Irradiated Fuel Behavior under Power Oscillation Conditions

In order to examine high burnup fuel performance under power oscillation conditions, two tests of irradiated fuels under simulated power oscillation conditions were conducted in the Nuclear Safety Research Reactor (NSRR). Irradiated fuels at burnups of 56 and 25 GWd/tU were subjected to four to seven power oscillations, which peaked at 50 to 95 kW/m with intervals of 2 s. The power oscillations were caused by quick withdrawal and insertion of six regulating rods of the NSRR with a computerized control. Impacts of cyclic loads on the fuel performance under hypothetical unstable power oscillations arising during an anticipated transient without scram (ATWS) in boiling water reactors (BWRs) were examined in the tests. Deformation of the fuel cladding of the test rods was comparable to those observed in shorter transient tests, which simulated reactivity-initiated accidents (RIAs), at the same fuel enthalpy level up to 368 J/g. The fuel deformation was mainly caused by pellet-cladding mechanical interaction (PCMI) and was roughly proportional to the fuel enthalpy. Enhanced cladding deformation due to ratcheting by the cyclic load was not observed. Fission gas release, on the other hand, was considerably smaller than in the RIA tests, suggesting different release mechanisms in the two types of transients.