Patent Number: 050911465
Section: summary

This invention relates to nuclear fuel bundles having part length rods. More particularly, part length rods are disclosed in combination with overlying steam vent tubes for permitting the high velocity localized discharge of steam from the fuel bundle. A system of apertures and devices is disclosed distributed along the steam vent tube to preferentially enable steam flow interior of the steam vent tube and preferentially exclude liquid moderator flow interior of the steam vent tube. BACKGROUND OF THE INVENTION Ueda Japanese Patent Showa 52-50489 disclosed the use of part length fuel rods for the creation of an improved fuel to moderator ratio in the upper two phase region of a fuel bundle, especially in the cold shut down state of the reactor. A fuel bundle was disclosed in which clustered part length fuel rods defined a large, central, generally conical shaped void in the upper two phase region of the fuel bundle. Two embodiments were disclosed in the Ueda reference. A first embodiment includes a large conical water rod occupying the large central conical void defined collectively by the part length rods. A second, and apparently preferred embodiment, disclosed the conical region otherwise unoccupied. Regarding this latter design, testing has established that while nuclear improvements in the upper two phase region in the cold state might be realized, adverse heat transfer performance, especially in terms of adverse critical power may be realized by large central void regions in a boiling water reactor fuel bundle. Specifically, the large defined void results in vapor being concentrated to the region. Unfortunately, surrounding portions of the two phase region tends to flow into the steam vent area. This results in the diversion of significant amounts of liquid coolant away from the heated rod surfaces adjacent the void, this liquid coolant being entrained in the accelerated steam flow within the large defined conical void. There results a reduced flow adjacent the full length fuel rods which surround the large void. This reduced flow rate has a corresponding reduced critical power on the rod surfaces adjacent the void. Overall fuel bundle efficiency is reduced. In Dix et al. U.S. Pat. No. 5,017,332 entitled Two-Phase Pressure Drop Reduction BWR Assembly Design, issued May 21, 1991 (formerly U.S. Pat. Application Ser. No. 07/176,975, filed Apr. 4, 1988), we maintained the nuclear benefits and removed the adverse thermal hydraulic effects by using generally smaller open flow channels dispersed across the two phase region of the fuel bundle lattice. The dispersed flow channels realized the natural tendency of the vapor phase of the two phase mixture to migrate ("drift") toward the low resistance flow paths where the realized flow was primarily vapor. It has been found that such dispersed flow paths are favorable for a BWR fuel assembly since preferential diversion of vapor away from the fueled rods has combined nuclear, stability, and thermal hydraulic advantages. SUMMARY OF THE INVENTION In a fuel bundle for a boiling water reactor having one or more part length rods in the two phase region, one or more steam vent tubes are introduced overlying the part length rods. The fuel bundle includes a lower tie plate for admitting water moderator and supporting a plurality of fuel rods in upstanding side-by-side relation, an upper tie plate for permitting water and steam to be discharged from the top of the fuel bundle and maintaining the fuel rods in upstanding side-by-side relation, a surrounding fuel channel for confining moderator flow along a path over the fuel rods and between the tie plates, and dispersed vertically intermittent spacers for maintaining the fuel rods in their designed side by side relation. One or more fuel rods extends from the lower tie plate vertically less than the full length to the upper tie plate ending interior of the fuel bundle at a disposition where the upper end of the part length rods is braced in the vertical position by a spacer. At least one of these partial length rods is provided with an overlying steam vent tube. This steam vent tube has openings and devices distributed along its length to encourage steam flow interior of the tube and remove liquid flow from the interior of the tube. The vent tube wall eliminates the interface drag between the steam interior of the tube and the surrounding water steam mixture. The presence of the steam vent tube as a high velocity steam escape path enables remaining portions of the fuel bundle to contain a higher liquid moderator fraction with flatter axial voids and power distributions during the operating state of the fuel bundle. The presence of high velocity escaping steam combined with distributed apertures along the length of the steam vent tube promotes resistance to fluid oscillations