Reload core designs for a 38.8-effective-full-power-month (EFPM) pressurized water reactor (PWR) cycle and a 45-EFPM boiling water reactor (BWR) cycle were developed to offer nuclear utilities the opportunity for economic benefit by permitting higher plant capacity factors and by reducing the required number of costly refueling operations. A key constraint on this work was the requirement to stay within current fuel burnup licensing limits. The designs use a single-batch reloading strategy and contain fuel with enrichments as high as 7.4 wt% 235U (exceeding the current licensing limit of 5 wt%). The PWR design uses Gd2O3 and an integral fuel burnable absorber as burnable poisons to hold down excess reactivity and control power peaking. The BWR employs only Gd2O3. Both core designs require higher-worth control rods to meet shutdown safety requirements.

Fuel performance issues were also investigated. The presence of high-burnup fuel assemblies at greater than core-average power leads to fuel performance concerns over the effects of waterside corrosion and increased fission gas pressure. Steady-state analyses of fuel pin internal pressure showed acceptable fuel pin performance. Fuel performance areas requiring further research were highlighted.

Extended-cycle cores have a fuel cost that is approximately $33 million/yr (or ~60%) more expensive than an optimized multibatch strategy. An economic analysis of these cores showed that extended cycles do not offer a significant economic benefit over conventional practice. Possible future scenarios that could make the subject loadings economically viable are a drop in separative work unit costs or a significant increase in the price of replacement electricity during shutdown.