Plutonium processing Ruthenium

Uranium stripping Dilute HNO3 solutions at 45-50°C are used to remove uranium from the TBP phase. Traces of the fission products ruthenium and zirconium are eliminated in the second and third cycles of the Purex process. Also, in the second and third cycles, neptunium and the last traces of plutonium are removed from the uranium product. 

Degraded TBP process solvent is typically cleaned by washing with sodium carbonate or sodium hydroxide solutions, or both. Such washes eliminate retained uranium and plutonium as well as HDBP and H2MBP. Part of the low-molecular-weight neutral molecules such as butanol and nitrobutane, entrained in the aqueous phase, and 90-95% of the fission products ruthenium and zirconium are also removed by the alkaline washes. Alkaline washing is not sufficient, however, to completely restore the interfacial properties of the TBP solvent, because some surfactants still remain in the organic phase. 

Distillation under reduced pressure allowed separation of the TBP-diluent mixture components into three fractions diluent, 60% TBP, and distillation residue. The first two fractions can be reused in the process, but the residue contains high-molecular-weight degradation products, which are not eliminated by alkaline scrubbing. Distillation removes the degradation products that are responsible for poor hydrodynamic behavior and for the retention of radioactive products such as plutonium, zirconium, and ruthenium (143). 

Separation and Purification. In the Purex process discussed here, the uranium, plutonium, and fission products are separated by solvent extraction into three different streams (Fig. 21.20). The plutonium stream goes through anion exchange (discussed later) to reduce traces of ruthenium, and the uranium stream goes through silica gel sorption to reduce traces of zirconium. The fission-product stream, which contains the fission products... 

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. 

FP-4 (zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony)—are only slightly soluble (<1 wt %) in the process alloy, thus will partition between both product streams. The process, as presented, offers no method of FP-4 removal and possibly an unwanted increase in these products would occur if the fuel were to be recycled. However, it would be possible to separate the FP-4 from the plutonium/thorium stream by recovering the plutonium/thorium by hydriding. The FP-4 do not form stable hydrides and would remain in solution. 

Effective purification of plutonium hexafluoride is proved through the selective adsorption of FPs on NaAlFi. A rather high decontamination factor (more than 5 x 103) is attained for ruthenium fluoride (36). A new separation process of PuF6 from UF6 by the selective adsorption onto UO2F2 is proposed (37). 

An extraction process for separating actinide elements (principally uranium, U, and plutonium, Pu) from fission products in an aqueous solution of spent fuel rods is illustrated in Figure 5.31. The extraction solvent is 30% tributyl phosphate (TBP) in kerosene. The most extractable of the fission products are zirconium, niobium and ruthenium. Zirconium, Zr, is used herein to represent the fission products. Determine the number of stages required in the wash section and in the extraction section. Determine the percentage of the Pu in the feed which is recovered in the extract product. V denotes the relative volumetric flowrate. 

From the above discussion it follows that tetravalent and hexavalent thorium, uranium, and plutonium can be separated from the trivalent rare-earth fission products by taking advantage of differences in complexing properties. More highly charged cation fission products, such as tetravalent cerium and the fifth-period transition elements zirconium, niobium, molybdenum, technetium, and ruthenium, complex more easily than the trivalent rare-earths and are more difficult to separate from uranium and plutonium by processes involving complex formation. 

One of the major issues of the Thorex process is the creation of a third phase between thorium and TBP if the thorium concentration in the solvent is too high. Furthermore, the partition of uranium and thorium is more difficult than the partition of uranium and plutonium. No change of the thorium oxidation state is required, but the separation of thorium from uranium in the IB contactor must be obtained entirely by a rather delicate adjustment of salting strength inside the contactor. It appears that the Thorex process has been variable in performance. Decontamination from ruthenium has varied and has been particularly poor when short-cooled thorium-based fuel was processed. 

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