Patent Number: 054917336
Section: summary

BACKGROUND OF THE INVENTION The present invention relates generally to nuclear fuel assemblies for use in nuclear reactors. More specifically, the present invention relates to a coolant vent duct for use in light water reactors, and more particularly to either a boiling water reactor or a pressurized water reactor. The coolant vent duct could be applied in conjunction with a part length fuel rod, with a water rod, with a water/fuel rod or simply by itself. In addition, the present invention relates to a hydraulic resistance strip positioned in the subchannel above a part length fuel rod to limit cross-flow from and between adjacent subchannels. In addition, the present invention relates to a part length fuel rod having an upper end fitting which functions to separate at least a portion of the liquid and the vapor portions of two phase flow. Furthermore, the present invention relates to a coolant diverter positioned within a fuel rod spacer of a nuclear fuel assembly for increasing the hydraulic resistance in the subchannels above part length fuel rods. It is known to generate large amounts of heat and energy through nuclear fission in a nuclear reactor. Energy is dissipated as heat in elongated nuclear fuel rods. Typically, a nuclear fuel assembly includes a number of nuclear fuel rods that are grouped together to form a nuclear fuel assembly. Such fuel assemblies include a number of elongated rods supported between upper and lower tie plates. It is known in boiling water reactor (BWR) fuel designs to include within fuel assemblies part-length fuel rods. Accordingly, some of the fuel rods in a bundle are truncated at some intermediate elevation in the core. This leaves an unfilled coolant channel above that elevation. By providing a truncated fuel rod, several important benefits are achieved. For example, there is a neutronic advantage in increasing the amount of fuel in the bottom of the core as compared to the top of the core. A more axial uniformity in water to fuel ratio is thereby achieved with an associated improvement in fuel cycle costs, increased shut-down margin, reduced pressure drop (principally because of increased flow area, but decreased wetted surface also reduces the pressure drop), and increased core stability because the pressure drop reduction occurs at the top part of the bundle where two phase pressure drops are most significant. Potentially, the part-length fuel rod could yield a small critical heat flux (CHF) benefit because of the reduced mass flux in the top part of the bundle. This potential is generally not achieved. An important factor is considered to be that the simple truncation of the part-length fuel rod results in large open subchannels that have less power density than the other subchannels in the bundle. This results in significant non-uniformities of subchannel enthalpy rises. Effectively, the flow in the other regular subchannels is reduced by a factor greater than one would expect merely from the increase in the bundle flow area that occurs above the top end of a part length fuel rod. Mixing devices and flow strippers have been utilized in an attempt to offset this problem somewhat at the expense of added pressure drop. A number of part length fuel rod constructions have been utilized in the prior art. U.S. Pat. No. 4,664,882 discloses a segmented fuel and moderator rod and fuel assembly for a boiling water reactor. The segmented rod has a lower fuel region and an upper moderator region for passing coolant through the upper portion of the boiling water reactor core which is normally undermoderated. The segmented rod displaces one or more conventional fuel rods in the fuel bundle. U.S. Pat. No. 2,998,367 discloses a core that includes short fuel rods, rods of intermediate length, and rods extending the full height of the core, immersed in light water. U.S. Pat. No. 4,789,520 discloses in an embodiment a fuel assembly having six fuel rods, each having a short fuel effective length portion in comparison to other fuel rods that are included in order to reduce the pressure loss within the fuel assembly. U.S. Pat. No. 4,957,698 discloses a fuel design that preferentially directs more unvoided water coolant into the upper region of the fuel assembly. This allows relatively more fuel to be placed in the lower portion of the fuel assembly. The arrangement is designed to allow moderation of neutrons in the upper portion of the assembly while preserving a higher volume of fuel in the lower portion. The larger number of fuel rods that can be used in the lower portion reduces the linear heat generation (power peaking) in the assembly. U.S. patent application Ser. No. 07/737,859, filed on Jul. 30, 1991, entitled: "IMPROVED FUEL ASSEMBLY FOR BOILING WATER REACTORS", and assigned to the assignee of this patent application discloses, in part, a fuel assembly. SUMMARY OF THE INVENTION The present invention provides a coolant vent duct structure to be preferably located above a part-length fuel rod portion that improves critical heat flux (CHF) performance with respect to typical part-length fuel rods without significant degradation of the benefits that are achieved by using such a system, e.g., improved fuel utilization, stability, and shut down margin. The present invention improves CHF performance by providing a better matching of subchannel hydraulic resistance to subchannel power in the top part of the bundle for the subchannels adjacent to these rods. A reduction of active flow channel enthalpies, i.e., void fraction, in the top part of the bundle is achieved by the structure of the present invention. The inventive coolant vent duct is not limited to use