Zion Reactor Components
The Core Barrel component of the Reactor Vessel Internals is an irradiated cylindrical structure that was remotely cut under water. The first phase included five (5) horizontal cuts through the Core Barrel and, where appropriate, the Thermal Shield as well. The equipment was utilized for Units 1 & 2.
The Core Barrel and Thermal Shield were removed by severing the barrel with circumferential cuts at the elevations pre-determined by the customer. The rings were then relocated for secondary vertical cuts determined by cask size.
Upper Core Barrel
The geometry of the Upper Core Barrel dictated its removal in 2 separate rings; (1) the upper reinforced flange and (2) the region above the fuel that included the reinforced outlet nozzles. The cuts performed isolated these two bodies. The equipment had to fit in this area and perform two (2) cuts prior to removal of the baffle plates and baffle former plates. The clearance between the top of the baffle plates and the second cut line was 17.6 inches.
Lower Core Barrel & Thermal Shield
Prior to execution of the third cut, the baffle plates and baffle formers were removed by additional equipment (discussed in the following sections). The Thermal Shield was supported via a combination of blocks and pins adjoining the Core Barrel to the Thermal Shield. As such, the Core Barrel couldn’t be removed without removing the combined cylindrical sections together. Thus, the third cut was performed just below the blocks and pins resulting in a combined body approximately 26 inches in height. Equipment cutting depth had to account for complete penetration through both the Core Barrel and Thermal Shield including localized additional depth for the ABumper Bars@ welded to the exterior of the Thermal Shield. Two (2) additional cuts (cuts 4 and 5) were performed producing 65 inch tall right cylindrical bodies to be further volume reduced with vertical cuts. The fifth cut was performed 21.8 inches above the top surface of the Lower Grid Plate Assembly.
CHORCE – Circumferential Hydraulically-Operated Reactor Cutting Equipment
The Circumferential Cutting concept included four (4) primary operating elements. A stationary frame used to provide the platform from which to deliver and deploy the cutter, a rotating table to position the cutter about the polar axis, a plunging sled used to control the depth of the cutter and a removable saw arbor and cutting module.
Several approaches were evaluated, for example; the internal lathe cutting process seems to have the fastest rate of removal through the un-interrupted vessel wall, but this process is not well suited to cut the thermal shield. We considered a combination of technologies; weighing cost, efficiency, safety, and risk. Our evaluation included reviewing available industry information, our experiences and what has been presented by the customer.
In the end, the customer determined a single machine concept with a mechanical plunge cutting sled mounted to a single rotating blade. This equipment was designed to accommodate the variety of different configurations anticipated at Zion. In addition, this style of machine was well suited to host alternate processes that may have become advantageous during the project.
The Circumferential Cutting Equipment with a single rotating table and a singular circular saw blade severws the vessel and thermal shield by performing a series indexing plunge cuts. The entire cutting process was remotely operated underwater.
All components were designed and manufactured incorporating lessons learned on other nuclear reactor vessel internals segmentation projects. ALARA principles were incorporated such as quick installation, minimum crud traps, ease of decontamination, material selection, and other engineering controls. Our simple and straightforward design allowed for less complex training, operation, and maintenance.
The installation and operation of the Circumferential Cutting Equipment was underwater a maximum of 35-feet. Operation of the machine was done remotely by an operator in a low dose area. A typical PTZ underwater camera supported all activities (installation, operation, and machine removal).
Materials used in machine construction were mainly carbon steel with painted or plated surfaces. As was typical of previous decommissioning machines, bearings and feed screws were alloy steel and required periodic maintenance to slow down the rate of oxidization.
The Hydraulically Operated Circumferential Cutting Equipment designed and built by Plant Decommissioning was comprised of the following main sections:
- Stationary Frame
- Rotating Table
- Plunge Sled
- Cutting Module
In addition, the following support equipment was provided:
- Hydraulic Control Unit (HCU)
- Maintenance Stand
- Blade Changing Station
- Drive Unit for Clamp Legs
Sequence of Events
This Circumferential Cutting Equipment was a modular design to facilitate shipping. The two main half sections of the Stationary Frame are pinned and bolted together. Once fully assembled on the Maintenance Stand all required hoses are connected for pre-deployment testing. It was then delivered into cutting position underwater by an overhead crane.
