Patent Number: 056687281
Section: summary

TECHNICAL FIELD The present invention relates in general to the steam vent region above the upper ends of part-length fuel rods in a nuclear fuel bundle and particularly relates to deflectors disposed above the part-length rods enabling improved thermal hydraulic performance in the upper regions of the fuel bundle and facilitating removal of the part-length fuel rods. BACKGROUND In a boiling water nuclear reactor, a plurality of nuclear fuel bundles are provided, each including a matrix of upstanding fuel rods. Each fuel rod contains a substantial number of stacked, cylindrical pellets of enriched uranium, the fuel rods being sealed at opposite ends. The fuel rods of each bundle are supported on a lower tie plate and extend upwardly from the lower tie plate to an upper tie plate. The fuel bundle is thus defined by a lattice of rows and columns of vertically extending, generally parallel, laterally spaced fuel rods. In addition to supporting the fuel rods, the lower tie plate permits entry of coolant/moderator water within the fuel bundle. The upper tie plate maintains the matrix of fuel rods upright from the lower tie plate and permits the discharge of heated water and generated steam from the interior of the fuel bundle. The fuel bundle also includes spacers, for example, seven such spacers, spaced along the length of the fuel bundle. The spacers serve to maintain the fuel rods laterally spaced from one another and are disposed at intervals along the lengths of the fuel rods. A channel surrounds each fuel bundle and extends from the lower tie plate to the upper tie plate. The channel confines the water flow from the lower tie plate to the upper tie plate. The fuel bundles are positioned in the reactor core between a lower core plate and an overlying top guide. In operation, water enters each fuel bundle through its lower tie plate. The water acts both as a coolant and a moderator. As a coolant, the water is heated and evolves to steam by heat exchange with the fuel rods. As a moderator, fast neutrons from the nuclear reaction are moderated and become slow-moving or thermal neutrons to continue the chain reaction within the fuel bundle. As the water is heated, the bundles have increasing fractions of steam in the upper two-phase portion of the fuel bundle. It has been found beneficial to provide a plurality of less than full-length fuel rods in the nuclear fuel bundle and these rods are called part-length fuel rods. These part-length fuel rods are supported on a lower tie plate and extend upwardly toward the upper tie plate, terminating short of the upper tie plate in or just above one of the upper spacers, for example, the fifth or sixth spacer. Thus, a vent volume is defined between the upper tie plate and the upper ends of the part-length fuel rods. It is known to provide in this vent volume a steam vent tube and liquid deflector. See, for example, U.S. Pat. No. 5,091,146, of common assignee herewith. The steam vent tube and liquid deflector of that patent was not provided with capability for removal to provide access to the underlying pad-length fuel rods. DISCLOSURE OF THE INVENTION According to the present invention, there is provided in a nuclear fuel bundle of the foregoing-described type a deflector for disposition in the volume above the part-length fuel rods for improving thermal hydraulic performance, while maintaining nuclear performance, and simultaneously facilitating removal of the part-length fuel rods. To accomplish this, a deflector is disposed across the steam vent region above the upper end of the part-length rods. As a consequence, all of the water flowing upwardly between the part-length rods is diverted away from the steam vent region into the interstices of the full-length rod region. The more dense liquid tends to remain in the interstices of the full-length rods, while the lighter, more mobile vapor moves toward the open steam vent region above the deflector. As a result, the upper two-phase region of the fuel assembly which contains the full-length fuel rods has a beneficially reduced void fraction and improved critical power limits. The deflector may take various forms in accordance with the present invention. For example, a flat plate overlying the upper ends of the part-length fuel rods may be used. A flat plate, however, causes significant pressure drop and is therefore not an optimum choice. An inverted pyramidal-shaped or cone-shaped deflector is preferable. The inverted pyramid may comprise four sides in alignment with the rectilinear sides of the steam vent whereby water is deflected laterally outwardly from its four sides into the interstices of the full-length fuel rods. The pyramid may also be rotated about its axis approximately 45.degree. so that the upwardly flowing liquid will be deflected toward the corners of the rectilinear vent volume. It will be appreciated that an inverted cone may likewise be used. An inverted cone has the advantage of affording uniformity of flow distribution issuing from the part-length rod region. The advantages of a cone-shaped deflector can be approximated by a pyramid having additional sides, for example, eight sides, whereby an approximate uniform laterally outward flow distribution is afforded. A swirl deflector may likewise be used. The swirl deflector can produce a vortex downstream of the deflector which will continue to separate liquid out of the steam vent path. The deflector may be releasably attached within the fuel bundle to the spacer or can be located above the spacer on a structural rod or rods depending from the upper tie plate for easy removal of the deflector through the upper end of the fuel bundle. In a further form hereof, the deflector may have openings for receiving the upper ends of the part-length fuel rods or at least the end plugs thereof. In a preferred embodiment according to the present invention, there is provided a fuel bundle for boiling water reactors comprising a plurality of vertically aligned, spaced-apart fuel rods within the fuel bundle for generation of a fission reaction in the presence of water moderator, a lower tie plate for admitting water moderator through the lower tie plate to the interstices of the fuel rods and supporting the vertically aligned, spaced-apart fuel rods, an upper tie plate for enabling water and steam to be discharged from the top of the fuel bundle and maintaining the vertically aligned, spaced-apart fuel rods in upstanding, spaced-apart, side-by-side relation, a surrounding fuel channel for confining moderator flow along the fuel rods and from the lower tie plate to the upper tie plate, at least one of the fuel rods being a pad-length rod extending vertically from the lower tie plate and terminating shod of the upper tie plate wherein the upper end of the one part-length rod defines with respect to the surrounding fuel rods a volume overlying the part-length rods under the upper tie plate, at least one deflector in substantial vertical registration with at least one part-length rod and disposed in the volume for deflecting flow of liquid laterally away from the volume and into the interstices of laterally adjacent fuel rods, enabling reduction in void fraction in the interstices, the deflector being releasably secured in the fuel bundle to enable access to and removal of the part-length fuel rod through the top of the fuel bundle. Accordingly, it is a primary object of the present invention to provide novel and improved deflectors for the steam vent region of a nuclear reactor fuel bundle, enabling improved thermal hydraulic performance, while simultaneously reducing the void fraction about the full-length fuel rods and facilitating access to and removal of the part-length rods.