The Zion Nuclear Power Station consists of two commercial nuclear reactors and is currently being decommissioned. This plant consists of two nearly identical Westinghouse 1085 MWe Pressurized Water Reactors (PWR’s). Each reactor was online for approximately 22 years, after which they sat idle for 15 years.
Once the reactor vessels and internals have been size reduced, they will be packaged in liners for disposal. Because of the high dose rates associated with the reactor internals, all segmenting is being done remotely underwater. Segmenting of the Unit-2 internals began in March 2012 by mechanical cutting processes. Work will begin with the Unit-2 reactor, with Unit-1 following. The schedule is to complete the segmentation of the Unit-1 internals in early 2014. Some of the major tooling will be utilized on both reactors.
ZionSolutions, LLC (a subsidiary of Energy Solutions) awarded the contract to segment both of the reactor pressure vessels (RPVs) and their internals (RVI’s) to Siempelkamp Nuclear Services, Inc. (SNS). The RPVs are to be segmented using thermal cutting as the primary process, and the RVIs will be segmented primarily by mechanical cutting. As has been done at previous segmentation projects, secondary cutting tools of various processes will be used as needed.
SNS and Siempelkamp have a history of segmenting reactors and reactor internals in the US and in Germany. In 2009, SNS acquired MOTA Corporation which was responsible for several successful decommissioning projects in the US (such as the RVIs at Rancho Seco and Fermi-1). In addition, Siempelkamp’s “NIS” group in Germany segmented the Stade reactor in 2009 primarily using a thermal cutting process.
The Zion reactor internals are characterized in waste classes of A, B, C, and GTCC (Greater Than Class C). SNS generated a Segmentation and Packaging Plan that was used to identify the final cut size based on the class of liner each component needed to be placed into. Besides the obvious goal to minimize the quantity of high level waste liners, the amount of cutting tools required to do this also needed to be minimized. This was not based upon a desire to save on costs, but rather due to logistics. The reactor cavity did not leave much room for anything other than the RVIs, waste containers and the tooling.
The RVI Tools
Decommissioning projects all desire to minimize high level waste. An arsenal of cutting tools makes segmenting and packaging more efficient, but limited space prevents unlimited tool selection. For Zion, the reactor vessel internals are being segmented using the following four custom designed tools:
VRS (Volume Reduction Station)
CHORCE (Circumferential Hydraulically Operated Cutting Equipment)
BMT (Bolt Milling Tool)
FaST (Former attachment Severing Tool)
Design, fabrication and underwater mock-up testing of three of these tools were performed in the fall of 2011. The final tool, FaST, is scheduled for underwater mockup testing during April 2012.
The reactor cavity is assigned with two cutting areas: the shallow end and the deep end. Simplified, the Upper Internals (UI) are cut in the shallow end utilizing the VRS and the Lower Internals (LI) are to be cut in the deep end utilizing the CHORCE, BMT and the FaST. To utilize space efficiently, the Lower Internals are cut while in the LI Storage Stand from the inside out. Some items are first size reduced in the deep end, then cut to final size in the shallow end using the VRS.
The VRS
The VRS offers the most functions and is largest in terms of mass (see Image 2). The Zion VRS is similar to a machine called the RMT used at Rancho Seco for similar work. With a foot print of 18’ 6” x 19’ 9” (5.6m x 6.0m), it utilizes all the space the shallow end of the pool has to offer.
The VRS will cut the Upper Internals and approximately half of the Lower Internals. Because of the size of the Lower Internals, they will first be size reduced by other tools while in the deep end. The VRS has multiple cutting tools for different types of cutting. Cutting configurations are both horizontal and vertical. A rotary turntable on a linear slide is used to position pieces relative to the cutting blades. Blades for both linear reciprocating and rotary saw cutting are utilized. There are over 10 axis of motion available to perform the various cut configurations. These motions are both hydraulic and electrically powered.
For most of the horizontal linear cuts, the VRS utilizes an 18-foot (5.5-meter) long blade. This blade has modular cutting segments that are replaceable. The cutting process is considered “reciprocating cutting” in the industry, and the cutting action is a cross between a guillotine saw and an arbor broach. The VRS also utilizes circular cutting blades. These “Rotary Milling” cutters are 38 inches (1 meter) in diameter with numerous cutting teeth. Depending on the item being cut, various saw blades with different numbers of teeth are used. The VRS can manipulate the rotary blades horizontally and vertically. For some cuts, two blades will be cutting simultaneously.
The CHORCE
The CHORCE (pronounced “Sea Horse”) makes 360-degree horizontal cuts in the Core Barrel and Thermal Shield. It utilizes a 38-inch (1-meter) diameter saw blade with 80 individual cutting teeth. This type of mechanical cutter is called a Rotary Mill and has previously been used by the SNS team in RVI segmentation at Rancho Seco, CVTR Parr and Fermi-1.
