Uranium separation from plutonium

Reprocessing is based on liquid-liquid extraction for the recovery of uranium and plutonium from used nuclear fuel (PUREX process). The spent fuel is first dissolved in nitric acid. After the dissolution step and the removal of fine insoluble solids, an organic solvent composed of 30% TriButyl Phosphate (TBP) in TetraPropylene Hydrogenated (TPH) or Isopar L is used to recover both uranium and plutonium the great majority of fission products remain in the aqueous nitric acid phase. Once separated from the fission products, back-extraction combined with a reduction of Pu(I V) to Pu(III) allows plutonium to be separated from uranium these two compounds can be recycled.2... 

Plutonium-239 forms plutonium oxide, which can be readily separated from uranium. 

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. 

In the partition contactor, plutonium was converted to inextractable, trivalent Pu (N03)3 by a reductant solution of ferrous sulfamate containing aluminum nitrate to keep uranium in the hexone phase. Plutonium was thus separated from uranium and transferred back to the aqueous phase along with the aluminum nitrate. Impure plutonium nitrate was purified by additional cycles of solvent extraction, not shown. 

Volatilization. Many fission-product elements, including krypton, xenon, iodine, cesium (normal boiling point 705 C), strontium (1380°C), barium (1500°C), the rare earths (3200 C), and plutonium (3235°C), are more volatile than uranium (3813°C). Cubicciotti [C17], McKenzie [M5], and Motta [M8], in laboratory experiments, showed that around 99 percent of these more volatile elements could be separated from uranium by vacuum distillation at 1700 C. Because of the high temperature and severe materials problems, volatilization has not been used as a primary separation process, but does contribute to removal of the most volatile fission products in conventional reprocessing. In fractional crystalUzation or extraction with liquid metals, distillation is used to separate uranium and plutonium from more volatile solvent metals. 

In the Aquafluor process [G4] developed by the General Electric Company, most of the plutonium and fission products in irradiated light-water reactor (LWR) fuel are separated from uranium by aqueous solvent extraction and anion exchange. Final uranium separation and purification is by conversion of impure uranyl nitrate to UFg, followed by removal of small amounts of PuF , NpFg, and other volatile fluorides by adsorption on beds of NaF and Mgp2 and a final fractional distillation. A plant to process 1 MT/day of irradiated low-enriched uranium fuel was built at Morris, Illinois, but was never used for irradiated fuel because of inability to maintain on-stream, continuous operation even in runs on unirradiated fuel. The difficulties at the Morris plant are considered more the fault of design details than inherent in the process. They are attributed to the attempt to carry out aqueous primary decontamination, denitration, fluorination, and distillation of intensely radioactive materials in a close-coupled, continuous process, without adequate surge capacity between the different steps and without sufficient spare, readily maintainable equipment [G5, R8]. 

In partition, step 5, plutonium is separated from uranium by reducing plutonium to the organic-insoluble, trivalent state with a reductant strong enough to act on plutonium but not so... 

In one respect, requirements for reprocessing fuel from an LMFBR is less demanding than from an LWR. It is not necessary to purify the uranium and plutonium products so completely. The uranium is not converted to UFg and does not have to meet the strict purity requirements of feed for a uranium enrichment plant. When the plutonium is recycled to an LMFBR it is diluted with uranium, so complete separation from uranium is not necessary. 

Figure 10.29 shows the principal steps in applying the Purex process to irradiated LMFBR fuel, step 7 of Fig. 10.28. The flow scheme and the compositions and locations of solvent, scrubbing, and stripping streams have been taken from the process flow sheet of a 1978 Oak Ridge report [Oil] describing a planned experimental reprocessing facility designed for 0.5 MT of uranium-plutonium fuel or 0.2 MT of uranium-plutonium-thoiium fuel per day. As that report gave process flow rates only for the uranium-plutonium-thorium fuel. Fig. 10.29 does not give flow rates for the uranium-plutonium fuel of present interest. This flow sheet shows the codecontamination step, in which flssion products are separated from uranium and plutonium the partitioning step, which produces an aqueous stream of partially decontaminated...

Hanfoid [D3]. Nitrite concentration in feed to the HA column of a standard Purex plant was adjusted to route most of the neptunium in inadiated natural uranium into the extract from the HS scrubbing column. Sufficient ferrous sulfamate was used in the partitioning column to reduce neptunium to Np(IV), which followed uranium. This neptunium was separated from uranium by fractional extraction with TBP in the second uranium cycle. The dilute neptunium product was recycled to HA column feed, to build up its concentration. Periodically, irradiated uranium feed was replaced by unirradiated uranium, which flushed plutonium and fission products from the system. The impure neptunium remaining was concentrated and purified by solvent extraction and ion exchange. 

In Germany in 1938, Otto Hahn and Fritz Strassmann, skeptical of claims by Enrico Fermi and Irene Johot-Curie that bombardment of uranium by neutrons produced new so-called transuranic elements (elements beyond uranium), repeated these experiments and chemically isolated a radioactive isotope of barium. Unable to interpret these findings, Hahn asked Lise Meitner, a physicist and former colleague, to propose an explanation for his observations. Meitner and her nephew, Otto Frisch, showed that it was possible for the uranium nucleus to be spfit into two smaller nuclei by the neutrons, a process that they termed fission. The discovery of nuclear fission eventually led to the development of nuclear weapons and, after World War II, the advent of nuclear power to generate electricity. Nuclear chemists were involved in the chemical purification of plutonium obtained from uranium targets that had been irradiated in reactors. They also developed chemical separation techniques to isolate radioactive isotopes for industrial and medical uses from the fission products wastes associated with plutonium production for weapons. Today, many of these same chemical separation techniques are being used by nuclear chemists to clean up radioactive wastes resulting from the fifty-year production of nuclear weapons and to treat wastes derived from the production of nuclear power. 

