Shoreham Nuclear Plant required custom built machinery to perform the destructive cutting necessary to decommission the Reactor Pressure Vessel. The equipment used to machine out all the ancillary piping was performed with portable pipe cutting equipment. This was the first undertaking of this type to be performed in the United States and the time allotted from mobilization to de-mobilization was only 27 days!
The Reactor Pressure Vessel, (RPV), had an inside diameter of 18’3″, Wall Thickness of 6″ and was additionally clad with a 7/32″ thickness of 308 Stainless Steel, bringing the total wall thickness to nearly 6-1/4″.
A custom designed, hydraulic powered RPV Segmentation Machine was built specifically for this project. Use of the RPV machine was specified due to the combined advantages offered in terms of cutting speed, and particulate contamination control. These factors served to address ALARA requirements, yielding an estimated radiation exposure level of only 2-5 millirem per man hour for this project.
Assembled and tested at the manufacturing facility in Illinois, the RPV machine frame had an outside diameter of 17′ and, with tool slides mounted, weighed approximately 8,000 pounds. The uniquely designed gear driven feed mechanism, precisely integrated with the radial feed tool slides, provided synchronized variable feed rates to accommodate the stainless cladding, embrittlement and carbon inconsistencies predicted in the RPV. Each of the two tool slides carry a standard 1/2″ carbide parting insert and are offset by 1/4″ vertically, along the axis of the vessel, to provide adequate chip clearance.
The project called for segmenting the RPV into seven sections ranging in height and weight from 4’10” @ 89,000 pounds per segment to 7′ @ 111,000 pounds per segment.
To prevent the tool from binding during the cutting operation, six “U” shaped lugs were attached to the outer wall of the vessel. Designed to span the cut line, these split lugs eliminated the possibility of unequal rigging affecting the cutting operation.
The fully assembled RPV machine, work platform and supporting structure, weighing approximately 23,000 pounds, was lowered into the RPV for each cut using the plant’s polar crane.
The RPV machine was then secured to the RPV liner wall with eight self contained
clamping legs. Six of these are hydraulically actuated. The remaining two are located on the X and Y axes respectively, and manually actuated to affect centering of the machine. As machine rotation begins and tool slide feed cams are engaged, the tool slides begin to feed outward, progressing further through the RPV wall with each machine revolution until the cut is completed.
In addition to the RPV machine, there were five custom hydraulic drills specifically designed for this project. These were used to bore 5 diameter holes for rigging in three of these vessel sections.
Upon completion of each cut, the RPV machine was lifted out of the vessel and the “U” shaped lugs were either torched apart or unbolted.
As each vessel segment was lifted off, it was transferred to a dry cutting station
where it was further cut into either three or four segments using an oxy-fuel track
mounted torch system to facilitate transport.