Patent Number: 053295633
Section: summary

BACKGROUND OK THE INVENTION 1. Field of the Invention The present invention relates generally to controls rods of a nuclear reactor. More specifically, the present invention relates to a control rod latch tool which facilitates the release and removal of the control rods when replacement is required. 2. Description of the Relevant Art FIG. 1 is a perspective partially cut-away view of a boiling water reactor of the type to which the present invention is applicable. This BWR (boiling water reactor) includes, merely by way of example, a head 40 which is secured to the top of the reactor vessel 41, a vent and head spray 42, a steam dryer lifting lug 43, a steam drier assembly 44, a steam outlet 45, a steam separator assembly 46, a feedwater inlet 47, a core spray inlet 48, a feedwater sparger 49, a low pressure coolant injection inlet 50, a core spray line 51, a core sprayer sparger 52, a top guide 53, a jet pump assembly 54, a core shroud 55, fuel assemblies 56, a jet pump/recirculation water inlet 57, a core plate 58, control rods 59 (enclosed in elongate tubular guide thimbles), a recirculation water outlet 60, a vessel support skirt 61, a shield wall 62, control rod drive mechanisms 63, control rod drive hydraulic lines 64, and an in-core flux monitor 65. In this type of BWR the control rods 59 have an essentially cruciform cross-section, and, in order to facilitate refueling (viz., replacement of the fuel assemblies), are disposed at the bottom of the reactor vessel in a manner wherein they can be selectively moved up between the fuel rod assemblies by the hydraulically operated locking piston type control rod drive mechanisms 63. FIG. 2 shows an example of a control rod 59 which is used in this type of reactor. This illustrated arrangement includes a lifting handle 66, upper guide rollers 67, a sheath 68, a plurality of neutron absorbing rods 69 enclosed in the sheath to define four blades 70, a coupling release or unlatching handle 71, a velocity limiter 72, lower guide rollers 73, and a coupling socket 74. Details of the construction of the lower portion of a fuel rod is shown in FIGS. 3 and 4. As shown, the coupling socket 74 houses a lock plug 75 and a lock plug return spring 76 which are arranged at the lower end of an actuating shaft 77. When coupled to a control rod drive, the coupling socket 74 is arranged to operatively engage with a spud 78, and an unlocking tube 79 which extend up from an index tube--drive 80, in the manner illustrated in FIG. 4. In order to disconnect a control rod 59 of the above described nature from an associated control rod drive, it is necessary to raise the unlatching handle 71. This releases the coupling arrangement shown in FIG. 4 and allows the control rod 59 to be lifted by the lifting handle 66 located at the top of the rod, up and out of the reactor vessel 41. As is well known, when a reactor of the illustrated type is opened for refueling or maintenance, the vessel 41 is flooded with water in order to suppress radioactive emissions. This, in combination with the fact that the control rods 59 are located deep in the reactor vessel 41, as will be appreciated from FIG. 1, renders access very difficult. Nevertheless, it is required to be able to periodically remove the control rods and/or the control rod drives from the reactor vessel. FIGS. 5 to 10 show an existing latch tool which is used to lift the unlatching handle and release the control rod 59 from its respective control rod drive. This tool basically comprises a stud 81 which extends upward from a frame 82 for connection to a crane line, a tube assembly 83, an actuator link air cylinder 84, a lifting hook air cylinder 85, and an actuator link 86 which is adapted to engage the unlatching handle 71. The actuator link 86 is operatively connected with the air cylinder 84 by way of an actuator rod 87. As shown, the actuator link 86 is supported near the bottom of a V-cross-section structural member 88. Details of a lifting hook structure 89 and the manner in which it is pivotally mounted and connected with the lifting hook air cylinder 85 are illustrated in FIG. 6. In order that the actuator link 86 be accurately located, the arrangement is provided with a short blade-shaped back guide member 90. Accordingly, when this tool 100 is put into use, the tool engages the unlatching handle of the control rod 59 from one side while the back guide 90 provides an aligning function on the other. The actuator link air cylinder 84 and the lifting hook cylinder 85 are supplied with air under pressure by way of air connectors 91. However, with this existing type of arrangement it is necessary for the fuel support piece to be removed before suitable disposition and alignment of the tool can be achieved. This removal is quite time consuming. Accordingly, it has been proposed to remove the back guide 90, from the latch tool in order enable its use without