Patent Number: 059498390
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 and 2 show a boiling-water fuel assembly 1 according to the prior art which comprises a long tubular container, with a rectangular cross section, referred to as a fuel channel 2. The fuel channel 2 is open at both ends so as to form a continuous flow passage, through which coolant flows. The fuel channel 2 is provided with a hollow support means 3 of cruciform cross section, which is secured to the four walls of the fuel channel. The support means comprises four hollow wings 3a and a hollow enlarged cruciform center 3b. The support means forms a vertical water channel 4 through which non-boiling water flows upwardly through the fuel assembly. The fuel channel with support means surround four vertical channel-formed parts 5a-5d, so-called sub-channels, with a substantially square cross section. Each sub-channel contains a sub-bundle comprising a plurality of fuel rods 6 arranged in parallel, which contain fuel in the form of a number of cylindrical pellets 7 of uranium dioxide stacked on top of each other and enclosed in a cladding tube 8. The upper part of the fuel rod is sealed by a top plug 9 and its lower part by a bottom plug 10. The active part of the fuel assembly consists of that part which contains fuel and its height is determined by the height of the stacks of pellets. The fission gas plenum 11 is arranged between the stack of pellets and the top plug 9. The fission gas plenum should correspond to 5-10% of the volume of the fuel. For a fuel rod whose diameter is substantially constant and whose active length is 4 meters, this means that the fission gas plenum should be 0.2-0.4 meters. A helical spring 12 is arranged in the fission gas plenum and the task thereof is to absorb movements in the pellets as well as to press down the stack of pellets against the bottom plug. The fuel rods 6 are kept spaced-apart by means of spacers 13 and are prevented from bending or vibrating when the reactor is in operation. The upper ends of the fuel rods are retained by a top tie plate 14 and their lower ends are retained by a bottom tie plate 15. Through two openings in the top tie plate 14, there extend two supporting fuel rods 6a which partially extend above the top tie plate. The two supporting fuel rods 6a are fixed to the bottom tie plate 15 and are each provided with a nut 16 at the upper side of the top tie plate 14. In this way, the top tie plate is prevented from being lifted out of its position by the water flowing through the fuel assembly. The other fuel rods in the sub-bundle are arranged resting on the bottom tie plate 15 and guided therein by pins on the bottom plugs 10 of the rods. The fuel rods make contact with the lower side of the top tie plate 14 by means of a helical spring 17 arranged around the respective top plugs 9 of the fuel rods. The helical springs 17 press the top tie plate 14 against the nuts 16, whereby these nuts limit the maximum distance between the top and bottom tie plates. The spaces between the fuel rods within each sub-channel are traversed by coolant. At the top of the fuel assembly, there is a top piece 18 provided with a handle in which the fuel assembly can be lifted. FIG. 3 shows a fuel assembly which, during operation, has a considerably lower pressure drop in its upper part as compared with the prior art. To avoid problems with turbulence, the reduction of the pressure drop occurs by allowing the coolant to expand successively in a number of steps. In a first step, the expansion takes place by part of the coolant, when reaching the end of the active part, flowing into the water channel 4 through a number of openings 20 and being conducted past the uppermost spacer 21. These openings 20 are arranged immediately below the level L--L where the active part ends. Above the level L--L there is thus no fuel. The openings 20 should not be arranged too far below this level since steam in the water channel deteriorates the moderation of the fuel. In a second step, the coolant expands due to the fuel rods 22 changing to a smaller diameter. The transition takes places immediately above the active part. The upper narrower part comprises a fission gas plenum and will be referred to in the following as a plenum part. The plenum part comprises a plenum tube 23 and a top plug 24. The upper part of the fuel rods are retained and supported by a top spacer 21 instead of by a top tie plate. The top spacer 21 is arranged immediately above the transition between the larger diameter of the cladding tube 8b and the smaller diameter of the plenum part. The plenum tubes 23 run freely through the top spacer 21 and extend about 10-20 cm above the top spacer. A spacer contains less material than a top tie plate and the top spacer which surrounds the plenum part can thus provide a smaller pressure drop than a top tie plate. The top spacer 21 comprises cells, for mutually fixing the fuel rods, which have an inside diameter corresponding to the outside diameter of the plenum tubes 23. The other spacers 13 comprise cells which have an inside diameter corresponding to the outside diameter of the cladding tubes 8b. The top spacer rests partly against the fuel channel 2 and partly against the support means 3. The fuel rods 22 are prevented from moving upwardly past the top spacer 21 since this surrounds the plenum tubes 23 which have a smaller diameter than the cladding tube 8b. In each sub-bundle there are two so-called supporting fuel rods (not shown), which have the same thickness along their whole length. These are intended to lift the sub-bundle out of the fuel assembly. In their upper ends, the supporting fuel rods fix the top spacer and in their lower ends they are fixed to the bottom tie plate. In a third step, expansion takes place due to the fact that the upper edge of the support means 3 terminates immediately above the top spacer 21 and somewhat below the upper part of the fuel rods. The support means as such causes a certain pressure drop. By terminating the support means further down in the fuel assembly, the pressure drop is reduced. Between the support means and the top piece, there is an interconnecting member 25 which distributes the lifting force so that the fuel channel 2 need not carry the whole lifting force when the fuel assembly is to be lifted. The connection between the fuel channel 2 and the top piece 18 may thus be given an advantageous design from the point of view of pressure drop. In a fourth step, a smooth expansion takes place at the end of the fuel rods. The top plug 24 is designed so that the transition may be as smooth as possible. At the top plug, a further reduction of the diameter occurs. The top plug 24 comprises a cylindrical pin 26 which has a diameter which is smaller than the diameter of the plenum tube 23 and which is arranged above the plenum tube. The final expansion occurs when the coolant leaves the fuel assembly via the top piece 18. FIG. 4a shows in more detail the composition of a fuel rod 22 intended for a fuel assembly according to the invention. In its lower part the fuel rod comprises a stack of fuel pellets 7 surrounded by a cladding tube 8b. Above the pellets there is a space 27 in which the pellets are allowed to expand. The height of the space 27 is about 10 cm. In the space 27 there is a short helical spring 28 adapted for locking during transport of the fuel. The upper end of the helical spring rests against the transition to the plenum tube 23 and its lower end presses against the stack of pellets. Above, and partly inserted into, the cladding tube, the plenum tube 23 is arranged. In the example, the plenum tube 23 has an outside diameter corresponding to the inside diameter of the cladding tube 8b. The plenum tube is hollow and sealed by a top plug 24 at its upper end. The top plug extends somewhat above the plenum tube and comprises a cylindrical pin 26 with a diameter which is smaller than the diameter of the plenum tube 23. The pin is intended to be engaged by means of a chucking tool. The plenum tube surrounds a space 29 which, together with the space 27, constitutes the fission gas plenum. The transition between the cladding tube and the plenum tube is open so that the fission gases which are formed in the fuel pellets may pass into the space 29. Since it is sufficient to use a short helical spring 28, the whole space 29 becomes available for fission gases. Only a small increase in length of the fuel rod is therefore needed to compensate for the smaller diameter. FIG. 4b shows an embodiment of a fuel rod for a fuel assembly according to the invention which differs from the fuel rod in FIG. 4a by the provision of a hollow intermediate piece 30 between the cladding tube 8b and the plenum tube 23. In its upper part, the top plug 24 has the shape of a sphere to facilitate lifting the top plug. FIG. 4c shows an additional embodiment of a fuel rod for a fuel assembly according to the invention where the plenum tube 23 and the top plug 24 form an integral unit.