Nuclear Fuel Life Cycle

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readily sublimes, that is, turns into gas from a solid state, bypassing the liquid one (Ferguson, 2011).

Uranium enrichment by gaseous diffusion is based on the phenomenon of molecular diffusion through a porous membrane with tiny holes. In an enclosed space at thermal equilibrium, all gas mixture molecules have the same kinetic energy. Less severe 235UF6 molecules have higher average speed of thermal motion and, therefore, are more likely to hit a barrier than the heavier molecules. As a result, lighter 235UF6 molecules penetrate (diffuse) through baffle holes more often. Accordingly, the heavy isotope molecules are concentrated before the barrier (Wilson, 1996). The gaseous diffusion method is extremely expensive, as it requires large areas for operation and a large number of fairly complex processes. In addition, the gaseous diffusion plants consume a lot of electricity.

As in any other technological process, during enrichment of uranium, there is some material loss. In particular, several tenths of percent of the total weight of enriched uranium remains inside the separation equipment and piping, accumulating as solids. When stopping and repairing the separation units, solid sediments are extracted from the process equipment. These losses are inevitable and are included beforehand (Hartigan, Hinderstein, Newman & Squassoni, 2015). Despite the fact that the product losses are small, they are significant in terms of radiological safety in the factories. Currently, these tailings come for storage until they can be used as a reproducing material in breeder reactors to produce plutonium.

IV. Fuel production

Enriched uranium is used as the starting material for the manufacture of nuclear reactor fuel. Nuclear fuel is used in the reactors as metals, alloys of carbide oxide, nitrides, and other fuel compositions, which are given a certain structural form. Constructional basis of the nuclear fuel in the reactor is a fuel element consisting of fuel and coatings. All fuel elements are structurally combined in a fuel assembly. Modern enterprises producing reactor fuel are: industrial complexes, the production cycle includes the following steps (Ferguson, 2011): obtaining uranium dioxide powder from the hexafluoride, manufacturing sintered pellets, preparation of tubular claddings and end pieces, packaging fuel pellets in the tube shells, end piece installation by sealing (welding), preparing and completing parts for fuel bundle assembly, removing defective fuel elements and assemblies, and the recycling of waste. The product at this stage of the fuel cycle, is nuclear fuel in a form suitable for direct use in a reactor.

The above production is the initial stage of the nuclear fuel cycle. Then, the fuel flows into the nuclear reactor and allows production of a predetermined amount of electricity. The processes taking place in a nuclear reactor are accompanied by burn-up of uranium nuclei and accumulation of fission products (new chemical elements) reproduction of plutonium. The fuel cycle does not end at the NPP; spent fuel assemblies must be unloaded from the reactor and placed in a cooling fuel pool to reduce the residual heat and radioactivity and then either safely and securely stored (open fuel cycle), or recycled (closed fuel cycle) (Wilson, 1996).

V. Nuclear Fuel Cycle after NPP

It is now hard to believe that, in the very first years after the birth of nuclear power engineering, almost all the radioactive wastes (RW) were thrown out almost like regular garbage. However, it is a nuclear waste problem, which for the first time was really recognized and tried to be dealt with correctly. The total global volume of radioactive waste compared to conventional waste is extremely small. During the year, about 300 tons of the world’s radioactive waste is being stored. If we add the waste of power plants of nuclear-powered submarines, etc., the total amount will be insignificant compared to the tens and hundreds of millions of tons of conventional waste (Hartigan, Hinderstein, Newman & Squassoni, 2015).

Storage of spent fuel

Burnt fuel elements just extracted from the reactor (using remote manipulators like a bridge crane) contain highly active isotopes. It is very dangerous to work with such material. Therefore, fuel elements are primarily directed to a cooling fuel pool (vault) available at each plant. There, they spend from 3 to 10 years, until the short-lived nuclides decay. After that, the activity of the spent nuclear fuel is determined by the fission products (FP) with a large decay time. Among them, the main contribution comes from strontium-90 (half-life T = 29.2 years), krypton-85 (10.8 years), technetium-99 (213 thousand years.) and cesium-137 (28.6 years). Besides the long-lived FP products, there are also transuranic elements like: neptunium, plutonium, americium, curium; as known, they are also radioactive with very long half-lives (tens and hundreds of thousands of years) (Wilson, 1996).

After ten (10) years of discharge activity of fuel elements, its source is reduced by about ten (10) times compared to what it was originally. After soaking in the pool, spent fuel can be placed in storage containers or be transported to the reprocessing plant to extract the remaining uranium and plutonium. For this purpose, aqueous dissolution technology is generally used and as a result, almost all radioactive waste becomes liquid (Ibid). It is pretty risky to keep it that way, even in special containers. After all, due to the remaining radionuclides, the fluid is constantly heated.

Radioactive waste activity will be negligible if it falls by, at least, six orders compared to the initial. It is easy to calculate that in 10 half-lives; it decreases 1024 times, and after 20, the same amount of time. This means that, for example, strontium and cesium should be stored under controlled conditions for 300-600 years. These huge periods may not cause doubt but, a situation in too distant future is too uncertain. Despite the complexity and high cost of processing and storage, the problem of radioactive waste cannot be considered solved completely. Not to mention the fact that the total waste or closed cycle are not met - the main method of disposal of dangerous products is still waiting for their spontaneous decay (Office of Coal, Nuclear, Electric, and Alternate Fuels, 1989).

Three categories of waste; storage and processing

Wastes are divided into three categories:

1) Waste type A - Has a short half-life (below 30 years) and weak radioactivity.

2) Waste type B - Also has a short half-life low radioactivity.

3)

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