, heating tests were conducted on micro fuel samples obtained from various radial positions of high-burnup fuel. The extent of the fuel fragmentation was evaluated via fission gas release rate and post-test microscopies. The results provide direct evidence for the contribution of the high-burnup structure (HBS) on the fine fuel fragmentation during a LOCA. The paper also demonstrates that the samples with a local burnup of lower than 72 GWd/tU did not fragment severely, which is in good agreement with the previously proposed burnup threshold. Heating tests incorporating stepwise depressurization from high pressure were also conducted to examine the evolution of fission gas release throughout the depressurization. The suppressing effect of hydrostatic pressure on fuel fragmentation is further confirmed. Additionally, the paper also provides results that suggest that most of the fine fragmentation at the HBS occurs during depressurization from 20 to 2 MPa.

KEYWORDS:

• LOCA
• FFRD
• fine fuel fragmentation
• high-burnup structure
• heating test
• depressurization
• fission gas release

Acknowledgments

The author gratefully acknowledges M. Aomi at Global Nuclear Fuel Japan and F. Mizusako and A. Suzuki at Nippon Nuclear Fuel Development for performing the works that are indispensable for this study. Furthermore, gratitude is expressed to F. Nagase at JAEA and experts in S/NRA/R, particularly K. Kitano and H. Akiyama for their advice and fruitful discussions.

Disclosure of statement

No potential conflict of interest was reported by the author(s).