Patent Number: 051695936
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to nuclear reactors and, more particularly, to tools for removing and installing control rod drives for commercial power nuclear reactors. A boiling-water nuclear reactor employs a plurality of fuel rods containing a nuclear fuel within a reactor vessel. The reactor vessel is filled with water to a level at least sufficient to cover the fuel rods. Fission in the fuel rods releases heat that boils the water surrounding them. This steam is used, either directly, or through an intermediate heat exchanger, to perform a useful function such as, for example, driving an electric turbine-generator. The intensity of the nuclear reaction in a nuclear reactor is controlled, in part, by moving control rods between fuel rods. The control rods absorb neutrons, thereby controlling the intensity of the nuclear reaction, and the rate at which steam is produced. The control rods are controlled by control rod drives inserted through the bottom of the reactor vessel. Control rod drives occasionally require maintenance or replacement. This has presented a problem because of the structure of the control rod drives and the working environment in which they must be handled. A typical control rod drive is about 16 feet long and weighs about 450 pounds. It is thus an awkward device that requires substantial mechanical handling assistance to install and remove. In addition, the sub-pile room below the reactor vessel typically has a headroom between the floor and the bottom of the reactor vessel of about 18 feet. This leaves little maneuvering room for lowering the control rod drive, rotating it into a horizontal position, and moving it out of the sub-pile room. Also, numerous fragile instrumentation cables hang down from the bottom of the reactor vessel. Such instrumentation cables can be damaged by contact with a control rod drive. If an instrumentation cable is damaged, the rules governing operation of a nuclear reactor require that work must stop until the damaged instrumentation cable is repaired. A further problem arises because the sub-pile room below a nuclear reactor is a high-radiation area. It is thus desirable to limit the amount of time that workers spend in that area. The following publications relate to devices which are used to lower and rotate a control rod drive in the sub-pile room. All of these publications are in Japanese, and full translations are not available. A translation of claim 1 is available and is provided for the use of the Patent and Trademark Office: Japanese Patent Publication No. 60-48715 PA0 Japanese Patent Publication No. SHO-60-49277 PA0 Japanese Patent Publication No. SHO-61-31839 PA0 Japanese Patent Publication No. 58-32359 PA0 Japanese Patent Publication No. SHO-61-36636 PA0 Japanese Patent Publication No. 61-42838 PA0 Japanese Patent Publication No. SHO-61-42839 PA0 Japanese Patent Publication No. SHO-61-36635 PA0 Japanese Patent Publication No. SHO-61-33158 PA0 Japanese Patent Publication No. SHO-61-25116 PA0 Japanese Patent Publication No. SHO-61-13198 PA0 Japanese Patent Publication No. SHO-57-39398 PA0 Japanese Patent Publication No. 57-49833 PA0 Japanese Patent Publication No. SHO-58-27880 PA0 Japanese Patent Publication No. 59-31034 PA0 Japanese Patent Publication No. SHO-60-35035 PA0 Japanese Patent Publication No. SHO-60-35036 PA0 Japanese Patent Publication No. 60-37439 PA0 Japanese Patent Publication No. SHO-60-46676 The length of the above list is regretted. However, the spirit of full disclosure requires the inclusion of each reference of which the applicants are aware. Also, as the seal between a control rod drive and the reactor vessel is broken during removal, a small amount of residual water spills from in the reactor vessel. Usually, the spilling water, which is contaminated with radioactivity, falls upon a worker in the process of removing the control rod drive. Although workers wear protective clothing and breathing apparatus in this area, it is considered undesirable to permit residual water to fall upon them. Japanese Utility Model Application Publication No. 57-49834, and Japanese Patent Publication Nos. SHO-58-15759 and SHO-53-18676 disclose water drain apparatus for use with control rod drives. Bolts securing a control rod drive are highly torqued during installation. Due to the cramped conditions in the sub-pile room, it is difficult to maneuver suitable tools into place to detorque these bolts to enable their removal. Japanese Patent Publication Nos. SHO-61-22274 and SHO-61-22275 disclose tools designed to remove such bolts. