Patent Number: 052805085
Section: summary

This invention relates to the insertion and replacement of fuel rods to fuel bundles. More particularly, and in the case of fuel bundles having part length rods, provision is made for the insertion and replacement of recessed part length rods to enable inspection and or replacement of the part length fuel rods without disturbing or requiring disassembly of the rest of the fuel bundle. BACKGROUND OF THE INVENTION In Dix et al. U.S. Pat. No. 5,112,570 entitled Two Phase Pressure Drop Reduction BWR Assembly Design, the concept of partial length rods is disclosed. This concept may best be understood by first understanding the construction of a conventional fuel bundle with conventional fuel rods and thereafter setting forth in summary format the part length rod invention of the above referenced patent, which patent is incorporated by reference in this disclosure. A conventional boiling water fuel bundle includes a lower tie plate for supporting a matrix of vertically upstanding fuel rods at their lower end and an upper tie plate for holding the same matrix of vertical upstanding sealed fuel rods at their upper end. The lower tie plate permits the inflow of liquid moderator (water) around the fuel rods, the upper tie plate permits the outflow of liquid moderator (water) and generated vapor moderator (steam) at the upper end of the fuel bundle. A channel surrounds the lower tie plate, surrounds the upper tie plate, and defines a confined flow path for the moderator between the upper and lower tie plates about the matrix of upstanding fuel rods. Fluid flow is thus confined between the upper and lower tie plates and isolated from surrounding liquid moderator in the so-called core by pass area immediate adjoined to the fuel channel on the outside of the fuel channel. The fuel rods within the fuel bundle are flexible and unless otherwise restrained would come into contact with one another under the forces of flow induced vibration and so-called "creep"--a differential growth in the fuel rods resulting from their heated, pressurized and radioactive environment. This being the case, a system of fuel rod spacers is distributed from the top to the bottom of the fuel bundles. These spacers form a matrix of individual cells discretely surrounding each fuel rod at spaced apart elevations within the fuel bundle holding the flexible fuel rods in their designed side-by-side relationship. During operation of the fuel bundle in a reactor core of a boiling water nuclear reactor, the fuel bundle can be dynamically described as having two regions of operation. These regions include a lower single phase region containing liquid moderator (water) and an upper two phase region containing liquid moderator (water) intermixed with increasing fractions of vapor moderator (steam). In a boiling water nuclear reactor, the moderator serves two purposes. First, the moderator moderates fast neutrons generated by the reaction into slow or thermal neutrons necessary to continue the reaction. Secondly, the dense water moderator turns to expanded saturated steam. Energy is extracted from the saturated steam by passing the steam through an engine, such as a steam turbine. Having set forth the conventional construction and operation of a boiling water nuclear reactor fuel bundle, the concept of part length rods can now be discussed. In the above reference Dix et al. U.S. Pat. No. 5,112,570, so-called part length rods were disclosed. In short, an invention is set forth in which "a plurality of fuel rods extending from (the) lower tie plate toward the upper tie plate , (the) part length rods terminating within the two phase region of the bundle before reaching the upper tie plate;". The invention makes the point that "at least two of the part length rods (are) separate from one another so as to define in at least two locations in (the) bundle spaced apart and separate vents commencing at the top of said partial length rods and extending to (the) upper tie plate." Specifically, "each of (the) spaced apart vents (is) immediately adjoined by adjacent full length fuel rods." Usefulness of the invention is set forth. Specifically, an improved fuel to moderator ratio is created in the upper two phase region of the fuel bundle. More importantly, pressure drop reduction in the upper two phase region of the fuel bundle is set forth. This enables greater stability of the fuel bundle and a reactor core including fuel bundles against thermal hydraulic and nuclear, thermal hydraulic instabilities. Additionally, the fuel to moderator ratio is improved, especially in the cold operating state. The reader will understand at this point that the disclosed part length rods relate to the required spacers in two ways. First, since the part length rods do not extend to the upper tie plate, it is the spacers that hold the part length rods vertically upright at their respective upper ends. Secondly, since the part length rods terminate before the upper tie plate, some of the spacers in the upper two phase region of the fuel bundle overlying the ends of the part length rods. As will hereinafter appear, it is these spacers that constitute an obstacle in the desired removal of the part length rods. It is common to inspect fuel bundles, and especially the individual fuel rods of fuel bundles during the operational life times of the fuel bundles. Unfortunately, the very presence of the part length rods renders the inspection of the part length rods inherently difficult. A brief understanding of the constraints of such inspections can be helpful. Inspections of fuel bundle parts are typically made during reactor outages. During such reactor outages, the power output of the reactor is lost. This loss of power outage carries with it a corresponding loss in revenue. Any delay prolonging the reactor outage can be costly--running into lost revenues of well over several hundred thousand dollars per hour! Accordingly, provision must be made for rapid inspection of all parts of a fuel bundle, including the new part length rods. Conventional