Patent Number: 048045160
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The curve of FIG. 1 shows the evolution of the thickness of the zirconia (oxide) layer of the surface of a fuel element sheath which change according to the measurement location along the longitudinal direction of an assembly of known type, with grids located and regularly spaced along the braces according to the prior art. The distance between the end of the lower part of the fuel element and the part of the element where the measurement is undertaken appears on the Ox axis, and the measured thickness of oxide layer on the Oy axis. The location of the grids spaced longitudinally along the fuel element is shown on FIG. 1 by vertical bars identified from A to H, the grid A being the grid situated closest to the bottom end part of the assembly and hence to the bottom cap of the element examined. The curve of FIG. 1, obtained from measurements carried out by Applicants, shows that: the oxide layer is substantially thicker between the grids E and H than between the grids situated at the bottom part of the assembly. The thickness of the oxide layer is smaller, compared to the thickness observed on the sheath element in the immediate upstream zone of the grids, not only at the level of the grids themselves but also immediately downstream of said grids. Finally, and according to Applicant's observation, depending on the location of the grid, this effect is observed over about 10 to 30 cm downstream of the grid, the zirconia or oxide layer becoming thicker on the part of the sheath element located in the intergrid space to reach a peak a little after the middle of the inverval, the highest peak being between the grids F and G. FIG. 2 shows diagrammatically an assembly 1 comprising a skeleton including a lower end part 3 and an upper end part 4, connected by elongated elements such as guide tubes 5. The assembly is provided with lower grids 6, 7, 8, 9 and upper grids 10, 11, 12, 13, 14, 15, 16, the distance between two adjacent upper grids situated in the upper half of the assembly 1 being smaller than the distance existing between two adjacent lower grids situated in the lower half of the assembly 1. In the particular case shown, the assembly comprises seven upper grids and four lower grids, these numbers not being limiting. To locate as shown the seven upper grids, allows a better mixing of the coolant in the upper part of the assembly than the mixing obtained with an assembly of the prior art and this for a same thermal power dissipated by the reactor and a same output temperature of the coolant, from the assembly; this results in a lower temperature of the sheath and hence permits the growth of the oxide layer thickness in this part to be slowed down. The formation of corrosion on the fuel elements on the part comprised between the grids E and H of FIG. 1 is therefore delayed. In one advantageous embodiment of the invention, the number of grids in the upper part is doubled, the fuel elements being for example tie braced by grids positioned axially every 25 cm. FIG. 3 shows diagrammatically a type of upper grid 10 in an assembly according to the invention. It comprises two parallel beds 21 and 22 of plates 23 for holding and spacing the fuel elements. The plates are provided with mixing fins 24, so as to ensure, in addition to their supporting function, a function of mixing the fluid streams. FIG. 4 shows a type of mixing grid with two beds 21 and 22. The beds are joined by rods or corner bars 25. Each bed comprises plates 26 mounted in two different directions. It will be noted that each bed of plates as appearing in FIGS. 3 and 4 is incomplete in the sense that the receiving cell for fuel elements 27 is only completely defined by plates belonging to two beds. The upper grids may be of the type described in French document FR No. 86 02 501, but other types of grids are also suitable for an assembly according to the invention. FIGS. 5 and 6 shows a possible embodiment of the upper grids with a single bed and show fastening blades 31 for fixing the grid 30 to some of the guide tubes. The grid 30 ensures a function of bracing fuel element 32 with the protruding parts or bosses 33, 34 located on each surface of the plates 35 at two levels. It introduces turbulences into the coolant flow passing upwardly through the assembly with fins 36, for example of the type described in French patent application FR No. 8416803. To increase further the rigidity of the grid, at least some of the plates are extended advantageously by a protruding portion 37 having a longitudinal stiffening fold 38. FIG. 7 shows, from above, lower grids advantageously used in an assembly of the invention. These grids comprise two sets of plates 40 in which are formed slots 41 (FIG. 8). Once fitted together, the plates are fixed by welding 42, for example, by electron gun. The plates 40 are stamped so as to create protruding parts causing each side of the plate to project into two adjacent recesses such as holes or cells 43 and 44 on FIG. 7. These protruding parts ensure the lateral bracing of the elements containing fissile material, such as the fuel element 45 on FIG. 7. The grid shown in FIG. 7, 8 and 9, comprises on each surface of a cell, a single boss. The bosses corresponding to the cell 43 are referred to by number 46 on FIGS. 8 and 9. The bosses on two surfaces are at a different level from that of the bosses born by the two perpendicular surfaces. Each element containing fissile material 45 is hence braced laterally due to permanent contact with four bosses. With each boss 46 is associated a boss 47 oriented in opposite direction, except in the case of the walls which define a cell occupied by a guide tube 48 (FIG. 7). The lower grids do not include fins. They hence only participate slightly in the mixing of the fluid streams and, consequently, only introduce a very small head-loss. The invention is in no way limited to the embodiments which have been described. It covers also particularly: Those where the grids have a polygonal non-rectangular cross-section, Those where the lower grids are also provided with mixing fins, Those where the upper grids and/or lower grids differ from one another.