Patent Number: 061480541
Section: summary

TECHNICAL FIELD The present invention relates to fluid separation devices for use in vent volumes within a nuclear fuel bundle and particularly to devices insertable into and removable from the fuel bundle for flowing liquid laterally outwardly into the interstices between and onto adjacent surrounding fuel rods. BACKGROUND OF THE INVENTION A typical boiling water nuclear reactor has a reactor core comprised of a plurality of fuel bundles in side-by-side relation to one another. Coolant/moderator flows upwardly within the fuel bundles and about the fuel rods within the fuel bundles and is converted to steam to produce power. In U.S. Pat. No. 5,112,570, there is illustrated a fuel bundle having a plurality of part-length fuel rods (PLR). These PLR's are supported on the lower tie plate of each bundle and extend upwardly toward the upper tie plate. The rods, however, terminate short of the upper tie plate and typically between a pair of spacers along the fuel bundle. Between the upper end of each PLR and the upper tie plate, there is defined in the upper two-phase region of the fuel bundle a vent volume. This vent volume preferentially receives vapor from the two phase mixture of liquid and vapor in the upper region of the fuel bundle during power producing operations. There are many advantages associated with the use of PLR's including the increased vapor fraction within the vent volume and the pressure drop reduction in the upper two phase region of the bundle. These advantages include increased stability from thermal hydraulic and nuclear instabilities. It will be appreciated that the mechanical hardware associated with fuel rod spacers causes local reduction in the flow area available for the vapor and liquid flowing through the fuel bundle. This causes significant pressure drops to occur as the flow passes each spacer. By using PLR's, the associated flow blockage effects of one or more of the full-length fuel rods extending through these spacers above the PLR is substantially eliminated. That is, because of the absence of a fuel rod at a lattice location above one or more PLR's, additional flow area through the spacer is obtained with consequent reduction in pressure drop across such spacer. As a consequence, significant flow diversion occurs into the lower pressure drop paths or vent volumes above the upper ends of the PLR's. Increased vapor and liquid are therefore pumped from surrounding flow passages, i.e., the interstitial regions around the adjacent fuel rods, into these vent volumes. The creation of vent volumes above PLR's, and flow diversions resulting therefrom, however, can cause some reduction in critical power performance in the fuel bundle. Additional water may accumulate in the vent volume region above the PLR and thus be shunted out of the vent volume without heat generating contact with the remainder of the full-length fuel rods. Separation devices have been utilized to drive the dense liquid or water out of the vent volumes in generally lateral directions onto the surfaces of and into the interstitial regions between the full-length fuel rods to improve heat transfer performance. Such separation devices have generally taken the form of swirlers disposed in the vent volumes. These swirlers create a helical flow pattern causing the dense liquid to be driven laterally outwardly of the vent volume by centrifugal force. Such separation devices have been located within the spacers and have extended therefrom above or below the spacers. However, the separation devices are typically connected to the spacers, at least in part closing off the opening through the spacer, preventing access to a part-length rod in the registering opening or openings below the closed opening(s) of the superposed spacer(s). This complicates bundle assembly because typically the separation devices are not separate entities which can be removed and then reinstalled into the bundle assembly at the assembly site or in the field. For example, in the case of a failed part-length rod underlying one or more superposed spacers containing separation devices, the part-length rod cannot be removed from the bundle without disassembly of the bundle. BRIEF SUMMARY OF THE INVENTION According to the present invention, and in one aspect thereof, there is provided a fuel bundle for a nuclear reactor having a vent volume above a part-length fuel rod wherein one or more separation devices are carried by a support structure, e.g., a rod which can be inserted through and removed from the one or more registering openings of the spacers in registration above a part-length rod. The support rod carries at its lower end a connecting structure for engagement and connection with a mating connecting structure on the upper end of the part-length rod. The separation devices are preferably in the form of swirlers, which are located within the vent volumes between adjacent spacers when the support rod is connected to the part-length rod. Particularly, the swirlers are spaced just above the underlying spacer. The separation device and spacer opening or openings are sized relative to one another to enable withdrawal of the support rod and separation device through the spacer opening or openings upon disconnection of the upper and lower connecting structures of the support rod and the part-length fuel rod, respectively. This facilitates assembly of the fuel bundle and permits withdrawal in the field of a failed part-length rod by removal of the overlying support rod and separation device(s). In another aspect of the present invention, it will be appreciated that the part-length fuel rod is typically coupled to the lower tie plate, for example, by a threaded connection. By coupling the support rod for the swirler to the upper end of the part-length rod prior to assembly in the bundle, the support rod with attached swirlers, together with the part-length rod, may be inserted into the bundle through the registering openings of the spacers upon assembly of the fuel bundle. The support rod may then be used to screwthread the part-length rod into the tie plate, thus facilitating assembly of the fuel bundle. The support rod and part-length rod