Spent fuel elements

If the spent fuel is processed in a nuclear fuel reprocessing plant, the radioactive iodine species (elemental iodine and methyl iodide) trapped in the spent fuel elements ate ultimately released into dissolver off gases. The radioactive iodine may then be captured by chemisorption on molecular sieve 2eohtes containing silver (89). 

Argon-40 [7440-37-1] is created by the decay of potassium-40. The various isotopes of radon, all having short half-Hves, are formed by the radioactive decay of radium, actinium, and thorium. Krypton and xenon are products of uranium and plutonium fission, and appreciable quantities of both are evolved during the reprocessing of spent fuel elements from nuclear reactors (qv) (see Radioactive tracers). 

AH operating facilities shear the spent fuel elements into segments several centimeters long to expose the oxide pellets to nitric acid for dissolution. This operation is often referred to as chop-leach. The design and operation of the shear is of primary importance because (/) the shear can be the production botdeneck, and (2) the shear is the point at which tritium and fission gases are released. 

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. 

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 ... 

Different countries have adapted various policies for handling the highly radioactive spent fuel elements ... 

In 1942, the Mallinckrodt Chemical Company adapted a diethylether extraction process to purify tons of uranium for the U.S. Manhattan Project [2] later, after an explosion, the process was switched to less volatile extractants. For simultaneous large-scale recovery of the plutonium in the spent fuel elements from the production reactors at Hanford, United States, methyl isobutyl ketone (MIBK) was originally chosen as extractant/solvent in the so-called Redox solvent extraction process. In the British Windscale plant, now Sellafield, another extractant/solvent, dibutylcarbitol (DBC or Butex), was preferred for reprocessing spent nuclear reactor fuels. These early extractants have now been replaced by tributylphosphate [TBP], diluted in an aliphatic hydrocarbon or mixture of such hydrocarbons, following the discovery of Warf [9] in 1945 that TBP separates tetravalent cerium from... 

The spent fuel element is still mainly UO2 and is dissolved in aqueous nitric acid, which is oxidizing enough to take the uranium to the VI oxidation state as UC>22+(aq) and Pu to Pu4+(aq) (the uranyl ion U022+ can be regarded as hydrolyzed U6+ see Section 13.6). Treatment of the solution of uranyl and plutonium(IV) nitrates with either an iron(II) salt or SO2 will reduce all the Pu to Pu3+(aq), which is not extractable with TBP, but will leave the uranium(VI) untouched (see Exercise 15.5). The solution is then equilibrated with TBP (which is immiscible with water) or TBP in an alkane solvent. The U022+ forms a neutral complex containing both TBP and the nitrate ions, which axe present in large excess ... 

The waste management program is therefore faced with the potential that the radioactive waste from the commercial fuel cycle could be in the form of spent fuel elements which have been declared to be waste or in the form of solidified high-level waste produced from the byproducts stream of the reprocessing plant. 

This project placed encapsulated spent fuel elements from an experimental AEG reactor into storage holes drilled into the floor of the mine located in a salt bed. Valuable experimental information was obtained about the interaction between the waste form and the salt in which the waste was emplaced. It was in fact this experiment, conducted in 1968, which revealed that inclusions of moisture, or brine, in the salt beds have a tendency to migrate up a thermal gradient towards a heat source placed in the salt. Quantities of brine were measured as migrating to the deposited waste canisters and the interaction of this brine with the canis-tered material was observed. 

We are presently constructing a near-surface test facility at the Hanford reservation which will be located in a basaltic flow in the side of a mountain there. This facility will have electrical heaters emplaced in it in 1979 and by 1980 we expect to emplace encapsulated cylinders containing spent fuel elements there. A second project under development will provide placement of spent fuel in similar containers in a deep granite facility at the... 

In the radiation grafting work, experiments were performed in spent fuel element and cobalt-60 facilities of the Australian Atomic Energy Commission, dose rates being determined by ferrous sulfate dosimetry (G(Fe)= 15.6). For the specific irradiations, which were carried out in quadruplicate, small strips (5 x 4cm) of cellulose (Whatman No. 41, W.R. Balston Pty. Ltd., double acid-washed chromatography grade) were equilibrated at 65% relative humidity for 24 h at 23 °C, folded into test tubes (16 x 1.2 cm), monomer solution (6ml) added to the tubes which were then lightly stoppered. After irradiation, the strips... 

The different redox stability of U and Pu is important in nuclear fuel reprocessing, one function of which is to separate unused uranium from 239Pu, which is itself used as a nuclear fuel. Dissolving the spent fuel elements in aqueous HN03 gives Pu(VI) and U(VI). Subsequent separation steps then depend on differences in complexing power and solubility of these ions. 

