A good summary of future fuel cycle issues and reactor strategies over the next few decades is given in Meneley (1998). This report considers short-, medium — and long-term time frames extending out for the next few decades. Clearly the choice of reactor and fuel cycle are inextricably linked. For example, the most widely operating reactor type is likely to be thermal reactors burning mixed uranium and plutonium fuel. As discussed earlier, the fast reactor could be operated as a stand-alone technology or in combination with thermal reactors. There is then the possibility of the thorium fuel cycle.

The largest change is the introduction of MOX fuel in LWRs and HWRs. PWRs are already being loaded with up to 30% MOX fuel. Higher percentage MOX fuel loadings are being considered but further technical work is required to establish whether fission gas release at high burn-up is a concern. There is also the question of high burn-up fuel under accident conditions. The capability of multiple recycle is also not assured; it may be that MOX fuel is limited to two or three cycles. MOX fuels are feasible for up to 100% loading in HWRs.

Further development and proof testing of fuel elements, either of MOX or uranium fuel, will be necessary for fuels capable of utilisation to higher burn-up. This will mean higher fresh fuel enrichment. It is expected that there will be a continuous drive towards higher burn-up because of the improved economics, certainly for batch rods.

For HWRs, the life of fuel can be greatly increased by a small amount of enrichment. Natural uranium imposes an inherent limit on fuel life. This enrichment leads to more flexibility in design and fuel management. RU can also be used in HWR since the U-235 content of uranium remaining after plutonium extraction is about 0.9%. A sequential once — through cycle in two different reactor types is under construction called ‘ double-burning’. The idea is to use discharged fuel from the first cycle for the second cycle without re­enrichment. Another cycle is the ‘DUPIC’ cycle, which aims to reform LWR pellets into HWR pellets.

In the short term over the next 15-20 years there will be an opportunity to conduct small-scale fuel development experiments, before prototyping in large-scale experiments in the medium term. It is likely that uranium-based fuel will take precedence over thorium — based technologies but there is the possibility for more consideration to be given to the latter. In the longer term, it is possible that recycling will be a more routine practice. Either the FBR or accelerator breeding could be used to convert fertile material to fissile material in large quantities. Thorium would have the advantage over uranium of a very high conversion ratio.

Future work programmes could, therefore, focus on increasing the reactor conversion ratio resulting in higher burn-up for a given enrichment, and reducing the need for burnable poisons. This could be achieved either through a thorium cycle in thermal reactors or FBRs utilising metal uranium-plutonium fuel. Other research will target increased fuel burn-up, and reduction of reprocessing costs. Finally, on-line fuelling carries with it none of the disadvantages of periodic shut-down of batch fuelling. Flexibility is much increased and parasitic neutron absorption is reduced for fuelling at full power.