In order to economically increase cycle lengths of pressurized water reactor, current regulatory burnup limit of 62 GWd/tU is expected to extend to ∼75 GWd/tU. According to the experimental observation in the Halden IFA-650 tests, high burnup fuel seems more susceptible to turn into fine fragments (pulvers) in loss-of-coolant accidents. Due to the cladding ballooning under temperature transients, the fragmented fuel can axially relocate into the ballooned region, which may lead to a local increase in cladding temperature and oxidation level, raising the licensing concerns related to core coolability as described in 10 CFR 50.46. Although fuel relocation has been extensively observed and well identified in various test programs, there is still data gap between experimental observation and numerical prediction. This paper proposes a three-dimensional simulation framework with discrete element method for fuel relocation, in which coarse fuel fragments are modelled with Voronoi cells and the high burnup structure is established through small isolated particles. Five tests conducted in the FR2 program were chosen to validate the simulation results, in which the predicted missing fuel lengths fitted well with the experimental observation, indicating the high fidelity of the simulation framework. Based on the validated simulation results, the mass fraction and filling ratio of fuel fragments along the axial direction were discussed to investigate the effects of cladding hoop strain on fuel relocation. Finally, a correlation of maximum mass fraction of fuel fragments with respect to cladding hoop strain was proposed to provide an implication to high burnup fuel performance evaluation under loss-of-coolant accidents for the future work.

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