Big Rock bows out3 July 2002
In 1999 Consumers Energy awarded BNFL's Reactor Decommissioning Group a contract for the Big Rock Point major component removal project. The group teamed up with MOTA and Sargent & Lundy to plan and execute the work, as part of the site restoration project. By Mick Papp, Tim Milner and Bart Slimp
The major component removal (MCR) project at Big Rock Point (BRP) at Charlevoix, Michigan, is now in its third full year of decommissioning operations with the reactor vessel removal scheduled for 2003 as per the contract. This challenging multidisciplinary project has benefited from an intensive pre-planning phase and an interactive dialogue with both the client and regulator. These factors cannot be over-emphasised in planning for future decommissioning projects.
BRP was operated by Consumers Energy from 1962 to 1997. The 75MWe BWR was originally procured and designed for early research into the irradiation of experimental high power density long-lifetime fuel, in addition to power generation. The plant was initially synchronized onto the local grid in August 1962 and declared commercially available in January 1965. After 35 years of power generation, on 29 August 1997, it ceased operation for economic reasons and declared a Nuclear Historic Landmark. At the time of closure, it was the oldest operating nuclear power plant in the USA. The BRP site restoration project will be completed by 2005.
Planning for and executing the removal and ultimate disposal of major components from a retired nuclear power plant is an aspect of the decommissioning process that requires a significant commitment of funding, project management capabilities, and engineering expertise. The most challenging component of nuclear decommissioning projects, however, is the removal and ultimate disposition of the reactor vessel (RV).
The RV is a 9.14m long vertically-mounted cylindrical unit with an outside diameter of 2.95m and 3.56m at the vessel flange. It is principally fabricated from steel, weighs 108.864t without water and internals but including the head, and is located in the containment sphere, within a massive concrete shield.
The RV insulation and minimal selected internal components remained in the RV for shipment and disposal. The mirror insulation provides a considerable contribution to the residual dose rate of the RV transport package, but it was left in place because of ALARA considerations for its dismantlement and removal. Using RV characterisation data, along with material sample data obtained during core bores taken in 2000 and industry precedence, regulatory criteria and guidelines were examined to control the packaging and transportation of components such as the RV. From this analysis it was determined that the RV would be packaged and transported as a 10 CFR 71 Type B package.
RV package design
Preliminary design of the RV transportation system (TS) package involved developing a container configuration that would satisfy regulatory requirements while considering the physical constraints of the transportation route, fabricator handling restrictions, and BRP containment sphere space limitations.
The regulatory requirement governing the 3m unshielded dose rate of the material to be shipped played a major role in pursuing a 10 CFR Part 71 Type B package design. While the radioactive material included in the RV package could be demonstrated to meet A2 limits under the definition of low specific activity (LSA), the calculated dose rate at 3m exceeded the 1Rem/hr 49 CFR Part 173.427 limit for a standard industrial package (IP). Consideration of the criteria governing shipment of greater than Class C (GTCC) materials, as well as the Barnwell disposal criteria for A2 fraction averaging, led to a decision to remove some internals from the RV prior to shipping. Among these were the upper grid bars and neutron windows, which were removed during summer 2000.
To maintain shipping width and height so that a combination highway and rail route would be feasible for the RVTS package, the package diameter was limited to approximately 4m. As the RV upper head could be shipped separately and disposed of without a Type B package, this was removed to make the package shorter and lighter. The package configuration selected was a right cylinder with flat endplates, approximately 3.8m in diameter and 7.7m in length.
The package top plate also serves as the lifting plate for removing the RV from the reactor cavity and transferring it to the upended container. Once placed into the container, the bottom of the RV will be cradled in a support ring that will provide vertical and lateral support. The top plate will then be welded onto the container to fully enclose the RV.
The shielding design resulted in a container with varying wall thickness. In the vicinity of the active fuel region of the RV, approximately 2.5m axially, the necessary wall thickness, around 17.5cm, is significantly greater than the 7.5cm thickness required in other side wall areas. To address design issues such as increased susceptibility to brittle fracture with increasing plate thickness and fabrication concerns with rolling 17.5cm-thick plate, the design adopted an outer shell of 7.5cm uniform thickness with a 10cm layer of additional shielding welded to the inside of the container.