The CHORCE has an 8-leg “spider” type of mounting frame that is approximately 12 feet (3.6 meters) in diameter. It is designed to clamp inside of the Core Barrel and cut outward through the Core Barrel and the Thermal Shield. The cutting head is detachable in order to bring it to the surface and perform a blade change. The machine is run by hydraulic power and utilizes a digital display for position location. Once the CHORCE completes a 360-degree horizontal cut, the severed piece is staged for further cutting on the VRS. Waste classification changes as the pieces get closer to the fuel zone, so final cut sizes will vary for the appropriate waste classification and liner size.
The BMT is used to remove the bolt heads that hold the Baffle Plates to the Former Plates on the Core Barrel. The cutter used is a traditional “End Mill” or rotary style cutter (similar to a drill bit). The BMT uses hydraulic power for the clamps and the 3-axis (X-Y-Z) motion. It is 34 inches (10.4m) tall x 4 inches (1.2m) wide. This height allows for most of the components to be elevated above the water, allowing for easier maintenance and tool changes. Although it only takes a few minutes to cut a bolt, this is an extensive process as each reactor requires over 1,200 bolt heads to be removed.
The BMT is a proven “Wall Mill” design that was first used in 1983 for the repair and modification of the St. Lucie Thermal Shield. Since then, Wall Mills have been used to repair and segment several RVs and RVIs.
The FaST
A tool was needed to separate the Baffle Former Plates from the Core Barrel. SNS had originally planned on a tool that pulled the Former Plates with enough force to break the retaining bolts (as was used at Rancho Seco), but Zion presented a few challenges. Units-1 and 2 each have different flow hole patterns in the Former Plates, making the reactors “nearly” identical. This alone required eight different mounting configurations of the tool. In addition, the Westinghouse Former Plates were of a different geometry than the B&W reactor at Rancho Seco. All of these factors would have required redesign of the proven ttool, therefore it was necessary to consider other approaches to remove the Former Plates.
The design approach that was ultimately selected was one that minimized risk. The proven Wall Mill cutting head would be incorporated to remove the Former Plates. The FaST tool would mount inside the Core Barrel and position the cutting End Mill above each Former Plate Bolt (see Image 5). A hole would be made into the Former Plate that would sever the bolt between the head and the threads. In effect this would “defeat” the bolt and the Former Plate would no longer be attached to the Core Barrel. Each reactor has eight layers of Former Plates with 80 bolts per level.
The FaST is approximately 12 feet (3.7m) long x 6 feet (1.8m) wide. The cutting head can be remotely detached from the frame and brought to the surface for repair or maintenance. All motion is powered by hydraulics and controlled by the operator using a touch screen. The technician can control the cut cycle manually or by running an automated computer controlled subroutine.
Reactor Segmentation
Of the few commercial power reactor pressure vessels segmented in the US, Zion has been one of the most challenging. Both Zion reactors are large and highly irradiated. After considering a few different cutting processes, SNS determined that a thermal cutting method would be used for both reactors. The NIS group of Siempelkamp in Germany had successfully used thermal cutting (oxygen-propane) for the reactor at Stade and is now preparing for Zion.
The approach is still in the planning stages at this time, but SNS will cut the vessel in the drained reactor cavity. A thermal cutting process will be primarily used, with some secondary processes for certain cuts. The reactor vessel will be cut from the top down, and lifted as the vessel sections are removed. Cutting will be done from the outside to minimize contamination spread. This also makes a thermal cut easier because it does not start on the stainless steel clad covering the inside of the vessel wall. Cutting will be performed by a robotic arm manipulating the cutting torch.
The top of the reactor will be covered with a steel structure that is designed for several functions. This structure will act as a radiological shield to reduce the background radiation level in the area. Besides the primary shielding benefit of the structure, removable shielding (either steel or lead) can be added as needed. This cover structure will contain fumes and allows for a negative pressure to be maintained inside the vessel.
Experienced Team Members
Invaluable to Success
Although the tooling is the most visible part of cutting, the team members involved play a large part. ZionSolutions has brought in an experienced segmentation company to perform the work and supply the tools. The SNS management and technicians have been involved in several reactor decommissioning projects. SNS has supplied a crew with decommissioning experience at Argonne EBWR, Millstone, Big Rock Point, CVNPA Parr, NASA Plum Brook, Rancho Seco and Fermi-1. This relevant experience is invaluable to a successful and safely completed project.
About the Authors:
Ron Richards is the deputy project manager for the RV/RVI Segmentation Project for ZionSolutions. He has over 35 years of experience in the domestic nuclear industry. Before joining ZionSolutions in 2010, Richards was previously with Sargent & Lundy and Exelon Nuclear in various engineering and project management positions.
Steve Larson started in nuclear power in 1983. He has served as a senior design engineer for several RV and RVI segmentations. As a project manager he was at NASA Plum Brook performing the RVI & RV segmentation. Currently, Larson is supporting the Zion Decommissioning effort as a project manager and a partner at PlantDecommissioning.com.
The authors would like to thank:
ZionSolutions, LLC;Siempelkamp Nuclear Services, Inc.;Force Manufacturing, Inc.;In-Place Machining Company; andPlantDecommissioning.com.
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