Patent Number: 055307299
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a fuel assembly 1 according to the invention which comprises an elongated tubular container, often of rectangular or square 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 the coolant of the reactor flows. The fuel assembly 1 comprises a large number of also elongated tubular fuel rods 3, arranged in parallel, in which pellets 4 of nuclear fuel are arranged. The fuel rods 3 are retained at the top by a top tie plate 5 and at the bottom by a bottom tie plate 6. The fuel rods 3 are kept spaced from each other by means of spacers 7 and are prevented from bending or vibrating when the reactor is in operation. A spacer 7 (FIG. 2a) comprises an outer spacer frame 8, with frame parts 8a-d, and cells 9 arranged inside frame 8. Inside these cells 9 there are arranged, for example, fixed and resilient supports (not shown), respectively, for the fuel rods 3 extending through the cells 9. The spacer frame 8 usually comprises a plate band standing on end. At the upper part of the frame 8 a plurality of guiding tabs 12 are arranged in order to prevent the bundle, that is, the fuel rods 3 inserted into the spacers 7, from being wedged in the fuel channel 2 when being inserted into or withdrawn from the fuel channel 2. The guiding tabs 12 are formed from parts projecting from the downstream edge of the spacer frame 8, which tabs are bent toward the centre of the spacer 7 around a folding axis coinciding with the edge of the spacer, that is, an axis perpendicular to the direction of flow. The guiding tabs 12 result in the flow resistance of the spacer 7 increasing since a large part of its side surface is presented across the direction of flow. The increased flow resistance downstream of the spacer 7 is advantageous since it creates a certain turbulence which facilitates the heat transfer between the fuel rods 3 and the coolant and hence retains the coolant film along the fuel rods 3. The spacer frame 8 is largely formed open at the bottom end by the arrangement of openings 13 and vertically folded tabs, referred to as guiding studs 14. The open bottom end permits excessive disturbance of the coolant flow to be avoided and permits the pressure drop across the spacer 7 to be limited. The guiding studs 14 are intended to guide the bundle in the fuel channel 2. The guiding studs 14 comprise a plate tab or the like, the plane of which is arranged parallel to the direction of flow of the coolant and one side edge of which runs in an inclined manner from the lower edge of the frame 8 in a direction downstream of the spacer 7 towards the centre of the spacer 7. Since the side surface of the guiding stud 14 is arranged in parallel with the direction of flow, only the cross-sectional area of the guiding stud 14 perpendicular to the plane of the frame 8 and the direction of flow will constitute an obstacle to the flow the coolant through of bundle and will thus not give rise to any significant flow resistance. The frame part 8 is provided with a first and a second portion 15, 16 arranged at different distances from the inner surface of the fuel channel 2, forming a spacing element between the bundle of fuel rods 3 and the inner surface of the fuel channel 2 in the fuel assembly 1. The second portion 16 constitutes a bulge in a direction towards the fuel channel 2, whereby it is located at a shorter distance from the fuel channel 2 than the first portion 15. FIG. 2b shows an alternative embodiment of the spacer 7 shown in FIG. 2a. The spacer 7 in FIG. 2b is intended to be used in a fuel assembly 1 comprising at least one position which is not occupied by a rod and which is located in a corner. The spacer 7 can then be adapted with a reduced corner portion and an extra frame part 8e. The frame part 8e comprises a long flat surface inclined towards the spacer centre for guiding water to the fuel rods 3. From FIGS. 2a, 2b and 3 it is clear that at the lower edge of the frame 8, the openings 13 and 23, respectively, are adapted to lead in coolant which flows upwardly along the fuel channel 2 to the inside of the spacer frame 8. At the corners of the frame 8 the coolant is led to the inside of the spacer frame 8 via the openings at the folded-in guiding studs 14. The coolant is led to an edge 18 which is obliquely positioned in relation to the longitudinal direction of the fuel assembly 1 and formed between the first and second portions 15, 16, which edge 18 causes the coolant to be deflected in a direction towards that fuel rod 3 which is arranged at the corners of the spacer frame 8. The oblique edge 18 extends in a direction downstream of the spacer 7 and towards the corners of the spacer frame 8. The short side of the frame part 8b is provided at the corner with a recess 19, extending along the direction of flow, which is limited at top and bottom by projecting parts 20 which are intended to form cells 9 at the four corners of the spacer frame. FIGS. 2a and 2b show an embodiment of a spacer frame 8 in which, at the mid-portion of a frame part 8b, the coolant is introduced into the inside of the frame 8 via the openings 13 and further upwards along the spacer frame 8 until it encounters an edge 21, extending transversely of the longitudinal direction of the fuel assembly 1 and formed between the first and second portions 15, 16, whereby the coolant is slowed down and directed inwardly toward the fuel rods 3 while the pressure is reduced. In the second portion 16, bottle-like openings are also arranged, partly in order to save material, with the "bottle-neck" facing downwardly and the "bottle-bottom" facing upwardly. These bottle-like openings constitute-an extension in the downstream direction of some of the openings 13 and assist in conducting coolant to the inside of the frame 8 at the "bottle-bottom" FIG. 3 shows another embodiment of a spacer frame 8 which, like the one shown in FIG. 2, is provided with a first and a second portion 15, 16, wherein the first portion 15, in the fuel assembly 1, is arranged at a larger distance from the fuel channel 2 than the second portion 16. The mid-portion of the frame part 8a is provided with alternately first and second portions 15, 16 arranged in parallel with the direction of flow, where in the first portions 15 openings 23 are arranged in which coolant is conducted, with the aid of guiding hoods 26, into the inside of the frame 8. The coolant is also led to the inside of the frame part 8a via the lower edge of the second portions 16, whereupon it flows upwards until it encounters an edge 21 formed between the first and second portions 15, 16 which is arranged in a direction across the direction of flow, whereby the coolant flow is deflected towards the centre of the spacer 7 for cooling the centrally arranged fuel rods 3. To further facilitate this deflection, a long flat guiding surface 24, inclined inwards towards the centre of the spacer, is connected to the second portion 16. The long and flat angle ensures a low pressure drop and the guiding surfaces have good mechanical strength when lifting out the spacers 7, and the fuel rods 3 arranged therein, from the fuel channel 2. FIG. 4 shows an advantageous embodiment of the invention in which, in the lower part of a fuel assembly 1, spacers 7 with a low pressure drop are arranged, so-called low-pressure drop spacers. The frames 25 of the low-pressure drop spacers may, for example, be designed as shown in FIGS. 5a-c. These low-pressure drop spacer frames likewise comprise a first and a second portion 15, 16, wherein the first portion 15, in the fuel assembly 1, is arranged at a larger distance from the inner surface of the fuel channel 2 than the second portion 16. The spacer frame 25 alternately comprises the vertically positioned first and second portions 15, 16. The frame 25 comprises openings 23 in order to reduce the amount of material. Both at the top and the bottom the low-pressure spacer frame is provided with folded-in guiding studs 14 intended for guiding the fuel rods 3 and the spacer 7 when they are inserted into, or withdrawn from, the fuel channel 2. The guiding studs 14 are arranged in parallel with the flow whereby the flow resistance of the spacer 7 is minimal. It is suitable to construct the spacer frames of the material inconel or of zircaloy.