Civil reprocessing of spent fuel utilizing the PUREX process has been successfully practised on a commercial scale for over 40 years without any occurrences of the diversion of nuclear materials.11 These operations have been for spent fuel management and for the recovery of the uranium and plutonium for recycling as UOX and MOX fuel for light water and fast reactors. Such a combination of spent fuel reprocessing and recycling may lead to benefits in ultimate waste disposal primarily due to reduced volumes of reprocessing HLW (the volume reduction factor of spent fuel to reprocessing HLW is a 4:1).

Irradiated nuclear fuel from research reactors was first reprocessed in the 1940s using pyrochemical and precipitation processes. These separation methods were soon replaced by the solvent extraction process (hydrometallurgy), which is better suited for continuous, large scale, remote operation, and can separate the three main streams of radionuclides (uranium, plutonium and waste, i. e. fission products and actinides). Different solvent extraction systems were explored before the discovery of an efficient extraction system. The combination, known today generically as PUREX, soon replaced all earlier solvent extraction methods because of its high performance in industrial-scale plants. PUREX utilizes the

15.15 Disposal facility concept for the CANDU heavy water reactor type of fuel (Courtesy of Nuclear Waste Management Organization in Canada).

extractant tributyl phosphate (TBP), mixed in a largely inert hydrocarbon solvent. The first plant for reprocessing based on the PUREX technology was built in Belgium in the 1960s. In the 1970s, there was some expansion of reprocessing capacity and its application to fuels from various types of reactor. In the 1980s, due to proliferation and other concerns, the strategy moved to a once-through cycle with the disposal of spent fuel. However, several countries including France,

Japan, UK, Russia and India continued to further develop, improve and adapt the PUREX technology. In France it was used for MOX fuel fabrication, in Russia for U recycling for the RBMK reactors and in India for U recycling of PHWR fuel and MOX to make fast breeder reactor fuel (FBR). Further consideration of spent fuel reprocessing has to be done within the current circumstances where long­term storage seems to be an interim strategy that will have to be combined with future advanced reactors and nuclear fuel cycles.

Sustainability is a major driver in developing advanced nuclear fuel cycle technologies.

Developments in advanced reprocessing technologies are directed toward the following goals:

• Reduction of reprocessing cost in comparison to the current PUREX process costs and in comparison to direct disposal costs of the once-through fuel cycle.

• Recovery of all actinides and long-lived fission products to reduce the volumes and radiotoxicity of the radioactive waste for disposal and hence a decrease in the expense of waste disposal and an increase in the long-term safety of the repository.

• Creation of flexible technologies that are adaptable for changing conditions and requirements such as new designs and materials for fuel and reactors of the third and fourth generations.

• Through a well understood reduction of safety risks and proliferation risks, to render nuclear power more acceptable to the public.

There are several national and international initiatives supporting the development of advanced nuclear fuel cycles. The International Atomic Energy Agency (IAEA) started the INPRO initiative and multinational approach in nuclear fuel cycles. There is also the generation IV international forum (GIF), the Russian initiative on development of international nuclear centres, the USA initiative Global Nuclear Energy Partnership (GNEP, lately renamed the IFNEC-International Framework for Nuclear Energy Cooperation) and some others.

Currently available and developing reprocessing technologies can be divided into groups according to their stage of maturity: [30]

• Evolutionary technologies (generation III reprocessing facilities) based on aqueous separation methods have been successfully tested and are ready for industrial implementation. The objective of these technologies is the co-management of U and Pu (or U-Pu-Np). One of the key features of these processes is that separation of Pu does not take place, which significantly reduces proliferation concerns. Furthermore, an integrated facility for reprocessing and fresh fuel re-fabrication can be applied. There is also flexibility in the processing systems to allow fabrication of MOX fuels both for LWRs and FBRs. There are also advantages like enhanced MOX fuel performance due to high homogeneity of fabricated fuel and the possibility of selective separation of some minor actinides and fission products. Figure 15.16 shows as an example the block diagram of the COEX reprocessing system developed in France.11

• Aqueous processes using new extractant molecules will provide two possible options for separation of actinides. One possibility is selective separation of minor actinides (MA) for interim storage allowing the postponed decision on transmutation in heterogeneous mode, either in fast reactor blankets or in accelerator driven systems (DIAMEX-SANEX is under development in France, TALSPEAK in the USA, TOGDA in Japan)

• The other option is group actinide separation using an integrated fuel cycle (on-line fuel reprocessing and fabrication) with the prospect of their homogeneous recycling in fast reactors (GANEX in France, UREX+ in the USA, NEXT in Japan).

• Innovative methods based on pyrochemistry (‘dry methods’) will allow reprocessing of different types of highly radioactive fuels such as metals, carbides, oxides or nitrides with high content of fissile material (in the fabrication of dedicated fuels for transmutation purposes or minor actinides targets), or fuels with high burn up. The advantage of the pyrochemistry method is that it is performed in inorganic media. The result of this is that the process is less sensitive to radiation effects allowing early reprocessing of fuel after discharge from a reactor. Furthermore, there is a low criticality risk compared to aqueous methods where water is an efficient neutron thermalization media. These ‘dry’ methods would also be very suitable for reprocessing as designed for molten salt reactors.

• A combination of hydro — and pyro-processes may have some advantages from each of the two processes but their efficiency may be affected by discontinuity between the two steps in the process.

• Other innovative processes are still more or less at the laboratory scale and include a process using Freon fluid or supercritical CO2 extraction, processes based on chromatographic methods and processes using precipitation methods.

In conclusion, there are a number of options for recycling of spent nuclear fuel.11 Some, including those that avoid separation of the pure plutonium stream, are at an advanced level of maturity. These could be deployed in the next generation of industrial scale reprocessing plants, while others (such as ‘dry’ methods) are at pilot scale, laboratory scale or a conceptual stage of development. Measures to improve the environmental protection of commercial reprocessing plants over the past 20-30 years have greatly reduced emissions and waste volumes11 in currently operating reprocessing plants.

The deployment of multi-national fuel cycle centres, operating under an international framework and most effectively implemented in those countries with a sufficiently large civil nuclear energy infrastructure, can serve to ensure a sustained supply of nuclear fuel and related services under conditions in which the risk of proliferation of technologies related to the production of nuclear weapons is minimized. Reprocessing of spent fuel will be an important function of these centres.

The next generation of spent fuel reprocessing plants will likely be based on aqueous extraction processes. The physical design of these plants will have to incorporate effective means of material accountancy, safeguards and physical protection. Innovative reprocessing technologies must be developed for the reprocessing of fuel types that will be used in future and that may be substantially different from the UO2 and MOX ceramic type fuel used today.