Patent Number: 054024579
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a fuel assembly for a pressurized water reactor as is disclosed, for example, in Published European Application No. 0 364 623. The fuel assembly 1 contains, for example, 17.times.17 rods, a fuel assembly top end piece 2 with hold-down springs 3 and a fuel assembly bottom end piece 4. A number n (for example n=24) of the rods are guide rods or tubes 5, while a rod 5a is an instrumentation tube and remaining rods 7, 8, 9 and 10 are filled with fuel in the form of cylindrical pellets. Those rods are usually situated in a cross sectionally square grid configuration, while in the reactor core there is generally provided a predetermined number of fuel assemblies which are identical to one another. The fuel assemblies are disposed in a reactor pressure vessel and in each case a coolant K flows through them the direction of an arrow from bottom to top as shown. In this case the rods are held in a supporting structure which includes the fuel assembly top end piece 2 with the hold-down springs 3, the fuel assembly bottom end piece 4, the guide tubes 5 lying in between, for non-illustrated control rods and, depending on the type of fuel assembly, the central instrumentation tube 5a. Disposed on the guide tubes 5 are grid-shaped spacers 6, which have a square cross section with square sheet-like meshes or openings. Each of the rods is guided by a mesh of the grid, so that the rods are combined to form a bundle or cluster and are held in such a way that they can freely expand axially, while their lateral position in the meshes is fixed by corresponding spacing means, for example springs and dimples on the webs of the spacer. The laterally open construction of the fuel assembly permits cross-mixing of the coolant K, which makes its heating-up more uniform. In the case of a boiling water reactor, this cross-mixing is prevented by a channel which laterally surrounds the rods and extends from the bottom end piece to the top end piece. Control rods or other controllable absorber assemblies are disposed outside the channel, while the coolant K enters in liquid form through corresponding openings of the bottom end piece and flows as a liquid/vapor mixture through the passages in the top end piece 2. In this case one or more rods may be replaced by a water tube, through which liquid coolant flows and, under certain circumstances, may take up the cross section of a plurality of meshes of the spacer. In order to achieve uniform temperature loading and high utilization of the fuel with optimum cooling, it is an aim to attain a temperature at the fuel rods which is as uniform as possible in the radial direction and in the axial direction. However, when operating the reactor, different parts of different fuel rods develop different temperatures, so that temperature peaks occur in the fuel assembly both in the axial direction and in the radial direction. Therefore, it has already been proposed in German Published, Non-Prosecuted Application DE 15 64 697 A1 to use spacers with mixing vanes, which are intended to produce a uniform radial temperature distribution in pressurized water reactors by inducing a cross-flow through the individual fuel assembly and over the interspaces of the fuel assemblies. In FIG. 2, the grid structure of such a spacer is represented diagrammatically. In that case, the fuel rods 7 . . . 10 are each situated in a respective mesh of a grid. The meshes are formed by longitudinal webs 11, 12 and transverse webs 13, 14 which, in the case of a fuel assembly of square cross section, cross the longitudinal webs at a right angle. The fuel rods extend in a transverse or crossing plane perpendicular to the webs, while lateral surfaces thereof are aligned parallel to the rods. That structure produces flow subchannels 15, 16, which are surrounded by four rods in each case. In FIG. 2, the flow subchannels are represented by being alternately hatched and unhatched, so that a checkerboard-like array of hatched channels, which are referred to below as "black" channels and unhatched channels, which are referred to below as "white" channels, is produced. In a white channel, which is surrounded by the rods 7, 8, 9, 10, for example, the longitudinal web 12 may have two non-illustrated lateral lugs on both sides of the crossing transverse transverse web 13, at an edge thereof facing away from the stream of coolant. The two non-illustrated lateral lugs are bent in opposite directions laterally into the stream of coolant in such a way that a swirl symbolically represented by an arrow is produced in the "white" channel 15. In the black channels, the transverse webs 13, 14 likewise have lateral lugs in each case being disposed on both sides of the crossing longitudinal web. The lateral lugs are constructed as mixing vanes and protrude oppositely relative to each other obliquely into the stream of coolant. However, an arrow indicated in FIG. 2 shows that there the alignment of the swirl being produced is opposite to the alignment of the swirl in the white channels. This produces cross-flows, which in each case lead diagonally through the meshes of the grid and cross one another in the flow subchannels to form a rotational flow. In FIG. 2 there is already shown a checkerboard-like pattern for the flow subchannels, in which the crossing points of the webs and the mixing vanes are located. According to the invention, not all of the crossing points lie in an axial cross sectional plane of the fuel assembly, but instead at least two such crossing planes and at least two groups of crossing points are provided, with the crossing points of the one group lying in the one crossing plane, and the crossing points of the other group lying in the other crossing plane. The checkerboard pattern of FIG. 2 in this case is produced when the one crossing point in each case belongs to the one group, and the other crossing point belongs to the other group, with the two crossing points lying next to each other in the grid. In FIG. 2, the flow subchannel 16 and the other flow subchannels with the crossing points belonging to the first group are emphasized by the hatching and in FIGS. 3 and 4 the longitudinal web 11 and the