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田崎 雄大; 成川 隆文; 宇田川 豊
Journal of Nuclear Science and Technology, 61(10), p.1349 - 1359, 2024/10
被引用回数：0 パーセンタイル：0.00(Nuclear Science & Technology)This study developed a probabilistic determination model with respect to cladding high-temperature burst conditions based on the Bayesian statistical method to reasonably evaluate fuel behaviors under loss-of-coolant accident conditions, including fuel fragmentation, relocation, and dispersal. The candidate models were based on the widely accepted empirical model established based on nonirradiated fuel cladding data. Explanatory variables were added to improve the applicability of these models with respect to irradiated materials and generalization performance. The posterior predictive distribution of each candidate model was evaluated using Bayesian estimation comprising 238 sets of high-temperature burst test data. The generalization performance was evaluated using information criteria. The results of model evaluation showed improved predictive performance by considering the effect of hydrogen content. A comparison with burnup as an alternative explanatory variable confirmed that hydrogen content was the better parameter and other burnup-associated effects, such as irradiation hardening of the metal matrix and oxide growth (reduction of the metal matrix), were less dominant under burst conditions.
成川 隆文; 宇田川 豊
Proceedings of TopFuel 2021 (Internet), 10 Pages, 2021/10
To clarify the mechanism and temperature threshold for fuel fragmentation during loss-of-coolant accidents (LOCAs), out-of-pile heating tests on bare fuel pellet pieces taken from a high-burnup PWR UO
fuel rod (segment average burnup: 81 GWd/tU) were performed. The fuel pellet pieces taken from various regions in the radial direction of the fuel pellet were inductively heated with no cladding restraint in vacuum up to 1473 K at a rate of 5 K/s. During the heating tests, the fission gases released from the fuel pellet pieces were continuously analyzed in-situ using a quadrupole mass spectrometer. Following the heating tests, microstructural observation of the fuel pellet fragments was carried out. Based on the relationship between the extent of fuel fragmentation and the terminal temperature, and the time history of fission gas release, temperature thresholds for minor fuel fragmentation and slightly more fuel fragmentation were estimated to be 973 - 1073 K and 1173 - 1273 K, respectively. The extent of fuel fragmentation and the amount of fission gas release became more pronounced with increasing temperature. Further, the microstructural observations after the heating tests revealed that most of the fuel fragments smaller than approximately 500 - 750 
m have microstructures consisting of many micropores and subgrains, which are characteristic of the dark zone or high-burnup structure. On the basis of these results, the mechanism of fuel fragmentation during LOCAs was discussed.
成川 隆文
no journal, ,
JAEA has performed a wide range of loss-of-coolant-accident (LOCA) studies in response to fuel-burnup extension, new experimental results, and the accident at NPPs. Currently, new LOCA tests are ongoing in JAEA to evaluate fuel behavior under LOCA conditions including the phenomena of fuel fragmentation, relocation, and dispersal (FFRD) and the mechanical strength of fuel cladding tubes during the post-LOCA long-term core cooling period. These test results, including those obtained from the future study, are expected to provide the necessary information for the future regulation on high-burnup fuels.
成川 隆文
no journal, ,
Currently, Japan Atomic Energy Agency (JAEA) is planning and conducting new loss-of-coolant-accident (LOCA) tests to evaluate the behavior of high burnup fuels under LOCA conditions including fuel fragmentation, relocation and dispersion (FFRD) phenomena. The current status and plan of these tests is presented.
