CRD milling machine

An apparatus for refurbishing the ends of control rod drive (CRD) penetrations used in nuclear boiling water reactor systems where the rods are disposed in a grid of rows and columns with a known predetermined spacing therebetween. The apparatus is rapidly positioned for support on a pair of CRDs adjacent to a CRD to be refurbished, coaxially aligning the end mill with the CRD to be refurbished. The end mill is then brought into contact with the CRD to be refurbished. After the refurbishment process, the apparatus is rapidly removed and ready to be positioned on another pair of CRDs adjacent to another CRD to be refubished.

BACKGROUND OF THE INVENTION 
I. Field of the Invention 
This invention relates to a machine for milling off old, cracked weld 
material from control rod drive penetrations used in nuclear boiling water 
reactor systems and more particularly to a machine that can rapidly be 
positioned and used so as to reduce the time exposure of workers to 
nuclear radiation. 
II. Discussion of the Prior Art 
The control rod drive (CRD) penetrations in a nuclear boiling water reactor 
(BWR) system have welded end caps. Intergranular stress corrosion 
cracking, (IGSCC), occurs at the weld interface because of the high heat, 
radiation, and fatigue environment of nuclear boiling water reactors. The 
cracked condition must be fixed before the function of the CRD is 
compromised. 
To prevent compromise of the CRDs, one must enter the BWR's dry well, a 
high radiation area, to remove the old weld material and end caps from the 
CRD's whose welds exhibit IGSCC and then weld on new end caps. Protective 
suits are required in this high radiation area and an individual's 
exposure time in such a high radiation environment is limited by federal 
regulations. The removal of the old weld material has heretofore been a 
highly labor intensive procedure using a hack saw and an abrasive disk 
grinding wheel. Such a method is time consuming, inaccurate and results in 
air-borne contamination in the form of radioactive dust particles. Because 
of the limits on radiation exposure mandated by federal regulations, the 
time involved removing the old weld material necessarily limits the time 
for welding on the new end cap. 
A need exists for a method which reduces time, eliminates inaccuracies and 
minimizes air borne contamination. 
The primary object of this invention is to provide a device which shortens 
the time for removing old weld material from the CRDs in BWR 
installations. 
Another object is to minimize the amount of air borne radioactive 
contamination in the form of radioactive dust. 
Another object is to provide a manually adjustable device which readily 
clamps onto CRDs adjacent to the CRD to be milled for support. 
SUMMARY OF THE INVENTION 
To achieve these and other objects, there is provided a milling apparatus 
for use on control rod drive penetrations in a nuclear boiling water 
reactor system. In such systems, CRDs are equally spaced in a grid 
pattern. The apparatus manually clamps onto CRDs adjacent to the CRD to be 
milled. The drill bit works straight down onto the CRD end to remove the 
cracked weld material and end cap. The waste product of the milling 
procedure is radioactive material in the form of chip-like scraps which 
are too heavy to remain air-borne and hence, there is virtually no dust. 
For a better understanding of the above and other features and advantages, 
reference is made to the following and detailed description of a preferred 
embodiment reflected in the accompanying drawings in which like numerals 
in the several views refer to corresponding parts.

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.