inspection of fuel bundles is typically accomplished in a submerged environment within a so-called "holding pool." The fuel bundle removed from the reactor is placed upright within the holding pool. Thereafter, the channel surrounding the fuel rods and the upper tie plate holding the fuel rods are removed. In the case of the fuel bundle having nothing but full length rods, the individual fuel rods may thereafter be accessed at the exposed top of each fuel rod, individually removed, inspected and replaced. Access to the fuel rods occurs at the top of the fuel rods. Such access is a routine matter. Unfortunately, part length rods present special problems. First, the part length rods have a length from the lower tie plate that is less than the full length rods. For example, the typical full length fuel rod is in the order of 160 inches in length; the typical part length fuel rod is in the order of 120 inches in length. In order to reach the part length rod, one has to penetrate a matrix of full length fuel rods. No view of the engagement of the part length rod is possible. All engagement is essentially "blind." Secondly, since the part length fuel rods do not extend to the upper tie plate, the part length fuel rods are held in their upright position by the spacers. Accordingly, any tool for the removal and replacement of the part length fuel rods can have a diameter dimension no greater than the diameter dimension of the fuel rods being inspected. It is required that the tool for the removal of the part length rods pass through the spacers overlying the part length rods. Finally, it is required in some applications that the part length rods be fastened against vertical movement to the lower tie plate. This is done by screwing the part length rods into receiving threads on the lower tie plate. Accordingly, the disassembly of such part length rods is correspondingly rendered more complicated. The screw threads stick. The part length fuel rod must be forcibly rotated. Accordingly, there is a need for both a tip on the part length rod and corresponding tools to render the rapid replacement and removal of such part length rods during reactor outages reliable and fast. SUMMARY OF THE PRIOR ART In the testing of the concept set forth in Dix et al. U.S. Pat. No. 5,112,570, entitled Two Phase Pressure Drop Reduction BWR Assembly Design, lead test assemblies were utilized having part length rods in certain locations of the fuel bundle. In the case of these lead test assemblies, the part length rods were not secured to the lower tie plate by a threaded connection; the part length rods rested with their weight being utilized to provided the connection to the lower tie plate. These lead test assemblies required the removal of the part length rods for inspection. In that case, each part length rod was equipped with a cylindrical tip. The cylindrical tip had a female screw driver indentation on the top and was provided with external male threads. Removal of the part length rods from the interior of the fuel bundle occurred by tool having an exterior tube with corresponding female threads and an internal, freely rotating male screw driver. In operation, the idea was to prevent the rotation of the part length rod through engagement of the male screw driver head with threaded engagement of the female threads of the extraction tool with the male threads of the part length rod tip. This prior art provision for the removal of the part length rods from the lead test assemblies was less than optimum. Considerable difficulty was experience with the registration of the male screw driver fitting of the extraction tool with the female screw driver indentation upwardly exposed from the tip. Further, threading of the male threads on the tip with the female threads of the tool was also less than optimum. The tool was pendulously suspended from a bridge a considerable distance to the particular part length rod to be remove and inspected. As a result, the female threads of the suspended tube were vertical. The part length rod, however, had been subjected to the forces of rod creep--a non linear differential growth--which often left the protruding threads of the part length rod less than vertical. Threading blind with the particular threads perfectly vertically aligned was difficult. The added complexity of the exposed threads of the part length rods being less than vertical made the part length rod removal task exceeding difficult in many cases. The reader will realize that the particular apparatus for the removal of the part length rods is prior art. The discovery of the difficulty in the intended removal of the part length rods is not prior art. This difficulty was only discovered when actual removal was attempted and is not known in the art. SUMMARY OF THE INVENTION A part length fuel rod tip and group of tip grasping tools is set forth for the removal, inspection and replacement of part length fuel rods from a fuel bundle having a part length rods interspersed with a majority of conventional full length rods. The part length fuel rod tip includes a longitudinal keyway allowing torque to be exerted on the fuel rod and a horizontal slot enabling grasping of the fuel rod for vertical withdrawal and replacement movement of the fuel rod. The tools include a torque socket for applying high torque forces to the part length rod for unscrewing the rod when the rod becomes stuck at its threaded connection to the lower tie plate, a tip grasping tool for permitting normal unscrewing torque and grasping for vertical withdrawal and replacement, and finally a tool having both high torque and grasping characteristics. In each case, the diameter of the tools is restricted to enable access to the part length fuel rods through the spacers overlying the upwardly exposed ends of the part length fuel rods. OTHER OBJECTS, FEATURES AND ADVANTAGES It is a first object of this invention to disclose a tip for a part length fuel rod which enables rotation under high torque forces as well as vertical withdrawal, manipulation for inspection, and