The SWELL fuel element lifetime code employs the “cumulative damage” approach to estimate when the cladding of an LMFBR mixed-oxide fuel element is likely to fail under normal and off-normal conditions. In the estimating process, properties and behavior of irradiated, as well as of unirradiated, cladding are considered. A unique feature of SWELL is its use of an empirical function, developed by calibration with experimental data, which relates the pressure-exerted-on-cladding-by-fuel-swelling to the pressure-of-fission-gas-retained-in-fuel. SWELL predicts that the lifetimes of some typical fuel elements will have to be reduced significantly if the elements are to be expected to withstand rather modest off-normal conditions near end of life. However, there are reasons why the predictions may be overly pessimistic. Early results from the newer and more-detailed LIFE fuel element behavior code indicate that the accuracy with which a fuel element's operating history is followed may be important for gaining understanding of the element's behavior. To conserve computer time, the best way to simulate actual history, as jar as cladding ΔD/D predictions are concerned, appears to be to use time-averaged power (excluding downtime) for a length of time sufficient to produce the actual burnup, which happens to be the SWELL practice.