Patent Number: 039403149
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a nuclear reactor fuel element which consists of a multiplicity of fuel rods, disposed or held in spaced relation by spacers arranged in several planes, and fastened to at least one support plate. Each fuel rod has a tubular casing which contains the nuclear fuel, for instance, in the form of pellets. Next to the fuel rods, these spacers constitute the most essential structural units within the fuel elements because by means of these spacers the fuel rods are maintained in a predetermined spacial relation. This is necessary so that over the entire life of the nuclear reactor, a completely uniform cooling of the fuel rods and the fuel elements is achieved by the reactor coolant. The reactor coolant may be either a gas or a liquid. 2. Description of the Prior Art It is known that spacers automatically interact with the fuel rods and the latter, in turn, interact with the tubular casings. For this reason, the type of material and the wall thicknesses of the parts in contact with each other must be matched to each other, taking into consideration the mechanical forces which may occur. Thus, the wall thicknesses of the tubular casing should be so large that mechanical damage to the tubular casings cannot occur. On the other hand, the total amount of structural material within the active zones of the reactor core should be kept as low as possible for reasons of neutron efficiency. In addition, the problems of heat removal and creep at the operating temperature of the reactor are important considerations. SUMMARY OF THE INVENTION The object of this invention is to find the optimum fuel element design which meets the foregoing requirements. According to the invention, this is achieved by designing the wall thickness of the tubular casings for the fuel rods without regard to the mechanical stress produced by the spacers and enlarging it according to the height of the spacers. Using this design criteria, it is possible to achieve the optimum design of the wall thickness of the tubular casing only on the basis of the operating conditions of the nuclear reactor, such as pressure, temperature, operational cycles and fuel expansion, as a function of fission gas pressure, swelling of the fuel, creep behavior of the material, etc. The mechanical stresses produced by the spacers are only taken into consideration at the contact points between the spacers and the tubular casings at the height of the spacers. At these points, the wall thickness of the tubular casings is larger. By using fuel elements of this design, instead of conventional fuel elements, substantially less tubular-casing material, such as Zircaloy, needs to be built into the nuclear reactor. As a result, losses caused by this material through neutron absorption can be reduced. This design criteria applies to practically all types of nuclear reactors. To illustrate this further, a fuel element for a heavy-water cooled nuclear reactor will be described in more detail. Such a fuel element has a circular cross section and is used in the reactor core within so-called separation tubes. The idea of the invention shown here can, of course, also be applied to fuel elements of other cross sections and geometry. So-called guide ducts are sometimes attached to the tubular casings of the fuel rods at the height of the spacers. Attachment of such guide ducts by means of soldering or welding always involves a heat treatment, which may in some cases bring with it the danger of premature defects in the material of the tubular casing due to metallurgical changes. The present invention provides a way of avoiding such defects and other influences on the structure of the tubular casings when such guide ducts are attached. The spacers can be brought into contact directly at the fuel rods, i.e., at their reinforced points, which facilitates a relatively simple design. Likewise, mechanical stress on the tubular casings when the fuel rods are inserted into or moved in the spacers is practically eliminated.