Once lowered to cut position the machine was clamped in place. The clamp mechanism consists of multiple hydraulically driven screw actuated legs. A centering device, attached to the stationary frame, provided machine concentricity to the Core Barrel ID. The screw mechanisms of the legs advanced and retracted by a hydraulic “Drive Unit”. Tensioning of the legs was maintained by the main HPC (Hydraulic Power Controller). To reduce the amount of hoses required, a single Drive Unit was used. This Drive Unit was positioned next to each leg using long handle tooling from the refueling bridge. A jib crane or other lifting device was used to help support the Drive Unit. Guide pins allowed for simplified locating using long handle tools (supplied). The time to move the Drive Unit from one position to the next was less than five minutes inside containment working off a bridge. Once the legs are in the clamped position, the machine no longer needs polar crane support. The design of the Screw Legs prevented loosening during machine operation.
Once all hoses are positioned and secured, the Rotating Table wa rotated to the desired starting location. The Rotating Table was locked in place to the Stationary Frame by activating locking pins or clamps (“Sled Lock”) from the HPC. The rotation motion was achieved by a ring and pinion gear and hydraulic motor. The rotating bearings were Vee wheel style with a circular Vee race. This is the same style as used on the field proven 204-inch OD “Shoreham” and 252-inch OD “Millstone” rotating cutting machines.
The cutting blade was moved towards the cut by the Plunge Sled. This linear motion was controlled by a ball screw and ball nut. The linear rails and bearings are a heavy duty “THK” style proven in previous segmentation equipment.
The Cutting Module consists of the saw blade, sub-frame, gear box, and hydraulic drive motor. This entire module was removable for changing the blade. This design has a clamp mechanism (“Cutter Clamp”) to hold the module in place. This reduced the amount of time, and frustration, of working with long handle poles to change a blade 35-feet below water. When removing the Cutting Module a long handle pole was used to hook or otherwise hold the Module. A jib crane or other lifting device was needed for the under 500-pound unit. The Cutter Clamp was disengaged and the module is free of the Plunge Sled. The Module was brought to a shallow level and placed in the Blade Changing Station.
If radiological conditions allow, the Module can be brought out of the water and hand tools used. At this closer level it is easier to spot any cutting debris that would compromise the correct mounting of the saw blade. Return and remote mounting of the Cutter Module was made easy and secure.
Removal of the machine required retracting the Clamp Legs. This was done in reverse of the clamping procedure with the Drive Unit.
Baffle Plate Removal
Reactor Core Barrel Assembly
The primary Core Barrel components consist of the Baffle and Former Plates, located within the Reactor Core Barrel. These components are irradiated and classified as greater than Class “C” (GTCC). The machining operations were performed remotely and under water.
The Baffle Plates were bolted to the Former Plates. Each of the fastening bolts was secured by a locking bar welded over the bolt. Each Baffle Plate was approximately 1 1/8” thick and 160” long. The Baffle Plate geometry is such that the components are classified as “W” Shaped or “C” Shaped.
Baffle Former Plates
The Baffle Plates were bolted to the horizontal Former Plates. There are eight (8) levels of Former plates located on the inside diameter of the Core Barrel. The Former Plates themselves were bolted to the Core Barrel.
The objective was to remove the GTCC Baffle Plates from the horizontal Former Plates. The process required machining (mill) through the Locking Bar and head of the bolt releasing / freeing the Baffle Assembly. Once the Locking Bar and Bolt Heads were removed, the Baffle Assemblies were in the “W” or “C” Shape configuration and available for packaging in their designated location.
BMT – Bolt Milling Tool
The evaluation began with reviewing both MDM and conventional cutting processes. Conventional cutting has successfully performed like operations on similar components and has proven itself on two previous decommissioning projects. MDM has also been successful in performing core barrel and thermal shield modifications, and has also been used on previous reactor decommissioning projects to remove the fasteners that attach the Baffle Plates to the Former Plates.
