Refueling Options, Considerations for Liquid-Salt-Cooled VHTRs
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The fuel elements in an RBMK are 3 m long each, and two of these sit back-to-back on each fuel channel, pressure tube. Following the Chernobyl accident, the enrichment of fuel was changed from 2. CerMet fuel consists of ceramic fuel particles usually uranium oxide embedded in a metal matrix. It is hypothesized [ by whom? This fuel has high heat transport characteristics and can withstand a large amount of expansion. Plate-type fuel has fallen out of favor over the years. Plate-type fuel is commonly composed of enriched uranium sandwiched between metal cladding.
Plate-type fuel is used in several research reactors where a high neutron flux is desired, for uses such as material irradiation studies or isotope production, without the high temperatures seen in ceramic, cylindrical fuel. Sodium-bonded fuel consists of fuel that has liquid sodium in the gap between the fuel slug or pellet and the cladding. This fuel type is often used for sodium-cooled liquid metal fast reactors. The fuel slug may be metallic or ceramic. The sodium bonding is used to reduce the temperature of the fuel.
These concerns became more prominent after the Fukushima Dai-ichi disaster in Japan, in particular regarding Light Water Reactor LWR fuels performance under accident conditions. The aim of the research is to develop nuclear fuels that can tolerate loss of active cooling for a considerably longer period than the existing fuel designs and prevent or delay the release of radionuclides during an accident. Used nuclear fuel is a complex mixture of the fission products , uranium , plutonium , and the transplutonium metals.
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In fuel which has been used at high temperature in power reactors it is common for the fuel to be heterogeneous ; often the fuel will contain nanoparticles of platinum group metals such as palladium. Also the fuel may well have cracked, swollen, and been heated close to its melting point. Despite the fact that the used fuel can be cracked, it is very insoluble in water, and is able to retain the vast majority of the actinides and fission products within the uranium dioxide crystal lattice.
Two main modes of release exist, the fission products can be vaporised or small particles of the fuel can be dispersed. It has several purposes.
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It is known that by examination of used fuel that the failure modes which occur during normal use and the manner in which the fuel will behave during an accident can be studied. In addition information is gained which enables the users of fuel to assure themselves of its quality and it also assists in the development of new fuels. After major accidents the core or what is left of it is normally subject to PIE to find out what happened.
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Materials in a high-radiation environment such as a reactor can undergo unique behaviors such as swelling  and non-thermal creep. If there are nuclear reactions within the material such as what happens in the fuel , the stoichiometry will also change slowly over time. These behaviors can lead to new material properties, cracking, and fission gas release.
The thermal conductivity of uranium dioxide is low; it is affected by porosity and burn-up. The burn-up results in fission products being dissolved in the lattice such as lanthanides , the precipitation of fission products such as palladium , the formation of fission gas bubbles due to fission products such as xenon and krypton and radiation damage of the lattice.
The low thermal conductivity can lead to overheating of the center part of the pellets during use. The porosity results in a decrease in both the thermal conductivity of the fuel and the swelling which occurs during use. According to the International Nuclear Safety Center  the thermal conductivity of uranium dioxide can be predicted under different conditions by a series of equations.
The bulk density of the fuel can be related to the thermal conductivity. Then the thermal conductivity of the porous phase K f is related to the conductivity of the perfect phase K o , no porosity by the following equation. Note that s is a term for the shape factor of the holes. Rather than measuring the thermal conductivity using the traditional methods such as Lees' disk , the Forbes' method , or Searle's bar , it is common to use Laser Flash Analysis where a small disc of fuel is placed in a furnace.
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After being heated to the required temperature one side of the disc is illuminated with a laser pulse, the time required for the heat wave to flow through the disc, the density of the disc, and the thickness of the disk can then be used to calculate and determine the thermal conductivity. For details see . The terms atomic battery , nuclear battery and radioisotope battery are used interchangeably to describe a device which uses the radioactive decay to generate electricity. These systems use radioisotopes that produce low energy beta particles or sometimes alpha particles of varying energies.
Low energy beta particles are needed to prevent the production of high energy penetrating bremsstrahlung radiation that would require heavy shielding. Radioisotopes such as plutonium , curium , curium and strontium have been used. Tritium , nickel , promethium , and technetium have been tested. There are two main categories of atomic batteries: thermal and non-thermal.
The non-thermal atomic batteries, which have many different designs, exploit charged alpha and beta particles. These designs include the direct charging generators , betavoltaics , the optoelectric nuclear battery , and the radioisotope piezoelectric generator. The thermal atomic batteries on the other hand, convert the heat from the radioactive decay to electricity.
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These designs include thermionic converter, thermophotovoltaic cells, alkali-metal thermal to electric converter, and the most common design, the radioisotope thermoelectric generator. A radioisotope thermoelectric generator RTG is a simple electrical generator which converts heat into electricity from a radioisotope using an array of thermocouples. It has a half-life of Some Russian terrestrial RTGs have used 90 Sr ; this isotope has a shorter half-life and a much lower energy density, but is cheaper.
Early RTGs, first built in by the U. Atomic Energy Commission , have used Po. This fuel provides phenomenally huge energy density, a single gram of polonium generates watts thermal but has limited use because of its very short half-life and gamma production, and has been phased out of use for this application.
Radioisotope heater units normally provide about 1 watt of heat each, derived from the decay of a few grams of plutonium This heat is given off continuously for several decades. Their function is to provide highly localised heating of sensitive equipment such as electronics in outer space.
The Huygens probe to Titan contains 35 devices. Fusion fuels include deuterium 2 H and tritium 3 H as well as helium-3 3 He. Many other elements can be fused together, but the larger electrical charge of their nuclei means that much higher temperatures are required.
Only the fusion of the lightest elements is seriously considered as a future energy source. Fusion of the lightest atom, 1 H hydrogen , as is done in the Sun and stars, has also not been considered practical on Earth. Although the energy density of fusion fuel is even higher than fission fuel, and fusion reactions sustained for a few minutes have been achieved, utilizing fusion fuel as a net energy source remains only a theoretical possibility.
Deuterium and tritium are both considered first-generation fusion fuels; they are the easiest to fuse, because the electrical charge on their nuclei is the lowest of all elements. The three most commonly cited nuclear reactions that could be used to generate energy are:. Second-generation fuels require either higher confinement temperatures or longer confinement time than those required of first-generation fusion fuels, but generate fewer neutrons.
Neutrons are an unwanted byproduct of fusion reactions in an energy generation context, because they are absorbed by the walls of a fusion chamber, making them radioactive. They cannot be confined by magnetic fields, because they are not electrically charged. This group consists of deuterium and helium The products are all charged particles, but there may be significant side reactions leading to the production of neutrons.
Third-generation fusion fuels produce only charged particles in the primary reactions, and side reactions are relatively unimportant. Since a very small amount of neutrons is produced, there would be little induced radioactivity in the walls of the fusion chamber. This is often seen as the end goal of fusion research.
However, there are no significant natural sources of this substance on Earth. Another potential aneutronic fusion reaction is the proton- boron reaction:. Under reasonable assumptions, side reactions will result in about 0. With keV, the optimum temperature for this reaction is nearly ten times higher than that for the pure hydrogen reactions, the energy confinement must be times better than that required for the D-T reaction, and the power density will be times lower than for D-T.
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