A key concept in the Generation IV Initiative is a mix of nuclear reactor designs, where the strengths of some reactors counterbalance the weaknesses of others, in order to achieve fully sustainable nuclear energy production. That is

particularly true as far as nuclear waste transmutation is concerned. A few different integrated cycles have already been proposed. Generally, they are rather complex cycles, starting from the current Generation III or advanced Generation III+ (e. g. EPR, AP-1000, ABWR) LWR range of reactors. The LWR SNF could, for example, provide feed fuel for emerging Generation IV reactors. Each cycle has its advantages and drawbacks, and an ideal solution has not yet been found. In addition, due to the extreme complexity of MA behaviour in terms of core kinetics, some integrated cycles also envisage a dedicated subsidiary process using ADS (NEA, 2006b). Some additional examples are shown in Fig. 13.4 . Three kinds of possible fuel cycle can be highlighted (Bomboni, 2009):

1 Cycles based on ‘current industrial technology and extensions’: only LWRs and, if necessary, CANDUs are involved and only one recycle of HM is envisaged.

2 ‘Partially closed fuel cycles’: these cycles are fully closed only for Pu; in some schemes a single recycle of some MAs is envisaged in LWRs or FRs.

3 ‘Fully closed fuel cycles’: all the advanced reactor concepts, ADS included, could be involved; only HM losses and FPs go to the geological repositories; pyrochemical reprocessing is envisaged.

For the complete transmutation of HMs all actinides are recycled continuously in a closed fuel cycle until they fission (Bomboni, 2009). A closed fuel cycle cannot

be achieved without multiple recycling of all HMs. An example of an advanced fuel cycle, based on Generation IV reactors, which may maximize the exploitation of natural resources, minimize the final mass and radiotoxicity of the waste and be proliferation resistant, is the symbiotic LWR-VHTR-GFR cycle (Bomboni, 2009). Although further analysis is still required, this example shows the potential of a kind of symbiotic cycle involving two of the most promising Generation IV reactor concepts, VHTR and GFR. It does not aim at being ‘the’ solution, but it should be considered as an interesting, reasonably feasible possibility that offers some useful advantages.