Patent Number: 061756067
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1a shows a boiling water fuel assembly 1 which comprises a long tubular container, of rectangular cross section, referred to as fuel channel 2. The fuel channel 2 is open at both ends so as to form a continuous flow passage through which the coolant of the reactor flows. The fuel assembly 1 comprises a large number of equally long tubular fuel rods 3, arranged in parallel in a bundle, in which pellets 4 of a nuclear fuel are arranged. The fuel rods 3 are arranged spaced from each other in four orthogonal sub-bundles by means of a cruciform support means 8 (see also FIG. 1b). The respective sub-bundle of fuel rods 3 is retained at the top by a top tie plate 5 and at the bottom by a bottom tie plate 6. The fuel rods 3 in the respective sub-bundle are kept spaced apart from each other by means of spacers 7 and are prevented from bending or vibrating when the reactor is in operation. The spacer 7 according to the invention may, of course, also be used in a boiling water reactor which lacks the cruciform support means 8 and instead is provided with, for example, one or more water tubes. At the lower part of the fuel assembly 1, a transition piece 9 is arranged. The task of the transition piece 9 is to guide coolant, flowing upwards through the core (not shown) of the nuclear reactor, to the fuel assembly 1 for cooling of the fuel rods 3 arranged therein. The transition piece 9 is arranged in the core in an opening in a so-called assembly supporting plate (not shown). Further, a bottom support 10 is arranged in the transition piece 9. The bottom tie plate 6 with the lower ends of the fuel rods 3 is arranged at least partly immersed into the bottom support 10. The bottom support 10 is provided with a plurality of through-holes 11 forming a filter. FIG. 2 shows the bottom support 10 in more detail. The bottom support 10 is substantially formed as a parallelepiped with a substantially square flat side. The direction of flow of the coolant is indicated by an arrow F. The parallelepiped is provided with a downstream and an upstream flat side 12, 13 and four narrow sides 14. The flat sides 12, 13 and the narrow sides 14 surround a cavity and form a parallelepiped. The downstream flat side 12 is formed with four openings 12a, where each opening 12a is adapted to receive the lower end of a sub-bundle. The holes are arranged in the upstream flat side 13 forming a filter through which the coolant F is forced to pass before it reaches the fuel rods arranged in the fuel bundle or in the fuel bundles. The holes 11 are substantially formed straight. Further, the bottom support 10 is provided with a passage opening 16 for passage of coolant to the cruciform support means 8. The cruciform support means 8 is formed as a channel for conducting non-boiling coolant up through the fuel assembly 1. Any foreign matter passing with the coolant through the fuel assembly does not run the risk of adhering to any unsuitable place therein. The embodiment of the invention shown in FIG. 3 is provided with so-called bypass holes 15. The bypass holes 15 are arranged in the upstream flat side 13 and at the respective corners of the bottom support 10. The bypass holes 15 have a substantially triangular shape. By arranging bypass holes 15 in the corners of the bottom support 10, it is ensured that coolant may always pass through the bottom support 10. In the event that the filter should be clogged by foreign matter, coolant flow is thus allowed to pass through the bypass holes 15. The bypass holes 15 are arranged such that the coolant flowing upwards through the fuel assembly 1 is forced to change its direction to be able to pass them (see the arrows F in FIG. 4). By this redirection of the coolant flow, any foreign matter is efficiently prevented from accompanying the coolant flow F up through the fuel assembly 1. The foreign matter which is possibly oriented such that it may pass with the coolant flow through the bypass holes 15 substantially maintains its orientation when the coolant flow F is forced to change its direction to be able to pass up through the bottom tie plate 6 to the fuel rods 3. Since the direction of the possible foreign matter is substantially maintained, the foreign matter will adhere to the bottom tie plate 6 and be prevented from passing up to the fuel bundle. The upstream flat side 13 of the bottom support 10 is shown in more detail in FIG. 3. FIG. 3 shows the filtering part of the bottom support 10 in a view from below. The filtering part, like the fuel assembly in its entirety, is divided into four parts. In principle, a filtering part is arranged below the respective sub-bundle of fuel rods 3. In the example shown, the through-holes 11 are arranged in rows, which are substantially parallel with an opening edge at the respective bypass hole 15. Further, the filter in the embodiment chosen in FIG. 3 is provided with a plurality of long and narrow grooves 17 (see also FIGS. 4-6). The grooves 17 are oriented so as to be arranged substantially parallel to an opening edge 15a of the respective bypass holes 15. By arranging grooves 17 in this way, an orientation of the possible foreign matter reaching the bottom support 10 is achieved, such that this foreign matter is prevented from being conducted out towards and through the bypass holes 15. The grooves 17 may, of course, also be oriented such that they are parallel to the respective narrow sides 14. The embodiment with grooves 17 also prevents clogging of the filter in that any foreign matter, which is arranged towards the upstream edge of the filter, does not make tight contact but there is a certain distance between the respective foreign matter and the filter, whereby coolant, possibly with a certain reorientation, is allowed to pass by the captured foreign matter. In FIG. 4, this is illustrated by foreign matter, drawn into the figure and marked with reference numeral 18. A space is thus formed between the foreign matter 18 and the bottom of the groove 17. The grooves 17 in the embodiments shown in FIGS. 3-6 have an additional function, namely, to form edges against which any foreign matter accompanying the coolant may be broken. The foreign matter which is broken when it reaches the surface provided with grooves 17 primarily consists of oxide flakes which have become detached from the fuel rods 3 and have accompanied the coolant. These oxide flakes do not give rise to any abrasion but may to some extent stop up and obstruct the flow passage if they have a size which is larger than the diameter of the holes 11 in the filter. Further, FIG. 4 shows that that part of the bottom support 10, in which the trough-holes 11 are arranged, is relatively thick. This provides for a stable design of the filter and ensures that vibrations, caused by the flow of the coolant therethrough, are avoided. In the embodiment shown in FIG. 4, the bottom support 10 downstream of the filter is formed with a flange 10a. The flange 10a has a substantially triangular shape which corresponds to the shape of the respective bypass holes 15. The flange 10a is arranged at a level between the upstream and downstream flat sides 12, 13 and above the opening of the respective bypass hole 15. The task of the flange 10a is to guide the coolant flow F, which has passed through the bypass hole 15, into the central parts of the bottom support 10 and then allow the flow to pass upwards through the bottom tie plate 6. The embodiment chosen, shown in FIG. 4, also shows the open structure in the bottom tie plate 6. The through-holes 6a in the bottom tie plate 6 have a diameter which compensates for the increased pressure drop which is caused by the filter. FIG. 5 shows an embodiment which corresponds to that shown in FIG. 4 apart from the fact that the bottom support is not provided with a flange 10a for guiding the coolant flow F. FIG. 6 shows an alternative embodiment of the invention, in which an additional level with a second filtering member (see reference numeral 19) is arranged in the bottom support 10. This second filtering member is, in the example shown, formed as a plate 20 with through-holes 21. The through-holes 21 preferably have their centre axis somewhat displaced in relation to the centre axis of the through-holes 11 arranged in the upstream flat side 13. The plate 20 is preferably provided with a centrally arranged opening for passage of coolant to the passage opening 16. FIG. 7 shows an alternative embodiment of a transition piece 9. The bottom support 10 is arranged as an integral part of the transition piece 9. The filter in its turn is made as an integral part of the bottom support 10. In the embodiment shown, the filter is provided with through-holes 11 and grooves 17 in a manner corresponding to that shown in FIG. 3.