In addition to determining the process, our team also considered availability of spare parts, the equipments’ ability to perform in an adverse environment and technician training / understanding of the implemented process.
At the end of the day, Plant Decommissioning proposed a mechanical cutting / milling process. Our design, however, was configured from a wall milling design developed by Horst Kwech in the early 1980’s.
The following Sections have individualized the major components of the Bolt Milling Equipment.
The Bolt Milling Equipment was a hydraulically powered, remotely operated and positioned, center cutting-plunge milling machine. The equipment was capable of remotely articulating into limited access corners, as well as close proximity to the Lower Core Plate. The three (3) main components included; Tower, Plunge Carriage, and Milling Carriage. In addition, a Maintenance Stand was provided.
The Tower concept included a dual mast (two columns) primary structure that rested on the Lower Core Plate and could extend out of the water to better facilitate tool changes. The primary structure rested on the 2” thick Lower Core Plates’ and was located on the 2 ¾” diameter holes for positioning and stability. The upper end of the primary structure had two fixturing arms used for positioning and stability. The arms attached to the upper most former plate. Each mast was designed around 8” square tubing with a 5/8” thick wall.
Mounted to the masts was one set of THK rails and an “X” Axis pinion rack. The mast had a top and bottom plate, as well as intermittent structural cross bracing.
The Tower’s provided a stable platform for the Milling Carriage to travel the entire length of the former plates. The Tower also enabled the Milling Carriage access to the eight (8) Former Plate elevations, the bolts attaching the “C” shape to the “W” shape, and extended the Milling Carriage to an elevation that facilitated tool changes without pulling the machine.
The Tower was constructed of structural steel and painted to deter rusting. Standard steel THK rails were provided. The rails required periodic maintenance to prevent rusting from impeding smooth and accurate operation. The pinion rack was provided with a black oxide coating to deter oxidation, but will not completely prevent rusting in submerged applications. The Tower included rigging points.
The Milling Carriage was the heart of the machine. The Milling Carriage traversed the tower and supported the Plunge Carriage with side to side movement and plunge cutting operations. The entire Milling Carriage was hydraulically operated in all three (3) axis.
A hydraulic pinion motor controlled the Milling Carriage X axis positioning and traversing operations. The side to side Y axis platform was constructed on a set of THK rails with the right to left axis movement actuated by a hydraulic controlled lead screw. The travel left to right or side to side was sufficient to reach two columns of bolts without having to reposition the BMT.
In addition, the Plunge Carriage was mounted to the Milling Carriage. The Plunge Carriage module bolted on to a Z axis linear motion table. Identical in principle to the side to side movement, the linear motion table had a set of THK rails, four linear bearings and a hydraulically operated ball screw for the plunging motion.
The Milling Carriage evolved into an assembly of THK precision rails, ball screws, hydraulic motors and gear boxes built on an arsenal of 1” thick, blanchard ground A36 plates. By design, the Mill Carriage exceeded the positioning, rigidity, stability and strength required to endure the challenge of cutting the 2,448 bolt heads and locking bars of the two unit project.
The Plunge Carriage was a modular component that served two functions. First, there is a left and right hand design. This mirror image module was necessary to access all of the corners existing within the Former Plate configuration. Second, the Plunge Carriage was the most vulnerable component located on the machine. The modular design supported easy replacement and quick recovery from a failure, crash or unexpected mishap.
The Plunge Carriage’s notable components included a quick-change tool holder, hydraulic powered spindle, plunge & retract functions and depth of cut positioning.
The Plunge Carriage was designed with adequate travel to articulate from a fully retracted (home) position through an extended position that can transcend through the thickness of the Baffle Plate and slightly into the Former Plate.
MDM Contingency Tool
An MDM (Metal Disintegrating Machining) cutting tool was included as an option to address a few contingencies. These are “work hardened” and “imbedded cutter” scenarios. Work hardening of stainless steel is common when using an incorrect federate, cutting with a dull blade, or when too many chips build up on the tool. If the work gets too hard, the blade will no longer be as effective. A second scenario was when a broken saw blade imbeds itself into the cut; parts of it may remain after freeing it.
