A study on the fracture pattern change of high-burnup fuel cladding failed by pellet-cladding mechanical interaction failure under reactivity-initiated accident conditions

ABSTRACT

In a part of the historical reactivity-initiated accident (RIA) simulated tests on high-burnup fuels performed at the Nuclear Safety Research Reactor, the fuel failure caused by pellet-cladding mechanical interaction (PCMI) led to splitting into upper- and lower-part pieces or even fragmentation of the cladding tube. A massive release of fuel fragments accompanied this fracture pattern change from previously known axial cracks and thus identified as a potential concern in safety evaluation regarding core coolability. Dedicated out-of-pile mechanical tests were performed with unirradiated Zircaloy-4 cladding specimens to clarify the condition of such fracture pattern change. The specimens were pre-hydrided and subjected to loading with axial-to-hoop strain ratios of ℇ_z/ℇ_θ = 0.5–1.25, simulating the effects of hydrogen embrittlement and pellet-cladding mechanical bonding of high-burnup fuels, respectively. The results indicate that higher biaxiality of the loading and lower ductility (failure strain level) assist the fracture pattern change. This study proposes a conservative criterion that a PCMI failure splits the cladding tube into more than two pieces when strain ratio ℇ_z/ℇ_θ >0.75 and a concurrent hoop strain < 1.7% at the failure instant.

KEYWORDS:

• RIA
• PCMI
• Zircaloy-4
• fracture pattern
• strain ratio
• zirconium hydride
• biaxial stress
• fuel dispersal

Acknowledgments

This study was conducted as a program sponsored and organized by the Nuclear Regulation Authority, Japan. The authors want to thank Mr. Fujisaku for his assistance in conducting the biaxial stress tests.

Disclosure statement

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