GIF has proposed an integral fuel cycle, with the reprocessed spent fuel from LWRs forming part of the feed for fast reactors (FRs). Spent fuel from FRs would then be reprocessed in situ (i. e. inside the same installation although outside the reactor). All HMs would be recovered together (i. e. without chemical separation of the different elements) and reused to produce new fuel for the same FRs (this is known as a multiple homogeneous recycle system), while fission products (FPs) would constitute the final waste (Generation IV International Forum, 2002).

There are, however, a number of challenges. There is a need to develop a cost — effective method to treat highly radioactive materials and to achieve efficient extraction of HMs (at least 99.9%). There are also radioprotection issues in treating significant quantities of MAs (particularly Cm, due to its strong у and neutron emissions) alongside significant quantities of other HMs. One solution is to recycle U, Pu and Np and, if appropriate, Am rather than the whole HM group. Np can be partitioned via the PUREX process, although this procedure has not yet been developed on an industrial scale. Instead, it may be best to store Cm until it decays into Pu, because 244Cm has a half-life of only 18 years or so.

Separating Cm from Am also poses challenges because the two elements exhibit similar chemical behaviour, making it potentially simpler to store Cm and Am together. Recovering Am and Cm would perhaps be viable in smaller dedicated facilities, where they could be heterogeneously recycled for critical reactors or for accelerator driven systems (ADSs). It is important to note that Cm recycling is very difficult to manage because it involves the creation of non­negligible amounts of 252Cf, an extremely strong neutron emitter (far more so than Cm itself, see Table 13.5) (Bomboni et al., 2009b). More research is also needed to investigate the possibility of recycling Am without Cm. Separating Cm and Am is a difficult procedure, and might not be particularly effective in terms of reducing radiotoxicity. Am reprocessing only reduces the long-term radiotoxicity by a factor of 10 or less (compared to a route without Am reprocessing), because Cm is produced by neutron capture. Finally, building the dedicated facilities needed for Am and Cm recycling might prove uneconomical.

A single reactor is unlikely to be sufficient for the burning of all the HMs. Successful transmutation is more likely to be achieved by a chain of reactors, each

performing different tasks. LWRs would be the first link in any possible chain, because their reliability has been proven internationally and LWR SNF is rich in fissionable elements. Nevertheless, for technological and neutronic reasons it is impossible to burn HMs completely in LWRs. Instead, FRs can substantially increase the availability of nuclear fuel through exploiting Pu by breeding 238U. The fast spectrum allows transmutation of both even-numbered Pu isotopes and MAs, due to its good neutron economy. The use of new TRU-based fuels will need careful investigation, focusing particularly on the dynamic behaviour of the core. Introducing large fractions of Pu and MAs tends to worsen safety parameters such as the fuel temperature coefficient (FTC) and the effective delayed neutron fraction eeff). Cores will need to be designed with neutron economy in mind, and should be able to reach and maintain criticality with small fractions of fissile Pu.