the need to remove the fuel support piece. However, this measure has not met with success in that, without the back guide tool engagement is unreliable and results in problems with the engagement to the unlatching handle. An alternate technique which has been proposed in order to save time and avoid the handling problems related with the removal of the fuel support piece, is such as to use a J-hook which is suspended from a cable, to unlatch the control rod 59. However, this technique suffers from the drawbacks that, as the unlatching handle is submerged in approximately 60 feet of water, in order for the operator to be able to observe the hook and control the maneuvering of the same, it is necessary to utilize a TV camera and immerse it to essentially the same depth as the hook. Correct positioning of both the hook and the camera is time consuming, requires at least two operators and requires a certain amount of experience and skill. SUMMARY OF THE INVENTION It is an object of the present invention to provide a new control rod latch tool which simplifies the unlatching of a control rod from its associated control rod drive. It is a further object of the present invention to provide a control rod latching tool which can be used for both unlatching and lifting the control rods. A further object of the present invention to provide a control rod latch tool which enables the tool to be placed in position on a control rod without the need to remove the fuel support pieces first. In brief, the above objects are achieved by a latch tool which features two elongate arms which extend down along either side of a control rod. Rotatable actuating rods are mounted on the arms. The upper ends of the actuating rods are provided with cranks which engage in slots formed in an actuating disc. As the disc is rotated, the cranks rotate the actuating rods and swing cam-like members, which are fixed to the rods into position under an unlatching handle. In the preferred embodiments, the actuating disc is threaded onto the exterior of a rotatable input member, so that after the cam-like members are swung into position, the actuating disc threads its way up the input member without undergoing any rotation and in a manner which pulls the cam-like members upwardly and lifts the unlatching handle. More specifically, a first aspect of the present invention resides in a tool which is designed to facilitate the unlatching of control rods from a control rod drive, and which features: a rotatable adapter member through which rotational energy can be input; an actuator disc operatively connected with the rotatable member so that rotation of the rotatable adapter member induces rotation of the actuator disc, the actuator disc having an elongate slot formed therein; and an actuator shaft having a crank at a first end and a cam-like member at a second end, the crank being received in the elongate slot so that rotation of the actuator disc induces the actuator shaft to rotate and cause the cam-like member to rotate through a predetermined angle. A second aspect of the present invention resides in a tool for use in a nuclear reactor and which can be lowered onto an essentially cruciform cross-section control rod having a lift handle at the top and an unlatching handle arranged near the bottom, the control rod being releasably connected with a control rod drive by a connection mechanism which is operatively connected with the unlatching handle, the tool comprising: first and second arms which extend down along opposite sides of the control rod from a cross-member which is seatable on the top of the control rod; first and second actuator rods which are supported on the first and second arms; first and second cam-like members which are fixed to the first and second actuator rods, respectively; a rotatable drive input member which is rotatably mounted on the cross member; and crank means operatively interconnecting the input member with the first and second actuating rods, for selectively inducing the first and second actuator rods to rotate and to induce the first and second cam-like members to engage and then lift the unlatching handle. As will be appreciated from the above, features which characterize the present invention reside in that the tool can be placed in an operative position without the need to remove the fuel support piece and when lowered down onto the control rod, becomes symmetrically located on both side of the control rod, in a manner which prevents separation of the control rod and the latch tool. This of course ensures reliable engagement with the unlatching handle. As the tool is used with a so called "rigid pole" system, precise location and operation is facilitated. The engagement, unlatching movement, and disengagement of the tool is easily carried out, without the need for air cylinders and valves, by a single operator who only needs to rotate the rigid pole through a number of turns .