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fuel element[′fyül ‚el·ə·mənt]
in nucleonics, a basic structural subassembly of a nuclear reactor. The fuel elements contain the nuclear fuel and are arrayed in the core of the reactor. The fission reaction occurs in the fuel elements, and the heat released in the reaction is carried away by the reactor coolant. The material constituting the core of a fuel element is hermetically sealed in a cladding.
The core of a fuel element contains a fissionable material, such as 233U, 235U, or 239Pu, and may also contain a fertile material, such as 238U or 232Th, which can be transformed into nuclear fuel. The material used in the core of a fuel element may be in the form of a metal, a cermet, or a ceramic. Metal fuel elements are made from pure U, Th, or Pu or from alloys of these metals with such other metals as Al, Zr, Cr, and Zn. Cermet fuels may be obtained, for example, from U and Al by pressing mixtures of powders of these metals. Ceramic fuels are sintered or fused oxides or carbides, for example, UO2 or ThC2.
The material used in the core of a fuel element must meet high requirements with respect to mechanical strength and the inalterability of physical properties and geometric dimensions at high temperatures and under intense neutron and gamma radiation. These requirements are best met by cermet, ceramic, and alloy cores. A large portion of such cores consists of filler material whose atoms do not take part in the fission process or in the production of nuclear fuel. For this reason, the fuel used in such cores is highly enriched; for example, the fuel may have a 235U content of 10 percent or more. The filler material generally has a small neutron-absorption cross section. Sometimes, however, the core material includes small amounts of metals, such as Mo, that strongly absorb neutrons. Such metals are added to the core material if increased resistance to thermal and radiation effects will result.
In the common power reactors using slightly enriched U, the cores of the fuel elements are most often made of sintered UO2. Such cores do not deform under high fuel burnup. Moreover, UO2 does not react with water. Thus, a cladding failure in a water-cooled reactor does not result in U entering the coolant.
The cladding of a fuel element must reliably separate the fuel material from the reactor coolant. If the integrity of the cladding is not maintained, the following may result: entry of fission products into the coolant; activation of the coolant; increased difficulty of servicing the reactor; and, in a number of cases, chemical reaction of the coolant with the fuel material. This chemical reaction may lead to “erosion” of the fuel element’s core and the core’s loss of its required shape. For these reasons, strict requirements are placed on the cladding material. It must possess high resistance to corrosion, erosion, and heat, must have high mechanical strength, and should not substantially alter the nature of neutron absorption in the reactor.
The most commonly used materials in claddings are stainless steel and alloys of Al and Zr. Alloys of Al are used in reactors with a reactor core temperature < 250°–270°C. Alloys of Zr are used in power reactors operating at temperatures of 350°–400°C. Stainless steel, which absorbs neutrons rather strongly, is used in reactors with temperatures > 400°C. Other materials, such as high-density graphite, are also employed in a number of cases.
To improve heat transfer between the fuel material and the cladding, a thermal bond is provided. In the case of metal fuel materials, the cladding is fused to the core of the fuel element. In other cases, a gas that conducts heat well, such as He, is introduced into the space between the fuel material and the cladding. Such a space is necessary when the fuel material and the cladding material have substantially different coefficients of volume expansion.
The shape of the core determines the structural design of the fuel element. Cylindrical, or rod, types are most common, but other types are also used, including tubes and plates. After being combined into clusters of various kinds, the fuel elements are placed in the reactor. In reactors with a solid moderator, the fuel elements or clusters of fuel elements are arrayed within the moderator in channels, through which the coolant flows. In reactors where the moderator is a fluid and also serves as the coolant, the clusters of fuel elements channel the flow of the fluid.
The principal measure of the effectiveness of a fuel element is the fuel burnup. In power reactors, the burnup reaches 30 mega watt-days per ton. The life of fuel elements in power reactors is up to three years. Used fuel elements may be reprocessed to recover unconsumed fuel and the newly produced fissionable material.
S. A. SKVORTSOV