Nuclear Fuel Life Cycle
Essay Preview: Nuclear Fuel Life Cycle
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Outline
Introduction
Uranium mining and processing
Uranium enrichment
Fuel production
Nuclear Fuel Cycle after NPP
Storage of spent fuel
Three categories of waste; storage and processing
Introduction
According to experts, at present, there is no alternative to nuclear power plants in terms of electricity production with minimal impact on the natural environment. A Nuclear Power Plant (NPP) is a huge complex including a nuclear reactor and associated equipment and it is designed to convert nuclear energy into electricity. In the NPP, fuel arrives in the form of structural units known as fuel assemblies. These fuel assemblies arrive and are ready to be placed into the reactor. Prior to the arrival at the plant, it must pass a number of technological processes at the enterprises of the fuel and energy complex. The nuclear fuel cycle is a whole sequence of repetitive production processes, from fuel production (including power generation) ending with the disposal of radioactive waste. Depending on the type of fuel and the specific conditions, the nuclear fuel cycles may vary in detail, but the overall concept is preserved. Nuclear fuel for reactors is uranium. Therefore, all the stages and processes of the nuclear fuel cycle are determined by the physic-chemical properties of this element.
Uranium mining and processing
The initial stage of the fuel cycle is manufacturing, where uranium ore is mined from the earth. The average uranium content in the earths crust is fairly high and is regarded as 75 * 10-6. Uranium is worth about 1000 times greater than gold and 30 times greater than silver. Uranium ores have extremely diverse compositions. In most cases, the uranium in the ore is presented not only by one but, by several mineral formations. There are about 200 uranium and uranium-bearing minerals. Uraninite, pitchblende, uranium black, and other minerals have the greatest practical importance. Uranium mining, as well as other minerals, is carried out mainly by ordinary mining or pit methods, depending on the depth of bedding. In recent years, they began to use in-situ leaching method. This allows the elimination of the actual mining and carrying out the extraction of uranium from ore on the spot of its formation. (Ferguson, 2011).
For example, in order to produce 1t of uranium with U3O8content of 0.1%, it is necessary to remove approximately 1,000 tons of ore from the depths, not counting the huge quantities of waste rock. This huge mass of ore is best to process and enrich uranium in the immediate vicinity of the mine. This will reduce the load of transport and greatly reduce transportation costs. Therefore, usually hydrometallurgical plants are located in the immediate vicinity of the open pit (Hartigan, Hinderstein, Newman & Squassoni, 2015).
The extracted uranium ore contains ore minerals and gangue. The goal is to obtain the greatest amount of ore possible. The hard part of the process is being able to separate the minerals from the gangue and obtain the chemical uranium concentrates. Required steps in the preparation of uranium concentrates are crushing, grinding, and leaching of the ore source. Very often, before leaching, the ore is enriched, increasing the uranium content by various physical methods. At all stages of the processing of uranium ore, there is a certain purification of uranium from accompanying impurities.
However, thorough cleaning cannot be achieved. Some concentrates contain only 60-80% as others may hold up to 95- 96% of the uranium oxide. The remaining percentages consist of various impurities. This uranium is not suitable for nuclear fuel. The following mandatory stage of the nuclear fuel cycle is refining, which includes the final cleaning of uranium compounds from impurities, and especially from the elements with a large neutron capture cross section (hafnium, boron, cadmium, etc.) (Wilson, 1996).
Uranium enrichment
Modern nuclear power industry with thermal neutron reactors is based on low-enriched uranium fuel (2-5%). The fast neutron reactors use uranium with even higher content of uranium-235 (up to 93%). Therefore, before producing the natural uranium fuel containing only 0.72% of uranium-235, it is necessary to enrich it. The goal of the final product is to be able to split isotopes of uranium-235 and uranium-238 with the end game of production of electricity. Chemical reactions are too insensitive to the atomic mass of the reacting elements. Therefore, they cannot be used for uranium enrichment; physical methods of isotope separation are needed (Ferguson, 2011). The main methods used for isotope separation are:
electromagnetic separation, gaseous diffusion, liquid thermal diffusion, gas centrifuge,
aerodynamic separation, chemical processing, distillation, and electrolysis.
Currently, a major and until recently, the only industrial production method of enriched uranium was gas diffusion. This method uses the difference in the velocities of different weight of the gas molecules. The substance must be in gaseous state in order for the diffusion to occur. This process uses a collection tube and at different speeds, molecules move through the tube. The lighter and faster molecules overtake the heavier ones. For this purpose, the tube must be thin enough to let molecules move through it alone. Thus, the key point here is the production of porous membranes for separation. They should not allow leakage and also withstand overpressure. For some light elements, the degree of separation may be large enough, but for uranium, it is only 1.00429 (output stream of each stage is enriched in 1.00429 times). Therefore, gaseous diffusion enrichment plants are huge in size and composed of thousands of stages of enrichment (Hartigan, Hinderstein, Newman & Squassoni, 2015).
In 1980, gaseous diffusion plants were responsible for as much as 98% of the total enrichment capacity. In recent years, competing centrifugal method is gaining ground, based on the use of high-speed gas centrifuges. Both methods use uranium in the form of UF6 hexafluoride. Uranium hexafluoride has interesting and important physical properties. Firstly, UF6 is the only uranium substance existing at ordinary temperature but under reduced pressure in a gaseous state. Secondly, uranium hexafluoride under normal conditions