The present invention relates to a fuel assembly for a nuclear reactor of the type comprising a framework or structure including two endpieces connected together by elongated elements or tie rods such as guide tubes, and several spacing grids comprising an upper end grid, a lower end grid and intermediate grids, said spacing grids forming cells for holding a bundle of fuel elements maintained at the nodal points of a square network. Each of said fuel elements is provided with a metal sheath closed at each of its ends by a fluid-tight plug arranged to engage the corresponding end grid.
The invention is particularly suitable, although not exclusively for pressurized water nuclear reactors, so-called PWRs, for which the intermediate grids disposed along the assembly are formed of zirconium-based alloy called "Zircalloy" having a low absorption cross-section for neutrons and weak mechanical characteristics under radiation.
In the above described type of fuel assemblies the spacing grids fulfill several functions. They ensure structral protection of the assembly against transverse shocks, they permit the mixing of the coolant streams and they support laterally and brace the fuel elements containing the fissile material.
Two types of spacing grids are possible, knowning that they must permit differential movements between the elongated elements of the assembly structure and the fuel elements. Either the grid is connected to the fuel elements, or the grid is connected to the elongated elements belonging to the structure or framework.
The document FR-A-No. 2 088 009 shows a rigid framework formed of two parallel end pieces, braced by tie-rods regularly distributed at the nodal points of a regular network. The diameter of the tie-rods is substantially equal to the diameter of the fuel rods. The spacing grids are slidably and not rigidly connected to the tie rods. The fuel elements being in greater number than the tie-rods, the spacing grids move with the fuel elements and, due to the expansion of the fuel elements which is different from the expansion or dilatation of the tie-rods during operation of the reactor, slide over the tie rods. The described assembly comprises abutment sleeves coaxial with the tie-rods between the different spacing grids for limiting their movements beyond predetermined limits. This type of assembly presents several drawbacks:
The upper grids sliding over the tie-rods can become too great if the fuel elements elongation reach consequent values; the guidance and support of the bundle of fuel elements risks then to be jeopardized and wear of the tie-rods will occur to much;
it is not possible to use "zircalloy" (of weak mechanical characteristics) as a material for the spacing grids in the mid-portion of the assembly;
the fuel elements rest on the assembly lower end piece which constitutes an abutment support for said fuel elements but in no case allows sufficient clamping for enabling axial support of the fuel elements.