Improvements in reactor performance can be gained by improving the fuel—for example, by increasing the maximum utilization of the reactor fuel’s fissionable content (its “burn-up”) or by using fuel geometries with greater efficiencies.
The development of significantly higher burn-up fuel for LWRs could allow operating cycles to be prolonged; it could also allow the long-term heat load of the used nuclear fuel and the total amount to be stored or disposed of to be reduced.13 R&D to increase fuel burn-up would focus on the materials issues associated with fuel integrity under long-term exposure to ionizing radiation as well as mechanical design issues which limit fuel lifetimes. For example, one issue requiring R&D is swelling of the higher burn-up fuel rods due to build-up of fission products, and the resulting risk of cladding breach. The development of higher burn-up fuel is a program of continuous improvement, but in order for significant breakthroughs to occur, considerable basic research, long-term irradiation of samples, and a sustained fuel qualification campaign are needed.
In current LWRs, the fuel rods have a cylindrical geometry in which coolant flows around the outside of the rods. An annular shape would increase the surface area of the rod in contact with the coolant by 60 percent, because coolant would flow through the center of the rod as well as along the outside surface. This would allow the coolant to be heated more efficiently, and fewer fuel rods could produce more power. Studies suggest that new plants may be able to achieve up to 50 percent more core power by using annular rather than cylindrical fuel rods (Kazimi et al., 2005). Annular fuel rods could be used in the current fleet of LWRs. Some modifications will be needed in these plants (for example, larger reactor coolant pumps, a larger pressurizer, and additional or greater capacity high pressure injection); however, the same containment, reactor vessel, and the majority of current equipment and piping could be used. There is interest in commercializing this technology (Westinghouse, 2006), but commercial-scale deployment in existing LWRs is unlikely to occur before 2020.
Many experts now believe that it is possible that both existing and advanced plants might be able to run for extended periods, perhaps as long as 80 years.