transverse web 13 are represented by solid lines, while the webs 12 and 14 lying behind are drawn with broken lines. The parts of these webs lying in these hatched channels are likewise identified in FIGS. 3 and 4 by hatching. The longitudinal webs 11 and 12 run approximately in zigzag form and have slots 15 in their upper and lower regions through which the transverse webs 13 and 14, which likewise run in zigzag form and have insert slots 16 in their upper and lower regions, are inserted. As a result, virtually two groups of longitudinal webs and two groups of transverse webs are produced, with the longitudinal web 11 belonging to the one longitudinal web group and all of the other longitudinal webs of this one group being completely covered in FIG. 3 by the longitudinal web 11, while the other longitudinal web 12 belongs to the other group and covers all of the longitudinal webs of the other group. The two longitudinal webs which neighbor the longitudinal web of one group then in each case belong to the other group. In the same way, in FIG. 4 the transverse webs 13 and 14 belong to two transverse web groups and a transverse web of one group neighbors two transverse webs of the other group. The longitudinal webs in this case pass through the transverse webs at lines of intersection determined by the slots 15, 16 within specified zones A and C which are perpendicular to the fuel rods. Corresponding cross sectional planes I and II which are perpendicular to the rods pass through the zones. The planes consequently describe the axial position of the crossing points. Thus, the longitudinal webs and transverse webs extend in zigzag form between these zones A and C or the corresponding crossing planes I and II as follows: End edges M facing the stream of coolant K, that is to say the lower edges of the webs 11, 12, 13, 14 of FIGS. 3 and 4, run between extreme points situated in a plane I' that is upstream, and extreme points situated in a plane II' that is downstream. Similarly, end edges N of these webs facing away from the stream of coolant run between extreme points on the upstream plane I" and the downstream plane II". As FIGS. 3 and 4 show, in this case the downstream extreme points (plane II') of the end edge M facing the stream of coolant lie further downstream than the upstream extreme points (plane I") of the end edge N facing away from the stream of coolant. The hatched or "black" flow channels lying in a lower crossing plane I then contain a group of crossing points, which are formed either by the longitudinal web 11 (or another longitudinal web of this group) and the transverse web 13 (or another transverse web of this group) or else by a longitudinal web and a transverse web of the other group (for example web 12 and web 14). The other crossing points lie in the upper crossing plane II and in each case are formed either by a transverse web of the one transverse web group and a longitudinal web of the other longitudinal web group (for example the webs 12 and 13) or a longitudinal web of the one longitudinal web group and a transverse web of the other transverse web group (for example the webs 11 and 14). It can be seen that in the direction of the stream of coolant K, the flow cross section in the "black" flow subchannels is initially constricted increasingly in the zone A by the crossing webs and therefore in a flow subchannel which contains the lower crossing points and is identified by reference symbol L an increasing compression of the stream of coolant takes place in the zone A. In the "black" channels, the webs cross in this zone A. In the neighboring "white" flow subchannels, which contain only upper crossing points and are denoted by reference symbol H, the full flow cross section is still available in the zone A. The zone A is adjoined by a zone B, in which the overall flow cross section of the two channels remains approximately the same, with the flow cross section of the white channels being constricted (or "contracted") approximately to the same extent as that by which the flow cross section in the black channels is enlarged (or "expanded") again. In the plane II' there is a maximum contraction of the flow cross section for the white channels. In the zone C, the webs intersect in the white channels, while in the black channels there is an expansion or even the full, undisturbed flow cross section is again available. In any event, the overall flow cross section increases in the zone C in spite of the intersection of the two grid webs. Thus, in the direction of the stream of coolant K there is initially a contraction (and possibly already an expansion) only in some of the flow subchannels, while the other channels still do not exhibit any contraction or expansion. In the direction of the axial flow K, the same process then follows, with roles reversed: in those channels wherein the flow cross section was not constricted by crossing webs until now, there now occurs contraction or expansion, while in the other channels, in which contraction or expansion has already occurred, no further contraction or expansion now occurs. This has the overall effect of producing a significantly smaller flow resistance, with a certain mixing of the flows in the subchannels also already taking place. If the grid structure described with regard to FIGS. 3 and 4 is used as a spacer, springs and dimples or other non-illustrated spacing means are fastened on the webs, for holding the rods in the meshes of the grid. According to the same principle, the spacing means may also be distributed over a plurality of axial planes, in order to break up the cross sectional constriction as far as possible and not concentrate it on one plane. The cross sectional constriction is caused, for example, by springs and dimples. FIG. 5 shows a plan view of a spacer that is constructed in this way from longitudinal webs and transverse webs, with springs 18 and dimples 19. However, the grid structure of FIGS. 3 and 4 may also be used as a support for mixing vanes, in order to permit an axial and radial temperature compensation by corresponding swirling in the flow. Particularly suitable locations for these mixing vanes are zones B and D (planes I" and II"), since for instance in zone