A Finite element analysis study on the fracture pattern change of fuel cladding under PCMI loading conditions

Li, F.  ; Mihara, Takeshi  ; Udagawa, Yutaka  ; Katsuyama, Jinya  

To better understand the failure behavior of high-burnup light water reactor fuel during the pellet-cladding mechanical interaction (PCMI) phase of a reactivity-initiated accident (RIA), biaxial tension tests of the cladding were performed in a previous study. These tests suggested that the degradation of ductility and increased strain biaxiality in the fuel cladding contribute to the fracture pattern transition from slit-like breaking to guillotine breaking, as observed in some historical RIA-simulated tests at the Nuclear Safety Research Reactor (NSRR) of the Japan Atomic Energy Agency (JAEA). To validate the applicability of the conclusion from the above mechanical tests in explaining the phenomena observed in RIA-simulated tests, this study employed two finite element models simulating PCMI loading conditions under different approaches, incorporating a ductile failure criterion for the fuel cladding. The models were used to examine the relationship between fracture patterns and key testing parameters, including the axial-to-hoop strain ratio of the cladding and the ductile failure criterion. The results indicated that as the axial-to-hoop strain ratio increased, the fracture pattern tended to change from a slit-like axial crack to cracks turning toward the hoop direction, ultimately leading to guillotine breaking. Under less ductile conditions, cracks tended to propagate at a tilted angle between the axial and hoop directions. This observation verified the importance of the strain ratio and the embrittlement status of the cladding in its fracture pattern, which provides an important basis for revising methods used to estimate fuel release during RIAs, particularly when accounting for high-burnup fuels.