Patent Number: 042241065
Section: summary

This invention relates to a nuclear fuel element consisting of ceramic material and designed in the form of plates which are specially adapted to the conditions prevailing in light water reactors. The fissile ceramic material constituting the fuel elements is divided into a number of small plates or wafers each provided with a thin independent cladding of metal alloy, said wafers being aligned in a number of rows in such a manner as to occupy the entire surface of the fuel plate. A fuel element of this type was described for example in U.S. Pat. No. 4,038,135 of July 26th, 1977 and results in particularly rugged and reliable designs. This fuel element is nevertheless subject to a disadvantage in that each fuel wafer has to be covered with thin metal foil which in turn calls for industrial developments of a somewhat complex nature. Moreover, there are some specific applications of plate-type nuclear fuels in which it is particularly advantageous to utilize the fuel in the form of very thin plates and especially plates having a thickness of less than 2 mm. This is the case, for example, when it is desired to equip a pool-type reactor core with fuel plates having a U-235 enrichment of less than 8% as will be explained hereinafter. In point of fact, the techniques developed in U.S. Pat. No. 4,038,135 of July 26th, 1977 do not readily lend themselves to the production of a plate element of small thickness. The present invention is precisely directed to a fuel element of small thickness which offers the advantage of permitting easier and more rapid industrial manufacture while at the same time retaining all the advantages attached to the plate-type nuclear fuel element disclosed in U.S. Pat. No. 4,038,135. The fuel element in accordance with the present invention is distinguished by the fact that a lateral covering is provided for each fuel wafer by means of a grid of thin wires fitted within a frame and each mesh of said grid has the shape and dimensions of a fuel wafer in order to serve as a housing for one of these latter. In accordance with another important characteristic feature of the present invention, the grid whose meshes serve to house the different fuel wafers of a fuel element is made up of thin wires of a metal having low neutron-absorption characteristics, said wires being on the one hand joined together by electric welding and on the other hand diffusion-bonded to the side plates and end plates constituting the frame proper as well as to the cladding plates. In an advantageous alternative embodiment of the invention, the ends of the wires are welded to a framing wire which extends on all four sides of the grid. In accordance with the invention, the wires constituting the grid have a circular cross-section of very small diameter which is preferably less than 2 mm and the grid meshes are preferably of either rectangular or square shape. The corresponding fuel wafers of the fuel element have a thickness which is within the range of 1 to 2 mm, for example, and is most commonly of the order of 1.5 mm. In a preferred alternative embodiment of the invention, the wires of the grid are of Zircaloy, but the grid, the side plates and end plates as well as the cladding plates can also be formed either wholly or partly of aluminum or of an aluminum alloy such as AG. Composite solutions--with some parts of Zircaloy and others of aluminum or aluminum alloy--are also of interest.