above a part-length fuel rod, it may be used above a water rod or a water/fuel rod or alone. To this end, in an embodiment, the present invention provides a duct structure that provides a hollow tube that is located above and is mechanically connected to the part-length fuel rod of a boiling water reactor fuel assembly. The duct structure can include an extension tube having at least one wall member defining an enclosed flow path therethrough, the extension tube being coupled to a portion of the part-length fuel rod so as to be disposed axially above the part-length fuel rod, and including at least one inlet opening, for allowing a portion of a fluid that surrounds the rod, that initially comprises a two phase mixture of steam and liquid, to enter the enclosed fluid path, and including at least one outlet opening located above the inlet opening, the extension tube can further include means for separating at least some of the steam located in the fluid from the liquid located therein. The structure allows steam to bypass the upper active portions of the fuel assembly. To accomplish this separation, the means can direct liquid water from the inlet holes while allowing steam to enter, or the means can entail directing water out of the duct while permitting the steam flow to continue flowing upwards inside the duct. In an embodiment, the duct includes a transition section and an upper section, the upper section having an outer perimeter greater than the outer perimeter of the fuel rod below. The inlet holes can be located in the upper section or the transition section or both. The upper section need not be round in cross section but preferably is round and is larger in diameter than the fuel rod below. In one form, the duct is a circular tube that has holes drilled into it and the transition piece merely provides a mechanical connection of the lower part-length fuel rod and the upper extension tube. This arrangement has some capability for separating steam from liquid because, owing to their greater inertia, the liquid drops flowing upwards towards the inlet holes are not as readily turned into the holes as is the steam. A drawback, especially when only a small amount of flow is taken into the extension tube, is that the liquid film that is on the surface of the rod can be drawn more readily into the holes, making separation performance less than desired. In an embodiment, the means for separating liquid from steam is located on an outer portion of the wall outside the enclosed area in juxtaposition to the inlet opening or openings. The means for separating directs liquid away from the inlet opening causing the fluid that enters an inlet opening to comprise a greater percentage of steam. For example, the means can comprise a V-shaped member extending from the outer wall upstream and adjacent to an opening. Assembly of this invention into the fuel bundle can impose dimensional limits on the radial extent of the V-shaped members or other means for diverting liquid films and drops from the inlet holes. For example, assembly of the bundle typically begins with a skeleton consisting of the lower tie plate, tie rods, and all the spacers. The remaining rods including the structure of this invention are then inserted through the openings in the spacers. The width of the openings for those cells which will contain this invention will be only slightly greater than the outside diameter of the upper tube. Thus, the radial extent of the means for diverting liquid away from the holes is limited to fall within the square envelope of the openings in the upper spacers. Once inserted through the spacers this invention may require rotation for optimum alignment of the inlet openings relative to the surrounding subchannels. An embodiment demonstrates that more flexibility as to the radial extent of the protruding members can be gained by locating the inlet holes into a hollow transition piece (hollow except at the bottom thereof) and which would have a local diameter (not including the protruding members) that is less than the diameter of the upper tube. An exemplary embodiment shows a different means of achieving sufficient separation performance. The liquid diverter is located upstream (i.e., below) of a group of inlet openings and serves to divert liquid away from all the openings in this group. For simplicity, the inlet openings are in the duct and a transition connector piece serves as the diverter. This means of liquid diversion includes one or more protrusions from the minimum section of the transition connector. Generally, there will be at least one protrusion at the top as the connector expands to the diameter of the upper tube. To be effective, the protrusions must have a reasonably sharp break in their surface so that the liquid film flowing radially outwards along the face of the protrusion will depart the surface and continue to move radially away from the transition connector piece. A double protrusion transition connector piece with a number of geometric characteristics to promote separation of liquid and steam can be utilized. The geometric characteristics are: (1) a smooth and gradual reduction in diameter moving axially upwards along the transition connector piece from the bottom end; the diameter is decreased so the subsequent protruding surfaces present a greater diversion of the overall flow from an axial to a radial direction; the diameter reduction is gradual so that the liquid film stays attached to the surface upstream of a first protrusion; (2) the first protrusion is shaped so that the liquid film will follow a smooth curved arc path as its flow direction is changed to have a large radial component; (3) the first protrusion