The sustainability of nuclear energy operations is partly determined by the choice of nuclear fuel cycle. The commonly used once-through cycle is the most wasteful, with less than 1% of the extracted U being converted into energy. This increases the use of U as a fuel source. According to recent estimates, the existing and estimated additional U availability is sufficient to support a once-through cycle until at least the end of the current century (Vezzoni, 2011). After the end of this century, unless new mining or extraction techniques are developed, U availability will be the limiting factor. The most recent available assessments (NEA and IAEA, 2010) have fixed a maximum limit value for the identified resources (IR) available at a cost of less than 130 $/kgn of about 16 Mton. This quantity can be augmented by adding the uranium dispersed in phosphates (about 22 Mton (Pool, 1994)), which gives a maximum limit of 38 Mton. This estimate is subject to change, depending on improved exploitation techniques at higher U prices (as has occurred with oil (Vezzoni, 2011)). As an example, it is possible to add a new resource category (available at a cost of less than 260 $/kgn) in response both to overall U market price increase and increased mining costs that enables a 15% extension of the (conventional) resources available (NEA and IAEA, 2010).

In the short term, three main options have been investigated to take account of finite U resources (Vezzoni, 2011):

• the adoption of reactor cores with a high conversion ratio (CR), defined as the ratio of TRU fissile production/TRU destruction

• the adoption of very high burn-up fuel

• the recycling of plutonium in LWRs by the use of mixed oxide (MOX) fuel

A key development is the transition from the open fuel cycle, where all the spent fuel is disposed of without recovering Pu (and MAs), to more optimized closed or partially closed fuel cycles based on partitioning and transmutation (P&T) (Vezzoni, 2011). Within a closed fuel cycle, fuel can be recycled, waste reduced and partitioned (e. g. MAs separated) so that each fraction can then be dealt with more effectively. Progressive waste management techniques include the transmutation of selected nuclides, cost-effective decay-heat management, flexible interim storage and customized waste forms for specific geologic repository environments. Because most of the heavy long-lived radioactive elements are removed, such methods significantly reduce the toxicity and decay heat of waste heading for geological repositories. This makes it easier to store and dispose of these radioactive wastes. The Generation IV roadmap fuel cycle crosscut group (FCCG) found that an important limiting factor facing the once-through cycle is the global availability of repository space, particularly as new repository capacity will be needed in a matter of decades. Closed fuel cycles have the potential to reduce the pressure on repository space and performance requirements.

There are number of challenges still to be overcome, including further development of separation technologies and the feasibility of advanced burners (Vezzoni, 2011). Another potential problem is the partitioning and transmutation of fissionable materials, which could be seen as adding to the risk of nuclear proliferation. Advanced partitioning technologies for Generation IV systems are better since they aim to prevent the separation of Pu from other actinides, and incorporate features that reduce the accessibility and possibility of creating weapons from waste materials. The use of fast-spectrum reactors and repeated recycling may make it possible to lessen the radiotoxicity of all wastes to the point where the confinement requirements can be reduced to less than 1000 years. However, realizing this goal would require substantial further research into fuel recycling techniques.

One important recent development has been the design of highly durable ceramics for the immobilization and possible disposal of MAs as well as Pu from dismantled nuclear weapons. Several possible hosts for actinides have been investigated, including complex oxides, silicates, and phosphates (Ewing, 2007). The most studied phase (Ewing et al., 2004) is pyrochlore (A2B2O7 where A and B are generally rare-earth or transition metal elements) because of its:

• ability to incorporate actinides

• chemical durability

• resistance to radiation damage (at least for some compositions)

It has been shown that compositions can be adjusted so that the dose at which the material becomes amorphous due to alpha-decay damage can be substantially reduced (Ewing, 2008), as shown in Fig. 13.1. Such investigations can be

13.1 Predicted temperature dependence of amorphization in pyrochlore-related phases containing 239Pu (Ewing et al., 2004).

considered the initial stage in the design of waste forms for particular waste stream compositions and repository conditions. For example, in the future it could be possible to choose the waste loading of a material based on the interplay between radiation damage accumulation and the anticipated thermal future of the repository. The nuclear fuel cycle will therefore become safer with the development of highly durable materials for the ‘back-end’ cycle (Ewing, 2008).