Uranium fuel, spent

A requirement, as stated in a recent Swedish law, for a continuation of the extensive nuclear power program in Sweden (6 operating power reactors at present and 7 more in operation in 1985) is that the engineering problems and safety aspects connected with the disposal of the high-level waste (HLW) or the unreprocessed spent uranium fuel (SUF) are thouroughly investigated. A completely safe disposal of either HLW or SUF must be guaranteed and technically proven by the nuclear power industry. 

The starting materials for uranium nuclear fuels are uranium compounds from natural uranium deposits and fissile material separated by reprocessing from spent uranium fuel rods. 

Because of such effects, spent uranium fuel elements from PWR, BWR, HWR, GCR and FBR differ in composition both from each other and between fuel batches from the same reactor. Furthermore, the composition differs betwe pins in the same fuel elemrat and for each pin also along its Iragth, especially when initial burnable poison concentration and enrichm t is graded along pins. The differraice is not so large that very different fuel... 

Develop a complete process and plant design for a chemical reprocessing plant to handle 5 metric tons per day of spent uranium fuel from nuclear process heat reactors of 20,000 Mw total heat output. The initial fuel was 1.6 per cent slightly enriched natural uranium and the average time the fuel was in the reactor at full power was 2.0 yr. 

The plutonium in spent uranium fuel from light water reactors (LWR) is 56% plutonium-239, 26% plutonium-240, 12% plutonium-241, 5% plutonium-242, and 1% plutonium-238 (Choppin and Rydberg 1980). This composition will vary with other types of reactor fuel, but this type is the most common in reactors operating in the United States. 

Spent nuclear fuel has fission products, uranium, and transuranic elements. Plans call for permanent disposal in underground repositories. Geological studies are in progress at the Yucca Mountain site in Nevada. Until a repository is completed, spent fuel must be stored in water pools or in dry storage casks at nuclear plant sites. 

Besides fission products, the various forms of known but newly formed elements in the spent nuclear fuel, there is a small but significant amount of fissionable, or fissile, material in the SNF. This is quite important. There is some unused, unfissioned U-235 that has become too dilute to use. Like natural uranium ores in which chain reactions do not... 

In 1976 the Swedish government stipulated that no new nuclear reactors should be charged until it had been shown how the radioactive waste products could be taken care of in an "absolutely safe manner" (8). Consequently, the nuclear power industry (through their joint Nuclear Fuel Supply Co, SKBF) embarked on a program referred to as the Nuclear Fuel Safety (KBS) Project (8). In one of the schemes (9) a repository for spent nuclear fuel elements in envisaged at a depth of 500 m in granitic bedrock. The repository will ultimately contain 6000 tonnes of uranium and 45 tonnes of plutonium. The spent fuel elements will be stored in copper cylinders (0.8 m in diameter and 4.7 m in length) with a wall thickness of 200 mm the void will be filled with lead. 

Spent nuclear fuel remains radioactive and consists of a mixture of uranium... 

The effect of irradiation on the extractability of sulfoxides towards plutonium, uranium and some fission products were studied by Subramanian and coworkers . They studied mainly the effect of irradiation on dihexyl sulfoxide (DHSO) and found that irradiation did not change the distribution coefficient for Ru, Eu and Ce but increases the distribution coefficient for Zr and Pu. When comparing DHSO and tributyl phosphate (TBP), the usual solvent for the recovery and purification of plutonium and uranium from spent nuclear fuels, the effect of irradiation to deteriorate the extraction capability is much larger in TBP. Lan and coworkers studied diphenyl sulfoxides as protectors for the gamma radiolysis of TBP. It was found that diphenyl sulfoxide can accept energy from two different kinds of excited TBP and thus inhibits the decomposition of the latter. 

Purex [Plutonium and uranium recovery by extraction] A process for the solvent extraction of plutonium from solutions of uranium and fission products, obtained by dissolving spent nuclear fuel elements in nitric acid. The solvent is tri-n-butyl phosphate (TBP) in... 

Reactor fuel consists of uranium that has been formed into a usable metal alloy and provided as small pellets, rods, or plates. The fuel is encapsulated with a metal cladding, such as zircaloy, which adds mechanical strength and also prevents radioactive contamination. Nuclear reactor waste or spent nuclear fuel consists of the fuel pellets that have been used... 

A recent and extremely important development lies in the application of the technique of liquid extraction to metallurgical processes. The successful development of methods for the purification of uranium fuel and for the recovery of spent fuel elements in the nuclear power industry by extraction methods, mainly based on packed, including pulsed, columns as discussed in Section 13.5 has led to their application to other metallurgical processes. Of these, the recovery of copper from acid leach liquors and subsequent electro-winning from these liquors is the most extensive, although further applications to nickel and other metals are being developed. In many of these processes, some form of chemical complex is formed between the solute and the solvent so that the kinetics of the process become important. The extraction operation may be either a physical operation, as discussed previously, or a chemical operation. Chemical operations have been classified by Hanson(1) as follows ... 

The solvent extraction process that uses TBP solutions to recover plutonium and uranium from irradiated nuclear fuels is called Purex (plutonium uranium extraction). The Purex process provides recovery of more than 99% of both uranium and plutonium with excellent decontamination of both elements from fission products. The Purex process is used worldwide to reprocess spent reactor fuel. During the last several decades, many variations of the Purex process have been developed and demonstrated on a plant scale. 

High-grade pitchblende ores are leached with nitric acid to recover uranium. Extraction of uranium from nitrate solutions is usually performed with TBP. TBP-based solvents are used in several areas of the nuclear industry, especially for reprocessing of spent nuclear fuels and for refining the uranium product of the Amex and Dapex processes. Extraction of uranium by TBP solvents is described in sections 12.3.4 and 12.5. 

High-level wastes consist of spent nuclear fuel and reprocessed wastes. Isotopes of uranium make up by far the majority of high-level wastes, accounting for about 94 percent of the mass of all such wastes. An additional 1 percent consists of plutonium isotopes, and the remaining 5 percent, of isotopes of other elements. 

Wronkiewicz, D. J. Buck, E. C. 1999. Uranium mineralogy and the disposal of spent nuclear fuel. In Burns, P. C. Finch, R. (eds) Uranium Mineralogy, Geochemistry and the Environment. Mineralogical Society of America, Reviews in Mineralogy, 38, 475-497. 

Bruno, J., Casas, I., Cera, E., Swing, R. C., Finch, R. C. Werme, L. O. 1995. The assessment of the long-term evolution of the spent nuclear fuel matrix by kinetic, thermodynamic and spectroscopic studies of uranium minerals. Materials Research Society Symposium Proceedings, 353, 633-639. 

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