ends in a sharp break and has only a small axial extent (i.e., the transition piece diameter is again reduced); this second reduction in diameter is done purposefully so that the recirculating flow behind the separating film acts to promote radial separation by providing a recirculating stream that joins with the separating film in a smooth tangential manner; the film separates as a continuous sheet of liquid that collides with the main axial flow of liquid drops and steam; this collision imparts outward radial momentum to the liquid drops that were flowing axially so as to move liquid in general radially away from the downstream inlet openings; (4) the second protrusion occurs as the transition connector piece is flared outwards to have the diameter of the upper tube. In this embodiment, the first protrusion takes the form of a ring shaped tapering disk located about the small diameter section of the transition connector piece. Another approach would be to use partial or segmented rings that are displaced axially that do not have a full 360.degree. extent at any one location. This approach can allow steam to pass more easily towards the inlet holes by flow around the ends of the liquid film sheets as opposed to having to penetrate the sheets as they start to breakup into drops. Another approach to separate liquid from steam ahead of the inlet holes would be to use turning vanes to give an azmathal or circular component to the flow in addition to its axial component. The addition of a swirling or twisting component to the flow is a common approach to achieving separation of liquid from steam since the liquid has more of a tendency to move outwards in response to centrifugal forces than does the steam. In another embodiment, the means for separating is located within the enclosed area of the duct, for example, the means for separating can comprise means for imparting a centrifugal force to the two phase mixture. If desired, at least two means for separating can be provided, one located within the enclosed area of the duct and the other on an outer wall portion of the duct to achieve a more complete separation of the steam from the liquid. In addition, the present invention provides a structure for a light water reactor fuel assembly having a part length fuel rod and a hydraulic resistance strip having a predetermined amount of hydraulic resistance, the strip being connected to an end of the part length fuel rod, the strip having an elongated continuous body defining an outer wall and having a cross-sectional area substantially less than the cross-sectional area of the part length fuel rod, the body being disposed axially above the part length fuel rod. The present invention provides a fuel rod for a light water reactor having a part length fuel rod portion and a reflex upper end fitting for separating at least a portion of the liquid and the vapor of two phase flow. The reflex upper end fitting is disposed axially above the part length fuel rod and in contact with the downstream end of the part length fuel rod. The reflex upper end fitting comprises a section having a diameter tapering downwardly into a reduced diameter and flaring thereafter into a second diameter terminating in a sharp break at a line around its perimeter, the tapering and flaring producing a smooth flow path for propelling fluid flowing therealong, radially inwardly and then outwardly from the downstream end of the part length fuel rod. In accordance with another aspect of the invention, a coolant diverter is provided for a nuclear reactor fuel assembly having a plurality of substantially parallel, elongated fuel rods. The coolant diverter comprises a stem portion and a flared diverter portion. The flared diverter portion is integrally formed with the stem portion so as to have a smooth, continuous, trumpet-like outer surface. The coolant diverter is for being mounted in a nuclear reactor fuel assembly for diverting at least a portion of the liquid coolant in a two phase coolant flow onto adjacent fuel rods. In a preferred form, the coolant diverter includes means for mounting the coolant diverter within a spacer for the fuel rods. The means for mounting preferably includes a diverter tube; the stem portion is disposed in the diverter tube; the diverter tube also is for guiding the two phase coolant flow along the coolant diverter. In accordance with this aspect of the invention, a nuclear reactor fuel assembly comprises a plurality of elongated fuel rods, means for supporting the fuel rods in spaced, substantially parallel relation where the supporting means includes a spacer for the rods, a coolant diverter and means for mounting the coolant diverter in the spacer. The coolant diverter has an elongated stem portion and a flared diverter portion integrally formed with the stem portion so as to have a smooth, continuous trumpet-like outer surface. The diverter portion terminates in a sharp break end portion. The coolant diverter is for diverting at least a portion of the liquid coolant in a two phase coolant flow onto adjacent fuel rods. In a preferred form, the nuclear fuel assembly has mounting means which includes a diverter tube for mounting the stem portion of the coolant diverter and for guiding the two phase liquid coolant flow along the coolant diverter. The diverter tube is mounted in the spacer. In one arrangement of the nuclear fuel assembly in accordance with the invention, the fuel assembly includes a part length fuel rod and the coolant diverter is positioned downstream of the part length fuel rod. In another arrangement of the nuclear fuel assembly in accordance with the invention, the coolant diverter is arranged between adjacent fuel rods.