Patent Number: 052271307
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, 1 designates a fuel assembly for a pressurized-water reactor. The fuel assembly 1 comprises a number of fuel rods 2 as well as guide tubes (not shown). These are retained into a bundle by spacers 3. The bundle is arranged between a top nozzle 4 and a bottom nozzle 5, which are provided with openings (not shown) for the coolant flow through the fuel assembly 1. How these openings can be throttled in order to control the coolant flow through them is shown in Swedish patent specification 8801141-6. According to the invention, the fuel assembly 1 has been provided with partial fuel boxes 6 and 7, each of which, according to FIG. 1, extending in over two ordinary spacers 3. The fuel boxes 6, 7 have been provided with bevelled corners 8 to prevent the fuel boxes 6, 7 from hooking into adjacent fuel assemblies 1. The lower fuel box 7 has been provided with a number of holes 9 at its upper edge to equalize the difference in cooling water pressure inside the fuel assembly when changing from a partial box to a boxless state. FIG. 2 shows the lower part of the fuel assembly 1 which is enclosed by a partial fuel box 7. This fuel box 7 is intended to comprise two ordinary spacers 3 but the upper one of them has been replaced by two partial spacers 10, 11 of the type described in more detail in Swedish patent 8802305-6. These spacers 10, 11 are welded to the box wall and at the same time constitute an inner support for the walls of the partial fuel box 7, which walls are conceived to be made from thin plate of Zircaloy (.about.1 mm). Thus, the box walls are conceived to serve as spacer frames. Otherwise, the upper part of the box walls is provided with inwardly-bent studs 12 to facilitate withdrawal of the fuel assembly 1 from the reactor core. FIG. 3 shows the peripheral partial spacer 10 seen from above and containing fuel rods 2 and guide tubes 13. In the spacer 10 only the outer fuel rods 2 and the guide tubes 13 are fixed in a lattice-work 14 whereas the fuel rods 2 and the guide tubes 13 in the centre of the bundle freely pass through the spacer 10. The frame of the spacer 10 is formed by the wall of the fuel box 7. FIG. 4 shows the central partial spacer 11. In this, only the centrally located fuel rods 2 and guide tubes 13 of the bundle are fixed in a lattice-work 15. Certain of the crossing plates 16 forming this central lattice 15 are extended to the walls of the surrounding fuel box 7 and fixed therein as support for the walls. When need arises, additional partial spacers may be arranged between the spacers 10 and 11 and the spacers 11 and 3, respectively, as support for the walls of the box 7. It would also be possible to arrange a simpler form of support structure at these locations. FIG. 5 shows a partial fuel box 7 seen from above in a section 5--5 in FIG. 6 which shows the walls of the fuel box 7 unfolded into a plane surface. The walls in FIGS. 5 and 6 have given the corresponding designations a, b, c, d. The figures show how a fuel box 7 can be perforated with holes 17. These holes 17 are intended to facilitate ocular inspection of the bundle inside the box walls and also to reduce the pressure difference on each side of the walls of a single fuel box. FIGS. 5-7 also show how the holes 17 are to be placed to prevent them from getting just opposite to each other when several partial fuel boxes 7 are placed adjacent to each other, as shown in FIG. 7. If the coolant flow in the fuel assembly 1 is throttled, the partial fuel boxes 6, 7 now prevent the coolant flow from adjacent unthrottled fuel assemblies from flowing over to the fuel assembly 1 already in the vicinity of the bottom nozzle 5. With both the partial fuel boxes 6, 7 in position, passage of coolant flow between adjacent fuel assemblies can only take place in the open area between the inlet box 7 and the outlet box 6. The effect of the inlet throttling is thus moved upwards and the effect of the outlet throttling is moved downwards in the bundle such that the cooling of the unthrottled highly loaded assemblies is considerably improved.