Patent Number: 051165660
Section: description

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 shows an overview of a typical nuclear boiling water reactor (BWR) system 10. It typically may have a height of approximately 100 feet and a diameter of 50 feet. The primary reactor vessel, reactor coolant, control rod drives and other systems are housed in the dry well 20. The suppression chamber 30 contains a pool of water to condense steam which may be released within the dry well in the event of an operating accident. A vent system 40 connects the dry well 20 to the suppression chamber 30. The approximate location of the CRD penetrations 50 through the dry well is shown in FIG. 1 by line I--I. The CRDs 50 penetrate through a wall surface 45 and are arranged in a grid pattern of equal x-y spacing as shown in FIG. 2. The tubular CRDs typically have a diameter of approximately 1.315" and are spaced on 5" centers. Each CRD has an end cap welded on the upper end. With reference now to FIGS. 3 and 4, the milling device of the present invention includes a support block, clamp block 60, and a milling block 120, both of which can be fabricated from cold rolled steel. Clamp block 60 is seen to comprise an upper, generally planar portion 61 which inclines to a higher planar portion 63 to which milling block 120 is mounted. Clamp block 60 also includes at least two integrally formed CRD holders consisting of two open ended vertical slots 62 and 64 and two parallel horizontal slots 66 and 68 which will be described in further detail. Milling block 120 is vertically reciprocable with respect to clamp block 60 and supports an hydraulic motor 130 and milling drill bit 145 which will be described in further detail. As shown in the FIGS. 3 and 4, the vertical slots 62 and 64 formed in block 60 are parallel to one another, with the same center-to-center spacing as two adjacent CRDs so as to be aligned with the longitudinal axes of the CRDs. The slots 62 and 64 may have the modified hexagonal shape which is larger than the CRD's diameter as shown in FIGS. 3 and 4. The CRD holder 64, adjacent to the milling block mount, is cut away laterally to form a rectangular notch 67 for receiving a support member 65 therein. Track slots 66 and 68 extend transversely across the length of clamp block 60 from the edge opposite the milling block 120 transversing slots 62 and 64 to intersect with vertical slot 103. Clamping bars 70 to 73 are pivotally hinged and set into the track slots 66 and 68. Each clamping bar has a generally planar outer surface and the inner surfaces thereof have a modified partial hexagonal recess as at 74. Each clamp bar has a manually adjustable thumb screw 80-83 fitting into a threaded bore in the clamp block (not shown) for securing the bars 70-73 transversely to the vertical slots 62 and 64 of the CRD holder. When the thumb screws are tightened with the clamp bars in the closed position, the planar outer surface is generally flush with the outer surface of clamp block 60 and the inner surface completes the hexagonal enclosure of the CRD. The hexagonal shape provides a secure grip on the CRD, withstanding the vibrations of the milling process while securing the apparatus during its operation. Although a hexagonal shape is used, it is understood that an alternative embodiment which provides a similar secure grip on the CRD may be utilized. The clamp block 60 contains three vertical longitudinal slots 100, 103 and 105 for the mounting of the milling block 120. A gear rack 110 is bolted into the groove 100 in the end face of the block 60. The groove 100 and gear rack 110 of a height such that the milling block 120 can be raised significantly above the height of the exposed end of the CRD to be milled when securing the clamp block 60 to two adjacent CRDs. Stem pinion 122 fits through bore 129 in the block 120 and engages gear rack 110 for imparting reciprocating movement to the milling block 120. Stem pinion 122 has a lever handle 125 with knobs 126 to allow sufficient torque to be applied to the pinion. The pinion fits into bore 129 and is secured therein by C-clips 127 and 128 on each end thereof. The milling block 120 consists of a vertical portion 123 and an integrally formed horizontal portion 121. As shown, vertical portion 123 houses the stem pinion 122 and is located adjacent to clamp block 60 for engaging stem pinion 122 with gear rack 110. The horizontal portion 121 provides a base or support surface for mounting a hydraulic motor 130. The hydraulic motor 130 may be a Series 5 motor available from Charrlynn Division of Eaton Corporation. The hydraulic motor has inlet and outlet hydraulic lines 133 and 134, each with quick disconnects as at 135-136. A motor shaft 132 extends from the motor 130 into the horizontal portion 121 of the milling block and a spindle assembly indicated generally by numeral 140 is secured to the shaft 132. A bottom plate 142 is bolted onto the bottom of horizontal portion 121 to hold the spindle bearings 141 in place. The milling bit 145 is attached to the bottom of the spindle assembly in an appropriate chuck and has a diameter which is larger than the CRD diameter. To secure the apparatus for operation, the thumb screws 80 to 83 of the pivotally mounted bars 70 to 73 are loosened. The milling block 120 is manually positioned near the top of gear rack 110 by appropriate rotation of stem pinion 122. The device is lowered over three CRDs, positioning two CRDs, 152 & 154 into the clamp block 60 CRD holders 62 and 64 and positioning the milling device above a third CRD, 150, to be refurbished. The thumb screws 80 to 83 are then manually tightened to secure the two support CRDs in their respective holder, as best shown in FIG. 4. Next, the milling bit is lowered onto the CRD 150 to be milled, via stem pinion 122 which engages gear rack 110. When the bit contacts the end cap and cracked weld material, the device is in a position to begin the milling operation for removing the end cap and cracked weld material. Next, the hydraulic motor is actuated. The milling bit -45 works directly down onto the CRD end. The end cap and cracked weld material are removed in the form of radioactive chips which are too heavy to remain airborne. Upon completion of CRD end treatment, the milling device is readily removed to be repositioned on an adjacent pair of CRDs for milling removal of another CRD's end cap and cracked weld material. Specifically, the device is removed by manually loosening the thumb screws 80 to 83. This releases the clamp bars 70-73 and the CRDs in holders 62 and 64, allowing the device to be lifted off. Upon manually raising the milling block by stem pinion 122 and gear rack 110, the tool bit may be repositioned over an adjacent CRD for refurbishment. While the above provides a full and complete disclosure of the preferred embodiments of the present invention, various modifications, alternate constructions, and equivalents will occur to those skilled in the art given the benefit of this disclosure, thus, the invention is not limited to the specific embodiments described herein, but as defined by the appended claims.