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to provide tools for handling control rod drives that overcome the drawbacks of the prior art. It is a further object of the invention to provide a handling tool for a control rod drive that permits positive control of the control rod during all stages of the removal process. It is a still further object of the invention to provide a handling tool for a control rod drive that reduces the likelihood of damaging instrumentation cables below a reactor vessel. It is a still further object of the invention to provide a handling tool for a control rod drive that reduces the time required for removing and installing a control rod drive. Briefly stated, the present invention provides a low-headroom tower that is pivotably mounted to a trunnion cart. The trunnion cart runs on rails in a slot in a work platform located in the sub-pile room of a reactor containment. An elevator in the tower raises an extension piece into contact with the bottom of a control rod drive. A detorquing guide is rotationally positioned to coincide with bolts holding the control rod drive in place. The elevator places an upward force on the control rod drive during detorquing of the bolts. This provides reaction torque to aid in bolt loosening and prevents leakage of contaminated effluent past the seal. A detorquing tool is fitted into the detorquing guide and is spring loaded to engage a selected bolt securing the control rod drive. An indexing device provides alignment for the detorquing tool with each succeeding bolt. The elevator is lowered until the bottom end of the control rod drive enters the tower. The load is transferred from the extension piece directly to the elevator. Lowering continues until the top end of the control rod drive emerges from the reactor vessel. A winch pivots the tower to the horizontal position about the trunnion cart, and rear wheels are engaged with the rails to permit rolling horizontal movement of the tower. An effluent container clamps around the control rod drive to channel away contaminated water that passes through the broken seal as the control rod drive experiences its first movement. A two-piece radiation shield pig is preset onto guide rods to clamp quickly onto the top end of the control rod drive. According to an embodiment of the invention, there is provided apparatus for handling a control rod drive for a nuclear reactor, comprising: a tower positionable below the nuclear reactor, means for lowering and raising the control rod drive a substantial distance within the tower, and means for rotating the tower, containing the control rod drive, between a horizontal and a vertical position, whereby transfer of the control rod drive is enabled. According to a feature of the invention, there is provided a method for handling a control rod drive for a nuclear reactor, comprising: positioning a tower below the nuclear reactor, lowering and raising the control rod drive a substantial distance within the tower, and rotating the tower, between a horizontal and a vertical position, whereby transfer of the control rod drive is enabled. According to a further feature of the invention, there is provided a method for removing a control rod drive from a nuclear reactor, comprising: positioning a tower below the control rod drive, engaging an upper end of an extension piece with the control rod drive, lowering the extension piece and the control rod drive a first portion of a distance required to clear the control rod drive from the nuclear reactor, removing the extension piece, continuing lowering the control rod drive a remainder of the distance until the control rod drive is clear of the nuclear reactor, and rotating the tower, with the control rod drive therein, to a horizontal position, whereby horizontal displacement of the control rod drive is enabled. According to a still further feature of the invention, there is provided a method for installing a control rod drive in a nuclear reactor, comprising: rolling a horizontal tower, containing the control rod drive, into position below the nuclear reactor, rotating the tower, and the control rod drive, into a substantially vertical position wherein a top end of the control rod drive is generally aligned with a predetermined point on a bottom of the nuclear reactor, raising the control rod drive a first portion of a distance required to install it in the nuclear reactor, transferring a load of the control rod drive to an extension piece, and continuing raising the control rod drive a remainder of a distance required to install it in the nuclear reactor. According to another feature of the invention, there is provided apparatus for removing a control rod drive from a nuclear reactor, comprising: a tower, means for positioning the tower below the control rod drive, an extension piece, means for engaging an upper end of the extension piece with the control rod drive, means for lowering the extension piece and the control rod drive a first portion of a distance required to clear the control rod drive from the nuclear reactor, means for removing the extension piece, means for continuing to lower the control rod drive a remainder of the distance until the control rod drive is clear of the nuclear reactor, and means for rotating the tower, with the control rod drive therein, to a horizontal position, whereby horizontal displacement of the control rod drive is enabled. According to still another feature of the invention, there is provided a torque breaker for breaking torque of a plurality of bolts securing a control rod drive of a nuclear reactor, the bolts being disposed in a first pattern, comprising: an extension piece, means for engaging the extension piece with a bottom of the control rod drive, a torque breaker tool, engagement means at a first end of the torque breaker tool, the engagement means being effective for rotationally engaging one of the plurality of bolts, an indexing guide affixed to the extension piece, support means at a second end of the torque breaker tool, pivoting means at a second end of the torque breaker tool, means in the indexing guide for pivotably engaging the pivoting means, the indexing guide including