Reprocessing of Spent Fuel Elements

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. 

The Purex process was developed between 1945 and 1949 in the USA for military purposes and since 1954 has been operated industrially in more than 10 countries in reprocessing plants of various sizes. Decades of experience with this process exist in the USA, Great Britain and France. In the Federal Republic of Germany operational 

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. 

With the plants UP 2-800 and UP 3 (each with a capacity of 800 t/a) in France and Thorp (with 700 t/a) in Great Britain, there is sufficient capacity in Europe to reprocess the waste of 100 nuclear power stations and thereby to cover the total demand for the next 30 to 40 years. Japan disposes over an operating pilot plant and an industrial plant has been in the planning phase for a number of years. 

Purex process The actual reprocessing process begins with the cutting up of the fuel elements taken from entry basins. This can be carried out in two process variants cutting up in ca. 5 cm long pieces with rod shears, whereby initially the head pieces are separated off and the individual fuel rods withdrawn from the rod bundle, or direct cutting with hydraulic bundle shears. 

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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). 

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 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. 

Homogeneous solutions of fissile nuclides are produced in the reprocessing of spent fuel elements, where care must be exercised that the critical size is not exceeded in any equipment or container in order to prevent an accidental chain reaction. Several such accidents have occurred in the past in fuel production and reprocessing plants in which very high doses were received by nearby personnel even though the duration of the chain reaction usually was very short and a violent explosion never occurred. 

Sulfuric Acid 6.0M Boiling Boiling — U — — 24 hrs.. processes for the aqeuous reprocessing of spent fuel elements Sulfex and Thorex processes... 

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. 

High-level waste that contains highly radioactive material, including fission products, traces of uranium and plutonium, and other transuranic elements resulting from the chemical reprocessing of spent fuel... 

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. 

During the eight years that the Babcock ft Wilcox Test Reactor (BAWTR) operated, five batches of spent fuel elements were shipped to the reprocessing plant at Savannah River. A typical batch contained 25 fuel elements (10 kg (rf U), which were shipped in a lead cask. 

Development efforts in the nuclear industry are focusing on the fuel cycle (Figure 6.12). The front end of the cycle includes mining, milling, and conversion of ore to uranium hexafluoride enrichment of the uranium-235 isotope conversion of the enriched product to uranium oxides and fabrication into reactor fuel elements. Because there is at present a moratorium on reprocessing spent fuel, the back end of the cycle consists only of management and disposal of spent fuel. 

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... 

Interest in the reprocessing of spent nuclear fuels has prompted studies of the solution photochemistries of Np and Pu. A recent review summarizes the photoredox behavior of the various oxidation states of these elements.169... 

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. 

The CTH actinide separation process was developed as a possible means to reduce the expected long term dose to man from a geologic repository containing solidified radioactive waste from the reprocessing of spent nuclear fuel The distribution data for the elements present in significant amounts in the high level liquid waste (HLLW) from a Purex plant, the general principles and the flowsheet have been described in detail elsewhere A... 

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... 

The separation of actinides has been studied for various purposes in Japan Atomic Energy Research Institute (JAERI). The works which have been carried out so far, are classified into four categories preparation studies of actinides nuclides, separation chemistry for chemical analysis, separation of actinides from radioactive waste, and studies on reprocessing of spent nuclear fuels. The present work is to review studies of actinide separation performed in JAERI, emphasizing the need of the separation for the main purpose of individual. Concern is focussed on the separation of transuranium elements and studies on thorium and uranium are put aside. 

The 1960s were marked by increasing interest of nuclear industry in volatile compounds of metals. Not to mention UF6, which had been exploited from the mid-1940s. Much effort was devoted to developments in the fluoride reprocessing of spent nuclear fuel. At that time, transition metals like Zr, Nb and Ta found many applications in nuclear industry. Some technologies for the extraction of these elements from ores and for the production of pure metals were based on the use of... 

Most of the present nuclear reactors have been burning solid fuel elements of either normal or enriched uranium. Thus far, it has been necessary to reprocess fuel in order to recover valuable fissionable or fissile material. It is possible that fuel elements will be developed for future reactors which can be burned to the point where it is not economically justifiable to recover fissionable materials. Obviously, this depends upon the value of these materials. Such a procedure would provide an optimum solution to the major part of the waste disposal problem. The fission products would still be locked in the fuel element, simple disposal techniques could be employed, and in fact, spent fuel elements would probably have secondary uses as radiation sources. 

Spent fuel elements contain appreciable amounts of tritium, partly produced by fission, partly by other nuclear reactions. About half of the tritium is released from the fuel upon dissolution. The rest is bound to the zircaloy of the hulls and is disposed of with them. The fraction of tritium that is released exchanges with water, forming HTO. The total annual input of tritium in a 1400 MT/year reprocessing plant is about 10 Ci. In West Germany a reprocessing plant of this size is supposed to retain 75 to 80 percent. 

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