Axial ratchetting of fuel under pressure cycling conditions

The various additions to the CYGRO fuel-rodanalysis technique in order to calculate ratchetting effects are described. These effects include fuel cracking, clad collapse, friction between fuel and clad, clad anisotropy, and effects of neutron flux on clad creep. By reasonable choice of parameters, good agreement can be obtained with tests on axial elongations of non-freestanding fuel rods. There is a pronounced sensitivity of these predictions to the value of creep enhancement as a result of neutron flux. Predictions of diameter changes are believed to be inherently less accurate because of the masking effects of ridging, wrinkling, and clad collapse.