The effect of spacer grids on the stress applied to a post-LOCA cladding tube under horizontal vibrations

ABSTRACT

There is a recognized need to discuss the preservation of the coolable geometry of fuel rods during long-term cooling after a loss-of-coolant accident (LOCA). Mechanical load induced by an earthquake may be a primary cause of loss of the coolable geometry after a LOCA. In this research, aiming to confirm the preservation of coolable geometry of fuel rods during an earthquake after a LOCA, we investigated the stress applied to the cladding tube with a rupture opening under a vibration condition via finite element analyses for a fuel assembly that considered the mechanical interaction between a fuel rod and spacer grids. The analyses demonstrated that the stress on the cladding tube increased with the axial holding force of the spacer grid on the fuel rod. The stress concentrating at the periphery of the rupture opening was compared with the strength of the post-LOCA cladding tubes determined via bending tests. This research concludes that the fuel rods are likely to be prevented from fracture due to bending arising from an earthquake during post-LOCA cooling unless the oxidation of the cladding tubes exceeds 15% equivalent clad reacted.

KEYWORDS:

• LOCA
• post-LOCA
• earthquake
• finite element analysis
• fuel rod
• fuel cladding
• spacer grid
• rupture
• stress

Acknowledgments

The authors are grateful to the staff of Hitachi Power Solutions Co., Ltd. for their considerable support in the finite element method analyses.

Disclosure statement

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