likewise can be removed from the bundle as a unit on-site, by unthreading the part-length rod from the lower tie plate and withdrawing the combined support rod, part-length rod and separation device(s) through the registering openings of the spacers. In both of the preceding aspects, and when using a ferrule-type spacer, the ferrule(s) of the spacer(s) in the vent volume above the part-length rod may be omitted. The swirler above each spacer may then comprise multiple blades having tips extending laterally to overlie the corners of the vent volume portion within the spacer. Thus, substantial portions of the entire vent volume area within the spacer are vertically aligned with the blades of the swirler. This requires substantially the entirety of the flow through the spacer opening left by the omitted ferrule to flow against the swirler blades for deflection laterally outwardly onto adjacent fuel rods. Moreover, the thickness of the spacer and the pitch of the swirler blades, in another form, can be such that the bladed swirler can be rotated, i.e., threaded past the spacer upon axial insertion and removal of the support rod carrying the swirlers. In this manner, an even larger area of the vent volume can lie in axial alignment with an overlying swirler. In another aspect of the present invention, the support rod may extend the full length of the fuel bundle, passing through the registering openings of the spacers between the upper and lower tie plates. One or more separation devices, e.g., swirlers, may be disposed at discrete locations along the support rod or, alternatively, the support rod may carry a separation device substantially along its full length. For example, a continuous helical swirler can extend about substantially the full length of the support rod for flowing a liquid laterally outwardly onto the surfaces and into the interstices of the surrounding full-length fuel rods. In this form, the support rod and swirler(s) may extend through the ferrules at those lattice locations in the spacers. In a preferred embodiment according to the present invention, there is provided a fuel bundle for a nuclear reactor comprising a plurality of fuel rods spaced laterally from one another in a matrix thereof enabling flow of liquid about the rods from a lower end of the fuel bundle toward an upper end thereof, a plurality of spacers spaced one from the other along the fuel bundle, each spacer having openings for receiving the fuel rods and maintaining the rods spaced from one another in the matrix thereof, at least one of the rods being a part-length fuel rod terminating in an upper end below upper ends of surrounding fuel rods and below at least one of the plurality of spacers, the part-length rod defining with respect to the surrounding rods a vent volume overlying the part-length rod and having a connecting structure adjacent an upper end thereof, a support structure extending through an opening in one spacer in registration with the part-length rod and having a connecting structure adjacent a lower end thereof, the support structure carrying a separation device disposed in the vent volume when the upper and lower connecting structures are connected to one another for flowing liquid laterally outwardly onto surfaces and into interstices of the surrounding fuel rods, the separation device and the opening of the one spacer being sized relative to one another to enable withdrawal of the support structure and the separation device through the opening of the one spacer. In a further preferred embodiment according to the present invention, there is provided a fuel bundle for a nuclear reactor comprising a tie plate, a plurality of rods including fuel rods spaced laterally from one another in a matrix thereof and connected to the tie plate, enabling flow of liquid about the rods from a lower end of the fuel bundle toward an upper end thereof, a plurality of spacers spaced one from another along the fuel bundle and having openings for receiving the fuel rods and maintaining the rods spaced from one another in the matrix thereof, a support structure extending through registering openings in the spacers and having a coupling structure adjacent a lower end thereof for releasably coupling the support structure and the tie plate to one another, the support structure carrying a separation device for flowing liquid laterally outwardly onto the surfaces and into the interstices of surrounding fuel rods, the separation device and the registering spacer openings being sized relative to one another to enable withdrawal of the support structure and the separation device through the registering spacer openings upon disconnecting the support structure and the tie plate relative to one another. In a still further preferred embodiment according to the present invention, there is provided in a fuel bundle for a nuclear reactor having (i) a plurality of fuel rods spaced laterally from one another in a matrix thereof enabling flow of liquid about the rods from a lower end of the fuel bundle toward an upper end thereof, (ii) a plurality of spacers spaced one from the other along the fuel bundle, each spacer having openings for receiving the fuel rods and maintaining the rods spaced from one another in the matrix thereof and (iii) at least one of the rods being a part-length fuel rod terminating in an upper end below upper ends of surrounding fuel rods and below at least one of the plurality of spacers, comprising the steps of providing a support structure extending through an opening in the one spacer in vertical registration with the part-length rod, providing a separation device on the support structure, locating the support structure in the fuel bundle with the separation device below the one spacer, withdrawing the support structure and the separation device through the opening of the one spacer enabling removal of the support structure and the separation device from the fuel bundle. Accordingly, it is a primary object of the present invention to provide removable separation devices for the vent volumes of nuclear fuel bundles enabling insertion and removal of support rods carrying separation devices as well as part-length rods relative to the fuel bundle.