T. KOYAMA, Central Research Institute of Electric Power Industry, Japan

Abstract: In this chapter, state of the art in nuclear technology development as well as basic principles of pyrochemical treatment, in other words, dry reprocessing, are described. Although this technology has not been commercialized yet, engineering-scale tests with real spent fuels and/or surrogates are underway in many countries, especially in the US, Russia and Asia. As the technology has an intrinsic nuclear proliferation resistance, due to its inherent difficulty in separation of pure plutonium, it is regarded as one of the most promising nuclear fuel cycle technologies of the next generation.

Key words: pyroprocess, dry reprocessing, electrorefining, molten salt, liquid metal, injection casting.

10.1 Introduction

What do we expect for pyrochemical treatment? As indicated by the prefix, ‘pyro’ (‘fire’ in Greek), we need high temperature, and we must use an unfamiliar liquid medium, a molten salt, in an inert atmosphere. However, pyrochemistry is expected to be the most promising method to solve the difficulties related to the treatment of spent fuel. For example, radiation damage of the solvent is not a concern at all because the molten salt is a fully dissociated ionic liquid. Hence, the treatment of relatively short-cooled spent fuel as well as a decrease in secondary waste due to radiation damage of the solvent are expected. Because water, which acts as a neutron modera­tor, is not present in pyrochemistry, a larger quantity of fissile materials can be handled compared with aqueous processing. In addition, simple but incomplete purification techniques such as electrorefining can be applied because the recycled nuclear fuel will later be used in fast neutron reactors. Hence, pyrochemistry is potentially more compact than aqueous technolo­gies, and a reduction of the fuel cycle cost is expected. The incomplete purification implies the group recovery of actinides in any step of pyro — chemistry, which results in an inherent difficulty in separating weapon- usable plutonium. Although purification is not complete, the recovery of minor actinides (MA) such as neptunium, americium and curium, separating them from fission products (FP), is sufficient to supply the prod­ucts required for fast reactors.

For these reasons, various types of pyrochemical processes using different molten salts have been proposed by many nuclear research institutes. Argonne National Laboratory (ANL) and Idaho National Laboratory (INL) have proposed a pyrochemical treatment, referred to as a ‘pyropro — cess’, suitable for metal fuel processing. Because the process uses chloride molten salts with relatively low melting points, for example, 352 °C for LiCl-KCl, without the evolution of a corrosive gas, engineering difficulties related to the materials are reduced. This might be the reason why this pyrochemical process is the most developed technology and has already been used to treat approximately 4 tons of heavy metals (HM) of spent fuels from the Experimental Breeder Reactor (EBR)-II, and has been applied in many institutes such as the Central Research Institute of Electric Power Industry (CRIEPI) in Japan and the Indira Gandhi Centre for Atomic Research (IGCAR) in India for spent fuel reprocessing, and at the Korea Atomic Energy Research Institute (KAERI) in Korea for spent fuel treatment.

Another pyrochemical process is electrorefining of oxide fuels in NaCl — CsCl molten salt bath, proposed by the Research Institute of Atomic Reactors (RIAR) in Russia. The recovery of plutonium and uranium as oxide forms was demonstrated using irradiated fast breeder reactor (FBR) MOX fuels. As the details of experimental results have not been reported, issues related to nuclear engineering at a higher temperature (650 °C) in the presence of chlorine gas are not apparent. The recovery of MA is another matter to be clarified. The fluoride processes being developed at the Commissariat a l’Energie Atomique (CEA) in France, and other insti­tutes are still at the stage of laboratory-scale experiments and require progress in engineering. The Nuclear Research Institute Rez plc of Czech Republic is developing a fluoride volatility process for the reprocessing of oxide fuels. Fluorination furnaces as well as UF6 condensers have operated well on an engineering scale; however, experiments with irradiated fuel are required to demonstrate the process. Thus, in this chapter, state-of-the-art nuclear engineering for pyrochemistry will be described, mainly focusing on the pyroprocess being developed at ANL, INL, CRIEPI, KAERI and IGCAR.

For newcomers to the study of pyrochemistry, the difficulties lie in the lack of specific technological knowledge on the handling of high tempera­ture molten salts, e. g. purification and control of the atmosphere, the selec­tion of compatible materials, the fabrication of stable reference electrodes, and so on. Although it is not described explicitly, the nuclear engineering discussed in this chapter has been developed on the basis of the technologi­cal knowledge accumulated in each laboratory. Unfortunately, know-how
is rarely described in published papers; thus, literature containing technical details are cited in Section 10.8 for further reference.