finally enabling convenient replacement of the part length fuel rod. The tip constitutes a generally solid cylindrical addition to the fuel bundle constructed from the same material as the cladding of the fuel bundle--usually the alloy known as zircaloy. The upper surfaces of the tip are rounded, imparting male gathering surfaces to the upwardly exposed end of the fuel rod. The tip includes at least one longitudinally extending keyway. This keyway enables the tip--and the fuel rod to which it is fixed--to be rotated under forces of high torque. Finally, at a portion of the tip relatively close to the fuel rod, a female cylindrical section is removed from the tip S to define a stop surface. This surface enables the tip to be firmly grasped to assure both rotation under high torque and vertical withdrawal and insertion of the fuel rod. Referring to the "female cylindrical section" removed from the tip S, it will be understood that those having acquaintance with the mechanical design arts would preferred to use other descriptive terms. This portion of the tip could be referred to as a "horizontal slot", "horizontal keyway", "fly cut notch", and other similar terms. We use the term "female cylindrical section" so that this specification can be geometrically descriptive for the purposes of describing this invention with precision. An advantage of the disclosed tip is that it does not interfere with the conventional portions of the tip design including provisions for charging the tip with gas under pressure to suppress gas discharge from the fission process occurring within the fuel rod. Stopping at this point, the reader must remember the circumstances of the required inspection of the part length rods. During the reactor outage, time is of the essence. Further, and due to the fact that the part length rods are radio active and therefore must be handled with relative rapidity under water in a holding pool, manipulation of the following disclosed tools occurs at the end of a long pole within an upright fuel bundle having its channel and upper tie plate removed. Finally, the pole must penetrate into the matrix of full length rods--and water rods--upwardly exposed in the pool. At the bottom of the pole--out of view of the personnel performing the inspection--all manipulation of the part length rod must occur. An additional object of this invention is to disclose an alternate tool for rotating the part length fuel rod under high torque. According to this aspect of the invention, a female socket is disclosed. The socket include an inside bore having an inside diameter equal to the outside diameter of the part length rod tip. This inside bore includes a protruding key. This key is dimensioned with respect to the bore and spaced with respect to the bore to fit interiorly of the tip keyway. By remote rotation of the tool, remote rotation under high torque can be exerted on the part length rod for unscrewing motion from the lower tie plate. Another object of this invention is to disclose a general purpose tool for the normal manipulation of a part length rod. According to this aspect of the invention, a female tool member is disclosed having a slit peripheral cylinder defining at least three discrete cylinder segments. The slits in the cylinder defining the cylinder segments are longitudinal and at preferable 120.degree. intervals about the cylinder. This cylinder is given a dimension enabling the cylinder segments to be separated overlying the part length rod tip so as to fit in a snug and slightly expanded relationship over the tip outer surface. A remotely actuated reciprocating sleeve moves to and from a surrounding and embracing relationship over the slit cylinder. When the sleeve is withdrawn from the slit cylinder, insertion and withdrawal of the sleeve from the partial length rod tip can occur. When the sleeve overlies the slit cylinder, locking of the sleeve to the part length fuel rod tip at the outer surface only occurs. In this locked state, normal torque forces as well as vertical forces of withdrawal and insertion can be exerted on the part length fuel rod through the tip. An additional object of this invention is to disclose a tool which locks to the keyway and cylindrical section of the partial length fuel rod tip. According to this aspect of the invention, a tool is disclosed having a spring biased tang with an attached key and male cylinder segment for fitting to the female cylinder segment of the tip. This tang fits to a tool shaft in a tang slot with the key and male cylinder section spring biased into a tip locking position at that end of the tang remote from the point of attachment to the shaft. A reciprocating sleeve operates over the tang sprung key and male cylinder section. In operation, when the sleeve is withdrawn, the tool may be placed over the tip, rotated until spring biased engagement of the key at the longitudinal keyway occurs, and thereafter advanced onto the tip until the male cylinder segment of the tool engages in and locks to the female cylinder segment in the tip. When the sleeve is advanced overlying the tang, key and male cylinder segment, locking of the tool at the tip occurs enabling rotation under relatively high torque as well as axial replacement and withdrawal of the part length fuel rod. Finally, a ramp is provided at the upward end of the tang attached male cylinder segment to enable climbing of the male cylinder segment of the tool free of the female cylinder segment of the tip to enable tool removal from the tip. An advantage of the spring biased key is that it permits relative rotation and tactile location of the keyway at the end of the part length rod from the required remote location of personnel inspecting the part length fuel rod. A further advantage of all disclosed tool constructions is that they can freely pass to and from the part length fuel rod tips without disturbing the surround full length fuel rods or (more importantly) the overlying spacers. As a result, inspection can be conveniently carried out in the short time confines of a reactor outage.