Nuclear Wastes: Technologies for Separations and Transmutation

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metals by reaction with lithium metal and prepared for further electrochemical processing. The oxygen present reacts to form lithium oxide, which may be removed from the salt by electrolysis with a carbon anode to regenerate the lithium metal at the cathode for reuse, and carbon dioxide which is discarded as waste. This avoids the formation of excessive solid process wastes.

Molten salts and alloys have a long history of use in the processing of nuclear weapons materials. They are used in processes for production of lithium metal and lithium compounds as well as of uranium and plutonium metals. Essentially all processes for the manufacture of fluorine and intermediate uranium compounds for uranium hexafluoride production are pyrochemical. Many of the recycle processes for uranium and plutonium, including residue recovery, are pyrochemical. There are no aqueous chemical analogues for many of the steps done by pyrochemical means, but there are advanced pyrochemical techniques that can replace many traditional aqueous processing methods.

The single-stage separation factors in pyrochemistry can be large for equilibria between liquid metal and molten salt phases, and cascades are not usually required for fuel recycle. Multistage equipment using short-stage-time centrifugal contactors originally designed for aqueous systems is being developed for those molten-salt/molten-alloy processes that require a high degree of purification. High-temperature liquid-liquid extraction equipment is similar in concept to that used for aqueous systems.

There is a large body of experience with the use of aqueous solutions and organic extraction phases for large separation cascades. There has been success using large and small separation factors, down to a stage separation factor of 1.002. This experience should be applicable to many of the proposed high-temperature molten-salt and alloy systems that generally show good separation factors for isolation of actinide elements from fission product residues.

AQUEOUS PROCESSES

Some insoluble products result from high burn-up fuel dissolution in nitric acid. The solids may be complex fission-product compounds or insoluble reaction products. High-fired pure plutonium oxide does not dissolve completely. Such solids would typically be removed from the solutions by filtration or centrifugation for further treatment or discard to waste. Both of these solid–liquid separation methods can be designed for easy recycle of valuable materials. The radioactive rare gases krypton and xenon can be removed in the off-gas system, perhaps by cryogenic absorption methods. Iodine can be chemically trapped from the gas phase during dissolution. A process to separate volatile ruthenium or technetium oxides could be included at the time of initial dissolution of the spent fuel.

After dissolving the fuel, the next step is to separate the uranium and plutonium from the very radioactive fission products and the higher actinides. With the uranium in the uranyl form (+6) and the plutonium in the +4 oxidation state, these elements may be separated from all other materials of concern to almost any degree desired using a multistage cascade that yields very low losses of uranium and plutonium to waste. PUREX accomplishes this task very well, but in most plants the actual recovery of uranium and plutonium is not as good as theoretical design

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