MA/Pu separation (possibly followed by further minor actinide separation, e. g. Cm removal) is mostly envisioned for dedicated transmuters (both ADS and low CR critical FRs) and ‘double strata’ scenarios (see Fig. 17.5). Where the objective is to use dedicated transmuters to phase out nuclear power and reduce the total TRU inventory (Fig. 17.6), grouped TRU separation (i. e. removal without separation into its constituent elements) is the most suitable approach. If, on the other hand, the objective is sustainability of nuclear power (Fig. 17.4) all options are available: grouped TRU separation (for homogeneous TRU recycling) or MA/ Pu separation (for heterogeneous TRU recycling) with the additional option of further MA separation.

Hydrometallurgical and pyrometallurgical chemical separation (partitioning) processes are under development in order to implement P&T strategies as indicated in the different scenarios described in Section 17.4. For both processes, a key issue is losses during reprocessing and re-fabrication, which must fall well below 1% and should probably approach 0.1%.

Many countries have, over the past four decades, developed hydrometallurgical processes to recover TRU elements so that, unlike the standard PUREX process, it is not only plutonium that is removed. Such processes would decrease the radiotoxic inventories of nuclear waste. While some of these processes have reached the stage of laboratory-scale demonstrations, none has ever been implemented at the industrial scale. Most of the partitioning strategies rely on a three-step approach:

• separation of U (and sometimes also Pu or/and Np) from spent fuel dissolution liquors

• actinide(III) + lanthanide(III) co-extraction

• actinide(III)/lanthanide(III) separation, which is the most difficult step because of the similar chemical properties of these element groups

The processes developed worldwide use a range of extraction systems. When considered in terms of process development (number of cycles, amount of secondary waste generated, scale-up of equipment, process control, robustness, safety analysis, etc.) or solvent management (treatment for recycling, by-product management, etc.) they are more or less suitable for industrial implementation. Nevertheless, all of these schemes require further development if not innovation. Even the most advanced processes may be in need of consolidation and optimization.58

Pyrochemistry has been studied for more than forty years, first for the reprocessing of fuels from molten salt reactors and then for the reprocessing of metallic fuels. These fuels were intended for the Integral Fast Reactor (IFR) proposed by the Argonne National Laboratory and used in practice in the EBR-II reactor at Idaho National Laboratory (INL), where the irradiated fuel of EBR-II has been processed using a pyrochemical facility.

Pyrochemical reprocessing has some advantages over hydrometallurgy since fuels foreseen for the new generation reactors would differ from present day commercial fuels and, in particular, would have lower solubility in acidic aqueous solutions. Other advantages are more compact equipment and the possibility of locating the reprocessing facilities close to the reactors, thus reducing considerably the transport of nuclear materials. In addition, the radiation stability of the salt in the pyrochemical process compared to the organic solvent in the hydro chemical process offers an important advantage (e. g. shorter cooling times), especially when dealing with highly active spent MA fuel. Until now, two pyrochemical processes have been developed to the pilot scale, both in chloride media, for the reprocessing and fabrication of oxide and metal fuels, respectively.

Pyrochemistry is a long-term objective and still requires much development and viability assessment before implementation at a larger scale. The research and development is thus often carried out in the framework of international collaboration. Specific issues that will need to be explored include corrosion of containment materials, online monitoring, the production of secondary waste and the motion of molten salts in pipes.

17.7 Conclusions

P&T has been historically associated with the waste minimization goal, and, in the last two decades, has been mostly explored in this context. Since about the year 2000, however, the Generation IV (Gen IV) initiative59 has, admitting some exceptions from a total consensus, defined a set of more general goals for future systems in four broad areas: a) sustainability (more efficient use of the available U resources and waste minimization); b) enhanced economics, c) enhanced safety and reliability and d) enhanced proliferation resistance and physical protection. The objectives of Gen IV do include P&T (waste minimization), consistent with sustainability and non-proliferation objectives and mostly associated with future reactor deployment.

Many recent studies have demonstrated that the impact of P&T on geological disposal concepts is significant even if not overwhelmingly high. 42 By reducing the decay heat of the waste, it becomes possible to utilize the repository volume more efficiently. As indicated in Section 17.5, deployment of P&T techniques reduces the thermal output of high-level waste by a factor of at least 3. This reduces the needed repository gallery length by the same factor and the repository footprint by up to a factor of at least 9.

The environmental impact of a deep repository is less affected by the use of P&T because the calculated doses to humans who might inhabit the land above the repository in the far future are mostly dominated by a few long-lived fission products for which no practical transmutation strategy is applicable. Nevertheless, by reducing the hazard associated with the emplaced materials, P&T would remove much of the uncertainty and (on the part of the general public) unease regarding the creation of a man-made Pu or U ‘ore body’ deep below the surface. This is especially the case for so-called intrusion scenarios that bring man in direct contact with the disposed waste. Other benefits arise from some less likely radionuclide release scenarios or unwanted events such as an increase in actinide mobility due to changes in geochemistry and (probably unlikely) nuclear criticalities due to actinide accumulation.

P&T will never remove the need for deep geological disposal but it has, nevertheless, the potential to significantly improve public perception of the ability to effectively manage radioactive wastes by largely reducing the TRU waste masses to be disposed and, consequently, to improve public acceptance. Both issues are important to the future sustainability of nuclear power.

While the physics of transmutation is well understood, major challenges are found in the fields of chemical separation, transmutation fuel development and impact on the fuel cycle. Regarding transmutation-dedicated reactors, key demonstrations are still expected, in particular in the field of external neutron source driven systems (as ADS), in order to prove their feasibility.

17.8 Acknowledgement

The author gratefully acknowledges the detailed, patient and appropriate comments of the editor of this book, which greatly contributed to the improvement of the present chapter.