Pyrochemical separations processes

Other Pyrochemical Processes. The chemistry of pyrochemi-cal separation processes is another fertile area of research e.g., new molten salt systems, scrub alloys, etc. and the behavior of plutonium in these systems. Studies of liquid plutonium metal processes should also be explored, such as filtration methods to remove impurities. Since Rocky Flats uses plutonium in the metal form, methods to convert plutonium compounds to metal and purify the metal directly are high-priority research projects. 

The term "Pyrochemical Processing" is commonly applied to a family of chemical processes that utilize oxidation-reduction reactions to effect chemical separations at elevated temperatures. 

The pyrochemical process of zirconium-hafnium separation is particularly attractive not only because it makes the entire process of nuclear-grade zirconium metal production from zircon more economical than that involving a hydrometallurgical separation stage, but also... 

Figure 4.20 Flowsheet of the pyrochemical process for zirconium-hafnium separation.

PYRO-A A pyrochemical process proposed for use in nuclear reprocessing for separating transuranic elements from fission products, once the uranium has been removed by the UREX process. The spent fuel is dissolved in a molten salt bath and electrolyzed. The transuranic elements deposit on the cathode, and the fission products remain in the melt. Developed by the Argonne National Laboratory. See also PYRO-B. 

Some of the pyrochemical processes have more potential for being proliferation resistant because of the great similarity of the chemistry of uranium, plutonium, and some of the fission products in the chosen systems. Ordinary processes are designed to maximize differences in chemical behavior in order to separate constitutents. For some of the pyrochemical processes the chemical equilibria are such that partial separations are possible but complete separations are thermodynamically limited. For example, excess uranium can be separated from plutonium by precipitation in a molten metal such as zinc only until both are present in about equal quantities in solution, but no further ( 3, 4). Likewise, the solubility of fission products is selectively limited. Only a portion of elements such as ruthenium will stay in solution and be removed 05). The majority of the ruthenium precipitates with the actinides. A complete separation is again thermodynamically limited. As a result only a modest dependence needs to be placed on process equipment and facility design for proliferation resistance. 

The chemical basis for the various separations used in this pyrochemical process is the differences in the partitioning of uranium, plutonium, and the fission product elements between molten salt and liquid alloy phases. This difference in partitioning is largely determined by the values of the standard free energy of formation (AGf°) of the chlorides of uranium, plutonium, and the fission product elements. 

Pyrochemical processes have the potential for low waste volume, but only if materials are recycled. No major problems are foreseen for recycle of the greatest bulk component, sodium nitrate. Regeneration will be required, but the presence of a considerable amount of nitrite is not a problem since nitrite also oxidizes uranium dioxide. Removal of the highly soluble fission products, such as cesium and iodine, will eventually require either a separation step or a bleed-off of the nitrate stream. 

The three types of nonaqueous processes on which most development work has been done are (1) pyrometallurgical processes, involving high-temperature processing of metallic fuels (2) pyrochemical processes, involving high-temperature processing of oxide or carbide fuels and (3) fluoride volatility processes, in which elements in fuel are converted to fluorides, which are then separated by fractional distillation. 

The current commercial reprocessing systems are currently based onnitric acid and organic solvents to separate and purify actinides from FPs. The primary difference between pyro-chemical process and the aqueous process is the use of high temperatures and the absence of aqueous process solutions. The pyrochemical processes offer several advantages ... 

The pyrochemical processes are assessed worldwide as a potential alternative to hydrometallurgical technology, especially in case of the reprocessing of minor actinide-rich materials. The pyrochemical R D programme, launched at the CEA Marcoule in the late 1990s, aims to demonstrate the feasibility of an innovative grouped separation of the actinides with sufficient decontamination of fission products. The core of the... 

PYRO-A A pyrochemical process proposed for use in nuclear reprocessing for separating transuranic elements from fission products, once the uranium has been removed by the UREX process. The spent fuel is... 

Brambilla, G. Caporali, G. "Process for the Pyrochemical Separation of Plutonium from Irradiated Nuclear Fuels", Ger. Offen. 2 611 333, 1976. 

Sathiyamoorthy, D., Shetty, S.M., and Bose, D.K. (1999) Pyrochemical separation of zirconium and hafnium tetrachlorides using fused salt extractive distillation process. High Temp. Mater. Process, 18(4), 213. 

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