For the grafting reactions, solutions were prepared in glass sample tubes (7.5 X 1.6 cm) solvent, sulfuric acid (to 0.2M), and monomer were added to give a total volume of 8 mL and preweighed cellulose acetate strips then were immersed completely in the solutions. Irradiations were performed in air in the spent Fuel Element Facility at the Australian Atomic Energy Commission to total doses of 2.0 X 105 rads or 1.0 X 106 rads, both at a dose rate of 1.84 X 105 rads/h. After irradiation, the grafted cellulose acetate strips were removed immediately from the monomer solutions and extracted with xylene for 24 h in a Soxhlet. The strips then were dried under vacuum for 24 h at room temperature and weighed. 

In both cases, the final waste (i.e. the spent fuel elements or the waste produced in reprocessing them) has to be stored permanently and safely somewhere. 

The main aim of reprocessing is the recovery of fissile and fertile material. If U or U-Pu mixtures are used as fuel, the fissile nuclides are and Pu, and the fertile nuclide is Reprocessing of these kinds of fuel closes the U-Pu fuel cycle. The U-Th fuel cycle is closed by reprocessing of spent fuel containing mixtures of U and Th. In the case of final storage of the spent fuel elements, the fuel cycle is not closed fissile and fertile nuclides are not retrieved for further use. 

The largest amounts of radioactivity are in the spent fuel elements, which represent high-level waste (HLW). For final disposal, the spent fuel elements may be taken apart and the fuel rods may either be caimed and put into casks or they may be cut into pieces and loaded into canisters and casks. 

Pii (about 2.2 kg), -" Pu (about 1.1 kg) and " Pu (about 0.4 kg) are generated by (n, y) reactions from Pii. Pu is a valuable nuclear fuel and may also be used for production of nuclear weapons. The global production rate of Pu in nuclear power reactors is of the order of 100 tons per year contained in spent fuel elements. Non-proliferation agreements should prevent uncontrolled distribution of Pu. Moreover, Pu is highly toxic. Am and Cm arc generated in smaller amounts in nuclear reactors by (n, /) reactions (about 0.15 kg Am and about 0.07 kg Cm per ton of spent fuel after a burn-up of 35 000 MW d per ton). 

As radiation sources y-ray emitters, such as Co or Cs, and electron accelerators are applied. Nuclear reactors and spent fuel elements have also been discussed as radiation sources. 

Disposal comprises the reprocessing of spent fuel elements i.e. the separation and recycling of unused uranium and of the plutonium produced in the reactor and the treatment and secure permanent storage of the radioactive fission products. 

Uranium-plutonium mixed oxides Uranium-plutonium mixed oxides (MOX) are becoming increasingly important, since plutonium is produced during the reprocessing of spent fuel elements. In these mixed oxide fuel elements a mixture of uranium(IV) and plutonium(IV) oxides with a plutonium content of 3 to 4% is utilized instead of ca. 4% 235u-enriched uranium(IV) oxide. Such fuel elements have similar nuclear physical properties to the standard elements with and can therefore be used in their place. 

U-Pu mixed oxides utili/e Pu isolated during the reprocessing of spent fuel elements. 

The spent fuel elements contain in addition to radioactive fission products considerable quantities of fissile uranium and plutonium, which are produced in the nuclear reactor. For a burn up, which gives the ratio of the energy produced with the nuclear fuel to the mass of heavy metals in the nuclear fuel, of e.g. 33,000 MWd per t uranium, a spent fuel, originally consisting of 3.2% 235y and 96.8% 238u, still contains 0.76% uranium, 0.9% plutonium (70% fissile) and about 3.5% of fission products. To recycle the not yet utilized and the bred plutonium, they have to be... 

The precedence of reprocessing spent fuel elements to dispose of the radioactive waste was abandoned in the middle of 1994. Since then direct permanent storage has been awarded equal legal precedence to a waste disposal option on the basis of reprocessing. It is expected that the electricity industry will make increasing use of this... 

Interim storage of spent fuel elements is initially carried out in pools of water at nuclear power stations... 

The essential functions of the reprocessing of spent fuel elements is to separate uranium and plutonium from one another and both of them from the radioactive fission products. For this purpose, the PUREX process (Plutonium and Uranium Recovery by Extraction), based on extractive separation, has become accepted worldwide. It is currently u,sed in all modern reprocessing plants. 

At the end of the 1970 s the civil reprocessing plant in the USA was closed down for political rea.sons. The spent fuel elements have been stored since then at interim storage sites. The only reprocessing plants operated in Western Europe, those in France and Great Britain, have successively expanded their capacities and have currently a joint capacity of 23001 heavy metal per year. 

The solid, liquid and gaseous radioactive waste (see marginal notes) produced during the reprocessing of spent fuel elements has to be safely stored, to prevent entry into the biosphere. 

The annual yield in vitrified high activity waste from the spent fuel elements of a 1000 MW nuclear power only amounts to 3 m-. ... 

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