Patent Number: 052456439
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, a perspective view of a fuel bundle B containing the top filled water region R is shown. The fuel bundle includes a lower tie plate 14 for supporting an upstanding matrix of fuel rods including full length fuel rods 16 and part length fuel rods 18. Only one spacer S is illustrated, this spacer being shown even with the top portion of the part length fuel rods 18 for both maintaining the full length fuel rods 16 and part length fuel rods 18 in their designed side-by-side relation as well as supporting the ends of the part length fuel rods 18. An upper tie plate 20 is only partially shown at the point of attachment of fuel bundle lifting bail 22. This upper tie plate 20 is the point to which full length fuel rods 16 attach. In a typical fuel bundle B construction, certain of the full length fuel rods (called tie rods) are threaded at the top and bottom used to tie the respective lower tie plate 14 and the upper tie plate 20 together. A channel C extends around the lower tie plate 14 at the bottom, the upper tie plate 20 at the top and serves to confine fluid flow between the tie plates to the exclusion of fluid flow exterior of the fuel bundle B. Since lower tie plate 14 admits single phase coolant and the upper tie plate 20 discharges both liquid coolant and generated steam, it will be understood that part length water region R is located in the upper so-called two phase region of the fuel bundle B. In the particular view shown in FIG. 1, a cruciform water region R is illustrated within a 9 by 9 matrix of fuel rods 16,18. Further, part length fuel rods 18 are also placed in a cruciform shaped matrix underlying cruciform shaped water region R. Thus, it will be understood that the part length water region R occupies the spatial interval between the top of part length fuel rods 18 and the bottom of upper tie plate 20. Referring to FIGS. 3 and 4, the construction of the top filled water region R can be further understood. Region R includes cruciform sectioned bottom wall 33 and side walls 35, 36. At least the bottom walls and side walls are sealed so as to define within water region R a fluid tight container. Thus water captured within part length water region R will remain within the water region, except for that converted to steam by neutronic heating. It will be understood that the part length water region R could possibly have an open top--although this is not preferred. Provision must be made for liquid entry into the top of water region R as well as the escape of generated vapor from the water confined within water region R. Accordingly, and referring to FIGS. 7 and 8, construction of water entry port 31 and vapor exit port 41 are illustrated. Referring to FIG. 7, the preferred construction of water entry port 31 is illustrated. A scoop opening 51 protrudes outwardly over side 35 of top filled water region R. As is known in the boiling water nuclear art, water layer 52 tends to accumulate on the side of side wall 35 and flow along side wall 35 in a thin film 52. This thin film 52--together with liquid particles 54 in the upward passing two phase flow of steam and water tend to be scooped to the interior of part length water region R. Thus, and with normal operation of fuel bundle B, part length water region R will fill with water. It is to be understood that water region R has a source of heating from the ambient neutron flux within fuel bundle B. Consequently, vent opening 41 is defined in top 38 for escape of generated steam. Fortunately, the amount of vapor present in top filled water region R is predictable--usually on the order of 20%. This being the case, approximately 80% of top filled water region R will be filled with liquid. Referring to FIG. 8, top filled water region R is shown with an alternate side scoop construction 31', Simply stated, wall 35 recess under top edge 61. Liquid film 52 and liquid particles 54 moving along side wall 35 are deflected into the top portion of top filled water region R. Again, vapor vent 41 in top 38 provides for the escape of neutronic heating generated vapor from the confined water 70. While we show a cruciform sectioned top filled water region R, it will understand that this invention will admit of a variety of constructions. For example, top filled water region R' is shown in FIG. 2 being essentially square in cross section and overlying nine part length fuel rods in a 3 by 3 array. Further, it will be understood with respect to FIG. 5 and 6 that the construction of the top filled water region R" may both be discrete with respect to each part length fuel rod 18, constituting a cylinder extending beyond the end 19 of each part length fuel rod 18. Further, the top filled water region R" can either terminate above upper tie plate 20 as shown in FIG. 5 or terminate below upper tie plate 20 as shown in FIG. 6. It will be understood that the length of the top filled water region with respect to the overall length of the fuel bundle is limited. Specifically, the generation of steam by neutronic heating within the top filled water region R will cause vapor to rise within the accumulated liquid. This being the case, a counter current flow limiting condition can occur within top filled water region R. If the region is too long, the upwardly rising vapor will carry contained water with its flow--rendering the amount of water within top filled water region R essentially unpredictable. As a consequence of this phenomenon, we prefer to terminate the top filled water region just above the 5th or 6th spacer--assuming that the heat generating part of the fuel bundle has about a 150 inch length. With this length, the amount of contained liquid within top filled water region R is predictable and can constitute a known in the nuclear design. Attachment of the top filled water region R can occur by any expedient. In the illustrated example, we have attachment of the top filled water region R to those spacer S which adjoin the top filled water region.