Metal concentration, fission product

Sepa.ra.tion of Plutonium. The principal problem in the purification of metallic plutonium is the separation of a small amount of plutonium (ca 200—900 ppm) from large amounts of uranium, which contain intensely radioactive fission products. The plutonium yield or recovery must be high and the plutonium relatively pure with respect to fission products and light elements, such as lithium, beryUium, or boron. The purity required depends on the intended use for the plutonium. The high yield requirement is imposed by the price or value of the metal and by industrial health considerations, which require extremely low effluent concentrations. 

The abundance of niobium in the earth s crust is estimated to be in the range 20 mg/kg and its average concentration in sea water is 0.01 mg/L. The metal also is found in the solar system including the lunar surface. Radionucleides niobium-94 and -95 occur in the fission products of uranium-235. 

If uniform mixing of the fission product vapors and volatilized materials results, the recondensed particles might be expected to have a constant specific activity of elements having similar boiling points. Note parenthetically that studies of fission-product incorporation into the metal and oxide products of vaporized iron wires (in which iron-metal spheres and iron-oxide irregulars are formed) indicate no simple relationship between specific activity and size. For example, a refractory element like zirconium is found most enriched in particles of intermediate size. This is probably in part caused by a concentration effect—i.e.y in these experiments the zirconium represented a mole fraction of about 10"9. As indicated earlier, the fission products are a minor constituent in the fireball, and a very complex pattern of incorporation can be anticipated, especially if coagulation with melted but unvaporized particles ensues. 

Deuterium is in very low concentration. Lithium has an atomic weight of 6.94 and the abundance of Li is around 7% in natural Li. The main reaction product of B is Li which does not generate but there are other, minor reactions that do. Except in boron steels, the activation of Li predominates. Another source of in fission reactors is the low yield, ternary fission of fuel (-130 x 10 atoms per fission product pair). In Magnox gas-cooled reactors, from ternary fission is mainly retained in the metallic uranium fuel and its cladding but some is released into the coolant circuits, where it may possibly diffuse into structures within the primary vessel. Tritium is a low energy /5 emitting radionuclide of low radio-toxicity and with a half life of 12.3 years. 

Stabilized as extractable compounds (VO -oxidation during the first cycle coextraction) the actinides will follow the heavy metal and can be separated from the intermediate product stream of the first cycle. This stream has a low fission product concentration, and the THOREX-process is discontinued anyhow at that step, but, of course, the insertion of a new component amidst an established process will cause unfavourable consequences. 

The dissolved metals and the extractor upstream wash waters are combined and analyzed. The uranium and plutonium concentrations are used to adjust the pH of the solution to optimize the separation from the fission products. Solutions about 1M in uranium and plutonium and 2-3 M HNO3 were the feed for the extraction. 

The sorption of the fission products Cs and Sr by the graphitic materials, from which the core and the fuel elements of High-Temperature Gas-Cooled Reactors (HTGRs) are made, is important for the prediction of fission product release in the case of an accident. Hilpert et al. [564, 565] determined, therefore, Cs and Sr partial pressures over such graphitic materials with different Cs and Sr concentrations. The vaporization enthalpies obtained showed a strong chemisorption of Cs and Sr by these materials. The vaporization enthalpy of Sr exceeds that of the pure Sr metal by about 210kJmol at 1500 K, if fuel element matrix graphite with a Sr concentration of 4.0 mmol kg is considered [564]. This value for Cs amounts to about 230 kJmol" at 1250 K for a similar concentration of 4.2 mmol kg[565]. In addition, sorption isotherms were evaluated. 

A solvent extraction process similar to Purex using TBP was developed by the Commissariat a I Energie Atomique [Gl] for use in the French plutonium separation plant at Marcoule. Since then, the Purex process has replaced the Butex process at Windscale [W3], has been used in the Soviet Union [Sll], India [S7], and Germany [S3], and by now is the universal choice for separation of uranium and plutonium from fission products in irradiated sUghtly enriched uranium. Fuel from the liquid-metal fast-breeder reactor (LMFBR) is also reprocessed by the Purex process, with modifications to accommodate the higher concentrations of plutonium and fission products. 

With a specific HLW concentrate volume of 600 liters/MT of heavy metal, the total fission product concentration will be on the order of 50 g/liter and the actinide concentration on the order of 10 g/liter. 

During the decontamination steps, acid streams containing small amounts of actinides and fission products are produced. These streams are evaporated to concentrate the metal ions and recycle them. Nitric acid is recovered from the condensates and recycled. Excess HNO3 may be destroyed by formaldehyde. Fission product concentrates are routed to the aqueous raffinate of the first extractor of the partitioning cycle which contains > 99% of the FP. This constitutes the high level liquid waste (HLLW, or alternatively called HAW, high active waste). All other liquid wastes can be subdivided into intermediate level waste... 

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide UjO (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a ceramic, which is the most common fuel material used in commercial power reactors. The UO2 is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity. 

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