means for indexing to a plurality of predetermined positions about a circle, the plurality of predetermined positions being of the same number as the plurality of bolts, means for permitting rotation of the indexing guide to an angular position providing vertical alignment of one of the plurality of positions with one of the plurality of bolts, means for maintaining a fixed relationship between the indexing guide relative to the plurality of bolts, and means for permitting engagement of the engagement means with successive ones of the plurality of bolts, whereby torque of the plurality of bolts is broken. According to a still further feature of the invention, there is provided a torque breaker for breaking a torque of a plurality of bolts in a control rod drive, the bolts being disposed in a predetermined pattern, comprising: an extension piece, means for engaging the extension piece with a bottom of the control rod drive, a torque breaker tool, an indexing guide affixed to the extension piece, the indexing guide defining a plurality of positions corresponding to the predetermined pattern, means for aligning the indexing guide in an aligned position wherein one of the plurality of positions is aligned with one of the bolts, whereby all of the plurality of positions are aligned with corresponding bolt positions, means for locking the indexing guide in the aligned position, the indexing guide including means for retaining a bottom end of the torque breaker tool at any selectable one of the plurality of positions, an engaging portion at a top end of the torque breaker tool, the engaging portion including means for engaging an aligned one of the bolts, means for exerting torque on the torque breaker tool, whereby the one of the bolts is loosened, and means for indexing the torque breaker tool to a next one of the plurality of positions, whereby a next one of the bolts may be loosened. According to a still further feature of the invention, there is provided an effluent container for catching a burst of effluent from a nuclear reactor when a control rod drive is removed therefrom: a rod, means for moving the rod into forcible contact with a bottom of the control rod drive, the forcible contact being effective for avoiding substantial leakage of the effluent from the control rod drive, first and second halves of a water container, each of the first and second halves including a semi-cylindrical sidewall and a semi-circular bottom, each of the bottoms including a semi-circular cutout, the semi-circular cutouts being fitted together to form a circular hole generally conforming to a peripheral surface of the rod, means for conducting a liquid from the liquid container, the liquid container being fittable over a bottom of the control rod drive including a location from which effluent leakage is expected, a clamp cylinder fittable over the liquid container, the clamp cylinder being effective for holding the first and second halves of the liquid container together, and means on the clamp cylinder for permitting retention of the clamp cylinder while the liquid container is slid downward therethrough. According to a still further feature of the invention, there is provided a radiation shield pig assembly for shielding a filter end of a control rod drive as it exits a nuclear reactor, comprising: first and second guide rods affixed below the nuclear reactor adjacent opposed sides of the control rod drive, a first hanger assembly, first means for temporarily affixing the first hanger assembly on the first guide rod, a second hanger assembly, second means for temporarily affixing the second hanger assembly on the second guide rod, a first semi-cylindrical half shield, first quick-release means for affixing the first semi-cylindrical half shield to the first hanger assembly, a second semi-cylindrical half shield, second quick-release means for affixing the second semi-cylindrical half shield to the second hanger assembly, means for clamping abutting edges of the first and second semi-cylindrical half shields to form a cylindrical radiation shield, means for clamping the cylindrical radiation shield to the control rod drive, and means for releasing the first and second semi-cylindrical half shields from the first and second hanger assemblies, whereby the cylindrical radiation shield may remain on the control rod drive during movement thereof. According to a still further feature of the invention, there is provided a method for shielding an end of a control rod drive of a nuclear reactor, comprising: affixing first and second guide rods below the nuclear reactor adjacent opposed sides of the control rod drive, temporarily affixing a first hanger assembly on the first guide rod, temporarily affixing a second hanger assembly on the second guide rod, affixing a first semi-cylindrical half shield to the first hanger assembly, affixing a second semi-cylindrical half shield to the second hanger assembly, clamping together abutting edges of the first and second semi-cylindrical half shields to form a cylindrical radiation shield, clamping the cylindrical radiation shield to the control rod drive, and releasing the first and second semi-cylindrical half shields from the first and second hanger assemblies, whereby the cylindrical radiation shield may remain on the control rod drive during movement thereof. The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.