Patent Number: 040381323
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is indicative of one half of a 36 nuclear fuel element designed for use in a Pressure Tube Nuclear Reactor and comprising a cluster of nuclear fuel rods 2 supported, axes parallel in spaced apart relationship by transverse grids. One such grid is shown at 3 and this comprises an outer band 4 joined at 60.degree. intervals by webs 5 which divides the space bounded by the band into six sectors each of which contains six fuel rod positioning ferrules 6. The ferrules 6 are welded together at their points of contact and with the webs 5. Correct positioning of the ferrules is assisted by C shaped spacers 7. The ferrules 6 receive the fuel rods 1 and together with end fittings (not shown) maintain them in spaced apart relationship, axes vertical and parallel to one another so defining interspaces between them through which a main coolant may be passed to remove heat from the fuel. In one of these interspaces an auxiliary coolant conduit 8 is located extending through and fixed to a central ferrule 9 to which the radially inner ends of the webs 5 are secured. The conduit 8 is closed at one, lower, end and has a number of lateral holes 10 in its wall so that when the conduit 8 is connected at its other end to a source of auxiliary coolant, the latter will emerge as a plurality of sprays directed transversely into the cluster of fuel rods at right angles to the flow direction of main coolant. This spray cooling facility is useful as an alternative or supplementary cooling to an axial flow of main coolant through the cluster from one end to the other. All the parts of the fuel cluster so far described are part of the prior art and do not of themselves form part of the invention. From the foregoing it will be understood that those fuel rods which do not lie in a line of sight with the holes 10 in the conduit 8 will be less effectively cooled by auxiliary coolant than those which do. In order to ameliorate the cooling of these fuel rods by auxiliary coolant, means are provided for modifying the direction of the jet of auxiliary coolant after injection into the cluster to deflect coolant on to the surfaces of fuel rods otherwise inaccessible to auxiliary coolant. Said means comprises a number of T-shaped target plates 11 attached by their cross pieces 11a to adjacent ferrules 6 by welds 12 and slotted to clear the adjacent radial web 5. Each stem 11b of the target plates 11 lie in the line of sight from one or more holes 10 in the conduit 8 and have a face directed, towards the conduit. This face is bounded by longitudinal edges 11c separated by a distance w equivalent to approx half the pitch of the fuel rods. The edges 11c of the cross piece are sharply defined; the edge surfaces making an acute angle .alpha. in FIG. 4 of preferably 60.degree., with the face of the target plate 11 (FIG. 4). In addition the target plates 11 are positioned so that the vertical centre line X--X (FIG. 3) through the face 11b of the plate directed towards the conduit lies in a plane passing through the said mid-pitch portion. In operation the nuclear fuel rods are normally adequately cooled by a flow of main coolant axially through the interspaces between the fuel rods. When auxiliary cooling is required, auxiliary coolant is injected transversely into the cluster via holes 10 in the conduits. In order to ensure that auxiliary coolant has access to wet rods in the outer row which are out of the line of sight of the auxiliary coolant jets, target places 11 depend from the grid so as to deflect a jet 13 from holes as at 10 on to parts of the surfaces of rods 2a, 2b which, being partially masked by other rods, may not otherwise be properly wetted by auxiliary coolant. It is important to provide sharp edges on the target plates 11 so that the jet 13 of auxiliary coolant water is deflected and properly spills off the target plate to envelope the adjacent pair of fuel rods 2a, 2b as shown in FIG. 5. The target plates need not be flat but are preferably made flat and as such are virtually unsensitive to any misalignment that may exist between the jet of auxiliary coolant and the larger plate which may set in after operation, say by the holes 10 becoming partially blocked.