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Fig. 3-1 Process of mechanical energy generation during high burnup fuel failure under RIA conditions High burnup fuel; i.e., fuel that has been used for a long period, contains large amounts of fission product (FP) gases at the fuel grain boundaries. The rapid fuel temperature rise during an accident causes thermal expansion of the FP gases, leading to fuel fragmentation. The enlarged fuel surface area resulting from this fragmentation brings about mechanical energy generation through thermal interaction between the fuel fragments and coolant.

Fig. 3-2 Relation between fuel fragment size and mechanical energy conversion ratio Test results show that the thermal-to-mechanical energy conversion ratio is higher for the fuel fragments with a larger surface area per unit mass (specific surface area), i.e., the finer fuel fragments. Test results for UO2 powder in a polyethylene bag (black circles in the figure) provided a tendency and conversion ratios similar to those for high burnup fuel rods (red squares), suggesting that the processes of mechanical energy generation are common in the two test series.

Extending the utilization period of reactor fuel, designated "burnup extension," has been promoted worldwide for efficient use of uranium resources, reduction of fuel cycle cost and other reasons. Burnup extension, however, may degrade fuel integrity due to cladding corrosion and undesirable changes in fuel states. Thus, the safety of high burnup fuel must be confirmed under accident conditions as well as in normal operations. To evaluate the influence of fuel failure on reactor safety, experiments simulating Reactivity Initiated Accidents (RIAs) are being performed in the Reactor (NSRR). In addition to high burnup fuel experiments, separate-effect tests were performed with unirradiated uranium dioxide (UO2) powder that simulated fuel fragment dispersion into the coolant, which is expected to occur in failures of high burnup fuel (Fig. 3-1). The powder fuel tests showed that energetic steam generation could arise from fuel/coolant thermal interaction, when fuel fragments having a large surface area were quickly dispersed into the coolant. Further, the conversion ratio from thermal to mechanical energy in this process depends on the fuel fragment sizes (Fig. 3-2). A similar tendency and conversion ratios were obtained from high burnup fuel tests. Consequently, the mechanical energy generated during high burnup fuel failure under RIA conditions was confirmed to be caused by the fuel-fragment/coolant thermal interaction, not by the ejection of fission product gases from the fuel rods.

Reference T. Sugiyama, Studies on Clad/Coolant and Fragmented-Pellet/Coolant Heat Transfer in High Burnup Fuel Behavior during RIA, JAERI-Review 2004-021, 59 (2004).

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