B, the contraction has already been completed there in each case in the one group of crossing points and the flow cross section is already expanding again ("black" flow channels), while in the neighboring ("white") flow channels virtually the full flow cross section is still available. Subsequently, turbulences and vortices may develop in the expanded stream of coolant, without leading to a high pressure loss. In FIG. 6, which is a plan view of the upper side of such a structure from above, facing away from the stream of coolant, it can be seen that at least some of the crossing points have at least one mixing vane inclined laterally with respect to the lateral surface of the webs. The mixing vanes are disposed on the edge of the webs facing away from the stream of coolant in each case. In the illustrated structure, in each case two neighboring crossing points have a pair of mixing vanes. For example, mixing vanes 20, 21 at a first of two crossing points, such as at the crossing of the webs 11 and 14 (or 12 and 13), are disposed on the edge of a first web 11, (or 12) while mixing vanes 23, 24 at a second of two crossing points are disposed on the edge of the web 13 (or 14) crossing the first web 11 (or 13). It is also possible to provide four such mixing vanes at each crossing point. FIG. 7 is a side view of the longitudinal web 11 and FIG. 8 is a side view of the transverse web 14. FIG. 9 is a side view without fuel rods and FIG. 10 is a perspective side view with fuel rods, through part of the mixing grid constructed according to FIGS. 6 to 8. In this case too, the (lower) end edge M of the webs facing the stream of coolant runs in zigzag form between the planes I' and II' on which its extreme points lie, while the extreme points of the (upper) end edge N facing away from the stream of coolant lie on the planes I" and II". In the case of this embodiment, the plane II' comes to lie virtually just as far downstream as the plane I". The lateral surfaces of the webs 11 to 14 extend between these end edges M and N and cross each other in the two zones A and C lying around the center planes I and II. The longitudinal webs are parallel to each other, but they form two groups which are offset from each other by one mesh width in each case, as is shown by the longitudinal web 11 of the one group and the longitudinal web 12 of the second group. A corresponding structure then also applies for the transverse webs. It can now be seen that the longitudinal webs in each case have the mixing vanes 20, 21 which are disposed at the upper crossings, i.e. in the plane II" while the mixing vanes 23, 24 disposed on the edge of the transverse webs 13 are disposed at the lower crossings, that is to say in the plane I". FIG. 10 shows a grid structure in a perspective representation together with rods situated in the meshes. In the case of pressurized water reactors, some of the rods are constructed as guide tubes for absorber assemblies or instrumentation tubes for guiding measuring lances or other instruments. Mixing vanes are preferably also provided on the webs supporting these rods, if the size of these guide tubes permits. In FIG. 10 it has therefore been assumed that all of the flow channels which can be seen have mixing vanes 30, 31, 32. These mixing vanes are situated on edges of webs having lateral surfaces 33, 34 which are aligned virtually parallel to the rods and which run transversely to the rods in the interspace between the rods. In this case, the mixing vanes 30, 31 belong to one group of crossing points, which lie in the lower crossing plane, while the mixing vane 32 belongs to another group, lying in the upper crossing plane. The number of mixing vanes depends on the flow conditions desired. Thus, under certain circumstances, a single mixing vane in each flow channel may suffice. However, it is advantageous if in each case a crossing point of a group having at least one mixing vane is respectively neighbored by a crossing point of the other group which likewise has at least one mixing vane. Often at least two mixing vanes, that are inclined in opposite directions with respect to the lateral surface of the webs, are considered necessary for each crossing point between fuel rods. In the case of the grid structure according to the invention, these two mixing vanes preferably lie in the same crossing plane. Thus, the edge of a web which has a corresponding, inclined mixing vane then also has a second vane, inclined in the opposite direction. In the case of the checkerboard pattern according to FIG. 2, a second crossing point which likewise has two mixing vanes comes to lie next to a first crossing point, in which a first web has the two mixing vanes. These mixing vanes of the neighboring, second crossing point are disposed on the edge of a second web facing away from the stream of coolant. The second web crosses the first web and is inclined in mutually opposite directions with respect to the lateral surfaces of the second web. In FIGS. 6 to 10, spacing assemblies are not represented, but they are always provided if such a mixing grid with its mixing vanes is used at the same time as a spacer for the fuel rods. It can be seen from FIG. 10 that--following the direction of a specific longitudinal web like web 11, i.e. following line I.sub.M --this web crosses the transverse web 14 and web 14' . . . of the first group of transverse webs at crossing points 114, 114' . . . which lie in a lower plane, while the crossing points 113, 113', 113" of longitudinal web 11 with transverse web 13 and the other webs 13', 13" . . . of the second group of transverse webs lie in an upper plane (line II.sub.M). Therefore, the crossing points are divided into two groups, according to different planes, and crossing points of the one group alternate with crossing points of the other group. This is true for the direction of each longitudinal web like 11, 12 as well as in the direction of each transverse web like 13, 13', 13", 14. This checkerboard-pattern applies for the mixing vanes, too: Vanes 32 (some of which are clearly visible in FIG. 10) lie in an upper plane and alternate with vanes 30, 31 (mostly invisible while covered by other elements in FIG. 10) lying in a lower plane.