Patent Number: 047175338
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to FIG. 1, those components relating to the invention of a nuclear fuel assembly are shown. The fuel assembly comprises an upper end piece 10 and a lower end piece 12 connected together by elongated cylindrical elements 14 and 15. The elongated elements 14 and 15 are fixed to a plurality of grids 16 spaced apart along the assembly. The grids hold the fuel rods in a regular polygonal network. The assembly as shown has a hexagonal overall cross-section and the grids hold the fuel rods 18 in position at locations distributed at the apexes of equilateral triangles whose sides are parallel to the side plates which form the girdle or frame 20 of grid 16. Some locations of the network are not occupied by rods 18 but by the elongated elements 14 which may form guide tubes. Other elongated elements 15, situated at the periphery of the network, are fixed to the girdles 20 so as to form a framework of the assembly. The elements 15 may be rods or tubes. Each grid 16 comprises a plurality of mutually parallel beds of wires for holding the fuel rods in position and spacing them, fixed to girdle 20 and offset in the longitudinal direction of the assembly. As shown in FIG. 1, grid 16 has two beds of wires, perpendicular to the longitudinal axis of the assembly. Each bed consists of two series of mutually parallel wires. The wires of one of the series forming bed 24 are at 60.degree. from the wires of the two series of bed 22. The wires of the other series of bed 24 are therefore parallel to the wires of one of the series of bed 22. In the embodiment shown in FIGS. 2 and 3, the girdle is formed from flat plates to which are fixed the two beds of wires which are either undulating or are tensioned so as to be better applied against the rods and thus generate holding forces. The wires of the two series may be connected together at the crossing points, for example by welding. The plates forming the girdle are fixed, generally by welding, to the elongated elements 15, three in number for each face of the grid in the embodiment shown in FIGS. 2 and 3. The girdle may be made of "Inoconel" or from stainless steel whereas the wires will typically be made from hyper cold drawn stainless steel, although other materials are suitable. The diameter of the wires will vary depending on the pitch of the network and the diameter of the fuel rods: a wire diameter of about 0.6 mm is often of advantage. In a modified embodiment, the wires are fixed to the elongated elements 14, for example by engaging the wire ends in grooves or notches in these elements. The elongated elements 15 may then either be kept or omitted. The reactor coolant will pass through each bed of wires, around a fuel rod, essentially through two passages of generally triangular shape, as shown at a in FIG. 3. Two successive passages associated with the same rod are offset by 60.degree. when passing from one bed to the next, whether there are two beds 22 and 24 (FIG. 1) or more. Thus stirring and mixing of the different coolant streams is obtained with temperature homogeneization. The ends of the interlaced wires which form the beds are fixed to girdle 20. Numerous types of connection may be used and those which will now be described only form examples thereof which are preferable in most cases. Referring to FIG. 4, the ends of wires 26 are engaged in openings in the girdle, then locked by thermal deformation giving rise to a small boss 27. The connection may be completed by brazing. FIG. 5 shows how the same method of fixing may be applied to a grid comprising a girdle 20 having corrugations at the pitch of the rods. A split washer 29, or circlip, may be engaged on the wire before thermal deformation and brazing, for completing retention thereof (FIG. 6 and wire at the top of FIG. 4). In FIG. 4 it can be seen that the outer bosses resulting from the thermal deformation form stops for the reciprocal spacing of the fuel assemblies at the height of the grids. The reciprocal bearing points of the peripheral bosses form stops which provide radial maintenance avoiding deformation during operation in the core of the reactor. The use of circlips 28 for mounting and fixing the wires to the girdle plates means that different materials not weldable together may be used for forming the girdle on the one hand, and the wires on the other and thus allows the girdle to be made from a material chosen for its low neutron absorption. Another solution, which may be used when the grid only comprises two beds of wires or for the endmost beds of grids comprising more than two beds, is shown in FIG. 7: the ends of the wires are jammed in notches 30 in the girdle 20 before being thermally deformed and/or brazed. Whereas the grid shown in FIGS. 1 comprises only two beds of wires, the one shown in FIG. 8 comprises three parallel beds. If we designate by 1, 2 and 3 three directions at 60.degree. from each other (FIG. 8), the wires of the first bed are in directions 1 and 2, those of the second bed in directions 2 and 3 and those of the third bed in directions 3 and 1. The small dimension of the wires reduces the pressure loss. The mixture is improved because, in two successive beds, half of the wires are orientated differently. The passages a (FIG. 3) will be offset by 60.degree. when passing from one bed to the next. The successive grids provided along the whole of the assembly may be disposed so that the coolant passages are aligned along the same helix or following helical elements in a quincunx arrangement. Whereas in the case illustrated in FIGS. 3 and 4, each bed provides three pinpoint supports per rod, which results in the case of a grid of the kind shown in FIG. 8 in three pairs of double bearing points spaced apart over the periphery, the grid shown in FIGS. 9, 9A, 9B and 9C only provides two pin point supports per rod in each bed. For that, the wires of each series have a spacing double that of the wires of FIG. 3. But the wires parallel to the same direction in two different beds are offset by a half pitch, so that all the rods have the same number of bearing points (three double bearing points per rod in the case of three beds). This arrangement increases the coolant passage section offered by each bed and allows the number of beds to be increased while limiting the pressure loss to an acceptable value. Instead of disposing the beds parallel to each other, they may be given different slopes with respect to the longitudinal axis of the assembly. This solution, one embodiment of which is shown in FIG. 10, further improves the mixing of the coolant streams and more especially allows a fraction of the flow to be diverted from one assembly towards the peripheral assemblies. Several methods of mounting may be used for forming the assembly. A first solution, which may be used more especially when the wires are previously undulated and do not comprise an excessive tension, consists in aligning them in a skeleton then fitting the guide tubes and the rod. Another solution, which allows the wires to be tensioned before final fixing, consists in mounting the wires in the grids while fixing them at only one end, the other being simply inserted in the opposite plate of the grid and held without tensioning. Once the rods are positioned, the wires are subjected to a calibrated traction force then secured.