Not until 1942 would they officially propose a name for the new element that fissioned like U235 but could be chemically separated from uranium. But Seaborg already knew what he would call it. Q>nsistent with Martin Klaproth s inspiration in 1789 to link his discovery of a new element with the recent discovery of the planet Uranus and with McMillan s suggestion to extend the scheme to Neptune, Seaborg would name element 94 for Pluto, the ninth planet outward from the sun, discovered in 1930 and named for the Greek god of the underworld, a god of the earth s fertiUty but also the god of the dead plutonium. 

It is emphasized that these values can be applied only in the case of plutonium separated from spent fuel from thermal reactors, where the original fuel comprised uranium enriched up to 5% in U-235, where the bumup was in the range not less than 10 000 MW d/t to not more than 40 000 MW d/t, and where the separation was carried out less than five years before completion of the transport operation. It will also be necessary to consider separately other contaminants in the plutonium. 

Once plutonium and uranium are coextracted and codecontaminated, plutonium is separated from uranium in the partitioning contactor by reduction to Pu(III) with a reduc-tant. Over the years, a number of plutonium reductants have been proposed. The most widely used reductant to partition plutonium from uranium in the PUREX process was (Fe(S03NH2)2) other alternates were proposed such as hydrazine-stabilized ferrous nitrate or uranous nitrate, and hydroxylamine salts. 

In a second aliquot of the same sample, the concentrations of the burnup monitor Nd and of the gadolinium isotopes in irradiated UO2—Gd20j fuel can also be determined. In this procedure, the rare earth elements are first separated from uranium, plutonium and the fission products by anion exchange from hydrochloric acid solution by addition of known amounts of Nd and Gd spikes to the sample solution, correction for losses of both rare earth elements during the separation process can be made. In the second step, the neodymium and gadolinium fractions are isolated from the other rare earth elements by cation exchange using... 

At the stage of spent fuel reprocessing, the accumulated plutonium is separated from uranium together with the accumulated minor actinides, which makes such plutonium ineffective for weapon devices. Also, the isotopic content of plutonium does not meet the requirements to weapons-grade plutonium ... 

Plutonium in irradiated fuel elements may be chemically separated from uranium. On treatment of a mixture of hexavalent uranium and tetravalent plutonium with divalent iron, plutonium is reduced to the trivalent stage. The stable uranyl nitrate is unaffected. In liquid-liquid extraction with tri-n-butyl phosphate (TBP) and kerosene, uranium goes into the organic phase, while plutonium stays in the water phase. 

It is easier to produce plutonium-239 from uranium-238 and then to separate plutonium from uranium than it is to enrich uranium. 

Pyro-processing is the most appropriate method for reprocessing of metal fuels. The process is relatively simple compared to a conventional aqueous process and has fewer steps, which could ensure economic benefits. In pyro-processing, plutonium is not separated from uranium, and most of the minor actinides are recovered [XX-3]. 

Initially, the only means of obtaining elements higher than uranium was by a-particle bombardment of uranium in the cyclotron, and it was by this means that the first, exceedingly minute amounts of neptunium and plutonium were obtained. The separation of these elements from other products and from uranium was difficult methods were devised involving co-precipitation of the minute amounts of their salts on a larger amount of a precipitate with a similar crystal structure (the carrier ). The properties were studied, using quantities of the order of 10 g in volumes of... 

The wastes from uranium and plutonium processing of the reactor fuel usually contain the neptunium. Precipitation, solvent extraction, ion exchange, and volatihty procedures (see Diffusion separation methods) can be used to isolate and purify the neptunium. 

MIBK is a highly effective separating agent for metals from solutions of their salts and is used in the mining industries to extract plutonium from uranium, niobium from tantalum, and zirconium from hafnium (112,113). MIBK is also used in the production of specialty surfactants for inks (qv), paints, and pesticide formulations, examples of which are 2,4,7,9-tetramethyl-5-decyn-4,7-diol and its ethoxylated adduct. Other appHcations include as a solvent for adhesives and wax/oil separation (114), in leather (qv) finishing, textile coating, and as a denaturant for ethanol formulations. 

Uranium-239 [13982-01 -9] has a half-life of 23.5 min neptunium-239 [13968-59-7] has a half-life of 2.355 d. Recycling or reprocessing of spent fuel involves separation of plutonium from uranium and from bulk fission product isotopes (see Nuclearreactors, chemical reprocessing). 

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

Spent fuel can be stored or disposed of intact, in a once-through mode of operation, practiced by the U.S. commercial nuclear power industry. Alternatively, spent fuel can be reprocessed, ie, treated to separate the uranium, plutonium, and fission products, for re-use of the fuels (see Nuclear REACTORS, CHEMICAL reprocessing). In the United States reprocessing is carried out only for fuel from naval reactors. In the nuclear programs of some other countries, especially France and Japan, reprocessing is routine. 

The last paragraph in this extract refers to work on the separation of uranium by ether extraction as a step toward obtaining a plutonium concentrate from a large sample of neutron-irradiated uranyl nitrate. 

Stanley G. Thompson joined my group on October 1, 1942 and it fell to his lot to discover the process that was chosen for use at Clinton Laboratories (in Tennessee) and the Hanford Engineer Works (in the state of Washington) for the separation of plutonium from uranium and the immense intensity of radioactive fission products with which it was produced in the nuclear chain reactors. Again I turn to my journal to tell the story ... 

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