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Jiang, Wen; Hales, Jason D.; Spencer, Benjamin W.; Collin, Blaise P.; Slaughter, Andrew E.; Novascone, Stephen R.; Toptan, Aysenur; Gamble, Kyle A.; Gardner, Russell (2021). "TRISO particle fuel performance and failure analysis with BISON". Journal of Nuclear Materials. 548: 152795. doi:10.1016/j.jnucmat.2021.152795. S2CID 234004805.
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Tristructural-isotropic (TRISO) fuel particle is a type of micro-particle fuel. A particle consists of a kernel of UOX fuel (sometimes UC or UCO), which has been coated with four layers of three isotropic materials deposited through fluidized chemical vapor deposition (FCVD). The four layers are a porous buffer layer made of carbon that absorbs fission product recoils, followed by a dense inner layer of protective pyrolytic carbon (PyC), followed by a ceramic layer of SiC to retain fission products at elevated temperatures and to give the TRISO particle more structural integrity, followed by a dense outer layer of PyC. TRISO particles are then encapsulated into cylindrical or spherical graphite pellets. TRISO fuel particles are designed not to crack due to the stresses from processes (such as differential thermal expansion or fission gas pressure) at temperatures up to 1600 °C, and therefore can contain the fuel in the worst of accident scenarios in a properly designed reactor. Two such reactor designs are the prismatic-block gas-cooled reactor (such as the GT-MHR) and the pebble-bed reactor (PBR). Both of these reactor designs are high temperature gas reactors (HTGRs). These are also the basic reactor designs of very-high-temperature reactors (VHTRs), one of the six classes of reactor designs in the Generation IV initiative that is attempting to reach even higher HTGR outlet temperatures.
TRISO fuel particles were originally developed in the United Kingdom as part of the Dragon reactor project. The inclusion of the SiC as diffusion barrier was first suggested by D. T. Livey.[1] The first nuclear reactor to use TRISO fuels was the Dragon reactor and the first powerplant was the THTR-300. Currently, TRISO fuel compacts are being used in some experimental reactors, such as the HTR-10 in China and the high-temperature engineering test reactor in Japan. Spherical fuel elements utilizing a TRISO particle with a UO2 and UC solid solution kernel are being used in the Xe-100 in the United States.
TRISO (from English TRistructural-ISOtropic ) is a form of nuclear fuel processing that consists of triple-jacketed pac-beads . In the center is a core of uranium(IV) oxide , or a uranium/ thorium mixed oxide , which is followed by a porous buffer layer with an inner layer of isotropic pyrographite , then a layer of high strength silicon carbide and finally an outer layer coated with isotropic pyrographite. The core of the German variant has a diameter of 0.5 mm, the entire particle is 0.91 mm in size.[2]
The additional, innermost carbon layer is porous and provides expansion volume for the absorption of fission products; the two pyrographite layers ensure gas tightness.[3]
TRISO was developed around 1970 in Great Britain for the Dragon high-temperature reactor ( 1967-1975), the inventor is considered to be DT Livey.[4] In Germany it was used in the AVR (Jülich) from 1981 , but not in the THTR-300 . The TRISO particles are clearly superior to the older, double-coated BISO particles with regard to particle breakage caused by radiation.[5] On the other hand, the effect of TRISO silicon carbide as a diffusion barrier for some nuclides such as cesium-137 and silver-110m at higher temperatures is unsatisfactory - even in comparison with BISO particles.[6] Therefore, only maximum working temperatures of 750 °C are currently envisaged for high-temperature reactors with TRISO fuel, and the planned application of TRISO fuel for high-temperature process heat generation (950-1000 °C) has been postponed.
Further development is currently only taking place in the USA.[7] In tests there, a short-term temperature resistance of the coatings of 1800 °C was achieved.
The fuel used in HTGRs is coated fuel particles, such as TRISO[12][13][14][15] fuel particles. Coated fuel particles have fuel kernels, usually made of uranium dioxide, however, uranium carbide or uranium oxycarbide are also possibilities. Uranium oxycarbide combines uranium carbide with the uranium dioxide to reduce the oxygen stoichiometry. Less oxygen may lower the internal pressure in the TRISO particles caused by the formation of carbon monoxide, due to the oxidization of the porous carbon layer in the particle.[16] The TRISO particles are either dispersed in a pebble for the pebble bed design or molded into compacts/rods that are then inserted into the hexagonal graphite blocks. The QUADRISO fuel[17] concept conceived at Argonne National Laboratory has been used to better manage the excess of reactivity.
The basic design of pebble-bed reactors features spherical fuel elements called pebbles. These tennis ball-sized pebbles (approx. 6.7 cm or 2.6 in in diameter) are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro-fuel particles called tristructural-isotropic (TRISO) particles. These TRISO fuel particles consist of a fissile material (such as 235U) surrounded by a ceramic layer coating of silicon carbide for structural integrity and fission product containment. In the PBR, thousands of pebbles are amassed to create a reactor core, and are cooled by a gas, such as helium, nitrogen or carbon dioxide, that does not react chemically with the fuel elements. Other coolants such as FLiBe (molten fluoride, lithium, beryllium salt)[18]) have also been suggested for implementation with pebble fuelled reactors.[citation needed] Some examples of this type of reactor are claimed to be passively safe.[19]
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