within the fuel bundles. Variations of steam vent tubes are disclosed including steam vent tubes overlying multiple clustered part length rods. There results a fuel bundle design in which the entire upper cross section of the fuel bundle is devoted to steam generation and coolant outflow as contrasted to a fuel bundle having conventional water rods. OTHER OBJECTS, FEATURES AND ADVANTAGES An object of this invention is to disclose the use of at least one steam vent tube overlying at least one part length fuel rod within a fuel bundle of a boiling water nuclear reactor. According to this aspect, the part length rod--resting on the lower tie plate and terminating before reaching the upper tie plate--is provided with an overlying tube. This tube opens overlying the end of the part length rod and extends from the vicinity of the end of the part length fuel rod at least part way the distance between the end of the fuel rod and the upper tie plate. Typically, the tube is open overlying the part length rods and open at its point of discharge to and through the upper tie plate. Because of the opening of the tube overlying the end of the part length rod, the steam vent tube preferentially admits initial vapor flow to the interior of the tube and thereafter rapidly vents the vapor to discharge at the upper tie plate. An advantage of the steam vent tube is that it forms a dividing wall flow interface between rapidly flowing, upwardly venting steam interior of the tube and the surrounding two phase steam water mixture exterior of the steam vent tube. There results an isolated steam flow channel having a reduction in flow friction and a local increase in the volumetric flux of vapor outflow. The turbulent interface between the upwardly flowing steam and the surrounding two phase mixture is removed. Consequently, entrainment between the upwardly flow vapor and the bounding two phase liquid vapor region is removed. A further advantage of the steam vent tube is that it establishes a localized high velocity steam vent within the interior upper two phase region of the fuel bundle. As a result, less of the upper cross sectional area of the fuel bundle within the two phase region is required for upward vapor flow. The upper volume of the fuel bundle not occupied by vapor flow can instead be occupied by two phase liquid vapor mixture having a higher water content. There results a fuel bundle capable of maintaining a higher overall liquid moderator content. Flatter axial voids and power distributions result. Yet an additional advantage of the steam vent tube is that the entire upper portion of the fuel bundle is utilized either for the generation of steam or out flow of liquid and vapor moderator. Large flow blocking water rods for increasing the fuel to moderator ratio--but also otherwise obstructing flow area--are not required. A fuel bundle concept is disclosed in which the steam vent tube in classifying water to the exterior of the vent tube functions to both provide improved moderator fraction while maintaining (and not obstructing) flow area. An additional advantage of the disclosed concept is that it provides additional moderator in a manner which is the inverse of moderator provided by the more typically used large water rods. By the expedient of providing a concentrated isolated flow path for the upward preferential flow of vapor, the remain portions of the fuel bundle in the upper two phase region can have increase liquid moderator content--equivalent in may cases to that moderator content supplied by a large water rod. At the same time, all areas of the upper two phase region of the bundle are available for moderator flow. As a result, no portion in the fuel bundle need be dedicated to flow obstructing water rods. An additional object of this invention is to disclose a series of openings along the length of the steam vent tubes, these opening designed to preferentially admit vapor interior of the tubes and to preferentially exclude liquid interior of the tube. According to this aspect of the invention, the steam vent tube has at least one--and is preferably manifolded--with openings along its length. These openings are designed with vapor liquid separation configurations for preferentially admitting vapor interior of the steam vent tube and preferentially excluding liquid interior of the steam vent tube. An advantage of the manifolding of the steam vent tube is that a natural resistance to hydraulic oscillations is present in the arrangement. Lower frequency natural resonant oscillations within the surrounding two phase region of the fuel bundle are not in phase with higher frequency natural resonant oscillations within the steam vent tube. The result is that the two different resonant frequency systems tend to dampen one another. The tendency of the fuel bundle to resonate at one frequency is reduced.