With over 1,200 bolts to be removed from each reactor, there was a reasonable chance a cutting tool will dull or break before being replaced. Based on our lessons learned, both the “work hardened” and “imbedded cutter” areas are very painful to recover from. The MDM tool would be mounted in place of the Plunge Tool Module. The MDM power supply, flush pump, and all other required items were included.
All of the cutting was designed to be performed in a single plunge motion on the Z axis. A “Plunge” cut method is understood to be where the tool is actuated from the inside of the vessel towards the outside of the vessel or into the work until reaching a desired depth. Once the cutting depth was attained (removing the bolt head and bolt up to the Former Plate) the tool was retracted and indexed to a new bolt location.
Feed screws, gears, and bearing rails were not corrosion resistant. Because of the relatively short life time of use, the large price increase to provide stainless steel for these components was not justified. Simple maintenance of these components when removed from the pool would be sufficient for both Zion Units. For long term storage additional care must be performed.
There were no safety guards on the machine because of it being operated remotely.
All operations of the equipment were controlled by the HCU (Hydraulic Control Unit). All primary directional controls (including; hydraulic solenoids and valves) were above water in the HCU for easy access. Hydraulic hoses from the HPU (Hydraulic Power Unit) attached to the HCU.
FaST – Former (articulating) Severing Tool
In addition to circumferentially cutting the core barrel and milling all of the bolt heads that were retaining the Baffle Plates, Plant Decommissioning devised two (2) Former Plate Bolt Milling Machines to defeat the bolts and dowel pins which retained the Former Plates.
Bolts from the outside diameter of the core barrel held the Former Plates in place. It was determined that the best access was from the inside diameter of the Core Barrel. Two machines were designed to reach all of the bolts and dowel pins on five (5) different elevations. The larger of the two machines had the capability to articulate in three axis plus a rotary spindle motion. Both Former Plate Bolt Milling Machines were held in place by two hydraulic cylinders which held the machines in place and countered the reactionary forces of the cutting process.
The machines remote capabilities included its ability to rotate a complete quadrant of the vessel in order to position its self over each cutting position. The cutting head also articulated vertically for easy remote tool changes, the Z axis had infinitely variable plunge speed controls and the spindles had camera mounting hardware for constant monitoring of the operations.
Zion Station Additional Services
We have been providing on-going support for the Zion Decommissioning Project for both Siempelkamp and ZionSolutions.
For Siempelkamp, we are providing the fabrication of fixtures and support stands for the on-going cutting operations as well as Long Handle Tools and End Effectors and Cutting Chip Collection Systems. For ZionSolutions, we are providing the design and fabrication of fixtures and parts in support of the Fuel Transfer Operations, GTCC Loading, RVI Segmentation, and Dry Storage. While also providing Staff Augmentation (Project Manager and Coordinator) for GTCC Loading and project support as Subject Matter Expert (SME) for RV/RVI segmentation and various cutting processes.
Non-Standard Decommissioning Equipment Options
MDM Contingency Tool
An MDM (Metal Disintegrating Machining) cutting tool is included as an option to address a few contingencies. These are “work hardened” and “imbedded cutter” scenarios. Work hardening of stainless steel is common when using an incorrect federate, cutting with a dull blade, or when too many chips build up in the blade. If the work gets too hard, the blade will no longer be effective. A second scenario is when a broken saw blade imbeds itself into the cut; parts of it may remain after freeing it.
With approximately 600 linear feet of cutting over 2” thick stainless steel, there is a good chance a blade will dull or break before being replaced. Based on our lessons learned, both the “work hardened” and “embedded cutter” areas are very painful to recover from. Starting a new cut an inch lower may be the fastest recovery even if a cut was 90% done. The MDM tool would be remotely mounted onto the Rotating Table in place of the Cutting Module. The MDM power supply, flush pump, and all other required items are included.