Historical Development of Pyrochemical Methods for Treating Used Nuclear Fuel
Authors: Motsegood Perry N , Willit James L , Williamson Mark A. 
Published: February 2020

Pyrochemical processing (often referred to as “pyroprocessing”) of used nuclear fuel utilizes molten salts and molten metals at elevated temperatures as process fluids to separate actinides from fission products. In broad terms, separations are achieved by taking advantages of differences in reactivity of the U, Pu, Am, Np, lanthanide and fission product metals as a function of pO2 and pX2, where X is a halide such as chlorine or fluorine.
The presentation will cover highlights in the development of pyroprocessing technology with an emphasis on how the technology has reached its current state of development1 and the remaining challenges that are driving the current R&D in this area.2

In the U.S., pyroprocess development for civilian applications began in earnest in the 1960s with the recycling of used metal fuel from the Experimental Breeder Reactor II (EBR-II). At that time, a technique of “melt refining” was employed in which the fuel was melted in a single-use zirconia crucible. The crucible reacted with the lanthanide and active metal fission products as well as a fraction of the transuranics (TRUs) to form an insoluble oxide skull leaving uranium, noble metals, and most of the TRUs in the molten metal phase. This approach was abandoned because of incomplete TRU recovery. Interest in recycling EBR-II fuel was revived in the 1980s during the Integral Fast Reactor (IFR) program. In the IFR program, there was shift away from the use of chemical redox agents and to drive redox the reactions electrochemically in an electrolytic cell. This approach allows for more precise control of the reaction driving force and potentially decreases the volume of waste generated. For example, electrorefining, was chosen as the main separation method.1 Used metallic fuel is placed in an anode basket and inserted in to a bath of molten LiCl-KCl-UCl3. Passing current between the anode and a cathode results in the anodic dissolution of uranium and all metals more active than uranium. At the cathode only metallic uranium or a U+TRU metal alloy is deposited because the uranium chloride and transuranic chlorides are the noblest species in the molten salt. Fission products less noble than the TRUs (e.g., lanthanides) remain in the salt as chlorides and fission products more noble than, uranium (e.g. Fe, Zr, Ru, Mo, Tc) remain as metals in the anode basket.
Improvements in equipment design and additional refinement to the overall process flowsheet continued through the 1990s and into the 21st century. Direct electrolytic reduction of used oxide fuel technology was developed to expand the application of the technology to pyroprocessing of light water reactor (LWR) fuel. Process development has not been limited solely to recovery of uranium and transuranics. Back-end processes for recovery of fission products and placing them in durable wasteforms for geologic disposal were also developed.

References

J.L. Willit, W.E. Miller, J.E. Battles, J. Nuc. Mat., 195, 3, 229-249

C.E. Till and Y.I. Chang, Plentiful Energy, 2011, ISBN: 978-1466384606

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