Nuclear energy fuel elements

Olander, D.R. Fundamental Aspects of Nuclear Reactor Fuel Elements, chap. 17. Technical Information Center Energy Research and Development Administration, Springfield (1976)... 

The absorption of radiation leads to an increase in die tenqierature of the absorber. An exanqile of this is the absorption of the kinetic energy of fission products in nuclear reactor fuel elements which is a main source of the heat production in reactors. The absorption of decay energy of radioactive nuclides in appropriate absorbing material can be used in a similar - albeit more modest - way as an energy source. 

Plutonium-239 is a fissile element, and vvill split into fragments when struck by a neutron in the nuclear reactor. This makes Pu-239 similar to U-235, able to produce heat and sustain a controlled nuclear reaction inside the nuclear reactor. Nuclear power plants derive over one-third of their power output from the fission of Pu-239. Most of the uranium inside nuclear fuel is U-238. Only a small fraction is the fissile U-235. Over the life cycle of the nuclear fuel, the U-238 changes into Pu-239, which continues to provide nuclear energy to generate electricity. 

In the field of nuclear energy, titanium has been used for processing of fuel elements, where this demands use of nitric acid or aqua regia ", and for control-rod mechanism, in which the short half-life of irradiated titanium is of advantage. 

Nuclear fuels, like chemical fuels, must be purified to be most effective. The purification of the seventh-row elements has presented some fascinating and difficult problems of chemistry— so difficult, in fact, that chemists have played as big a role in the development of nuclear energy as have physicists. 

Fissile materials are defined as materials that are fissionable by nentrons with zero kinetic energy. In nuclear engineering, a fissile material is one that is capable of snstaining a chain reaction of nuclear fission Nuclear power reactors are mainly fueled with manium, the heaviest element that occurs in natnre in more than trace qnantities. The principal nuclear energy soiuces are maninm-235, plutonium-239, uranium-233 and thorium. 

Fuel Element. Any material which may undergo the appropriate reaction and be the source of energy in a fusion or fission nuclear reactor is known as nuclear fuel At the present state of technology such fuels are uranium, thorium or plutonium, either as natural materials or enriched or synthesized isotopes of these elements. They are used as solns or as solids (metals, oxides, or carbonates) shaped as plates, rods etc and known as fuel element (fuel plate or fuel rod)... 

The fission neutrons at birth have energies of approximately 1 to 2 MeV In a thermal reactor the neutron energy is rapidly reduced through collisions with light nuclei to thermal (—.02 to 1 eV), to promote for more efficient capture. Besides the nuclear fuel, there are many other materials in the reactor core also competing for the neutrons, including the moderator (the material used to slow down or thermalize the neutrons), fertile nuclides that produce additional fissile material (discussed in a later section), neutron poisons present in control rods, the coolant, fuel element cladding, and other structural materials. 

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- -butyl phosphate (TBP) in kerosene. First operated by the U.S. Atomic Energy Commission at its Savannah River plant, SC, in 1954 and at Hanford, WA, in 1956. Now in operation, with modifications, in several countries. Sites include Savannah River (SC), Cap de la Hague (France), Marcoule (France), Sellafield (England), Karlsruhe (Germany), and Trombay (India). See also Recuplex. 

As the disintegration rate of the fission products with ti/2 > 1 s is about five times the rate of fission, the activity of the fuel several seconds after shutting off the reactor is xl7 10 Bq (a 5 10 Ci) per MW of thermal energy produced. The p activity per MW and the heat production of the fission products are plotted in Fig. 11.19 as a function of the time after shutting off the reactor. The heat production requires cooling of the fuel elements, because melting of the fuel and volatilization of fission products may occur under unfavourable conditions. produced by the nuclear reactions U(n, y) U(n, and U(n, 2n) U causes a relatively high initial activity of uranium. As decays with a half-life of 6.75 d ... 

Reactions with fast neutrons, such as (n, 2n), (n, p) and (n, a) reactions, are only of minor importance for production of radionuclides in nuclear reactors. However, special measures may be taken for irradiation of samples with high-energy neutrons. For instance, the samples may be irradiated in special fuel elements of ring-like cross section as shown in Fig. 12.1, or they may be irradiated in a receptacle made of enriched uranium. In both cases, the fast neutrons originating from the fission of enter the samples directly and their flux density is higher by about one order of magnitude than that at other places in the reactor. 

Suripto, A., Yuwono, I., Badruzzaman, M., Nasution, H., Kusnowo, A., and Amini, S.S., Proceedings of the Second Scientific Presentation on Nuclear Fuel Cycle Nuclear Fuel Element Development Centre, National Atomic Energy Agency, 1998 89-94. 

In nuclear power stations electrical current is produced from nuclear energy. Efficient operation requires provision of the nuclear power station with fuel elements and the disposal of spent materials. These operations are brought together in the nuclear fuel cycle, which embraces on the provision side the extraction and dressing of uranium ores to uranium concentrates, their conversion to uranium(VI) fluoride, enrichment of the uranium isotope from 0.7% in natural uranium to ca. 3%, the conversion of uranium(VI) fluoride into nuclear fuel and the production of 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... 

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