For the RVTS to meet the normal conditions of transport (NCT) and hypothetical accident conditions (HAC) as defined in 10 CFR 71, it was designed in accordance with the requirements of the ASME Boiler and Pressure Vessel Code Section III Subsection NB. NUREG 1609 and Regulatory Guide 7.9 were also used to provide general guidance on other aspects of the design. While NCT and HAC presented a variety of challenges in the design phase, for the BRP RV package the specific HAC for a 9m drop to an essentially unyielding surface required the greatest effort to meet. In order to satisfy an HAC of 10 CFR 71 for package performance, the RVTS design team employed sophisticated finite element analysis techniques to perform a series of nonlinear, elastic-plastic analyses, determining potential areas of package containment breach, and assessed the radiological consequences.
As a Type B shipping package designed per ASME BPV Code Section III Subsection NB, stringent fabrication practices were required. For elements that form a part of the package containment boundary, or are welded integrally to the boundary, materials are specified to ASME Section III criteria. To ensure low temperature performance, notch toughness requirements at a lowest service temperature of -30°C were specified. To guard against laminations in the thick plates used for the package, ultrasonic testing of the base metal was required. Welding joints qualified to ASME requirements were specified, including a specialised field weld using narrow gap groove weld technology. This specialised weld will result in lower total heat input into the materials and a shorter welding time, while still providing a container closure weld that achieves full penetration.
The contract for fabrication was placed with Precision Components Corporation (PCC) at York, PA, in July 2001. All fabrication, vendor welding procedures, welder qualifications and purchase orders were reviewed by BNFL to verify that code requirements were met. Vendor surveillance during fabrication is ongoing to ensure that a consistently high quality is met. To date, the vendor, PCC, has met all schedule targets and the RVTS is on schedule to arrive at Big Rock on 8 October, 2002.
Transportation of the RVTS to the disposal site in Barnwell, will involve a combination of highway and rail transport. This requires designing a support structure that is adequate for inertial loads from both AAR and ANSI standards for tie down systems, has provisions for lifting or jacking as required (to move the package from one transport vehicle to another), and maintains shipping clearance profiles. The transport tie down system employs readily available structural plate material, wire rope, and turnbuckles. Since the tie down system is not an integral part of the package containment boundary, it will be fabricated to AISC/AWS/ANSI standards using pre-qualified welding procedures and joint configurations on ASTM specified materials. The fabrication of the tie down and handling devices is also under contract to PCC.
To accommodate the shifting of the package from the highway transport to the railcar, the RVTS tie down structure has been designed with identified jacking points. By jacking the package to allow placement of hydraulically driven slide beams, the transfer from highway transport to railcar can be accomplished without the need for a crane at the rail siding location.
To facilitate handling of the RVTS package prior to shipment from site, a lifting lug will be bolted to the top plate and two trunnions will be bolted to the side of the package. These devices are intended for handling prior to shipment only and will be removed before the highway transporter leaves the site with the RVTS package. The trunnions, machined from solid billets, allow the container to pivot about cradles at the tops of A-frame supports. These facilitate upending the empty container from its horizontal delivery position using the existing crane. The A-frames/trunnions and the lifting lug, fabricated from plate material, will be used to down-end the package once the RV is in place and the top welded. The loading of the RV into the shipping container and its subsequent removal from the containment sphere, loading onto the road transporter and shipping to the rail siding is contracted to Barnhart Crane and Rigging of Memphis, Tennessee.
The work completed to facilitate RV removal includes:
• Removal of the RV internals top grid bars and loading them into GTCC containers.
• Removal of the neutron windows and placement into GTCC containers.
• Evaluation of the existing structure for floor load and crane reactions associated with major component removal.
• Providing an opening in the containment sphere.
• Upgrading the containment crane to single failure proof and 125T from 75T.
• Removing a section of the concrete pedestal to facilitate RV removal.
• Removing the concrete block surrounding the RV.
• Preparing the nozzles.
• NRC approval of the BNFL 10 CFR 71 subpart H quality assurance programme.
• NRC approval of the RVTS shipping package - a Certificate of Compliance was issued on 8 April, 2002.
Future work scheduled will include:
• Removing the stabiliser arms and support brackets.
• Receiving and placing the RV shipping package.
• Closure welding of the shipping package following RV loading.
• Acceptance testing of the shipping package prior to shipment.
• RVTS road and rail shipment to Barnwell.
• Disposal and final burial of the RV.
While the existing structure could be considered robust for supporting the loads to which it was subjected during normal operation, providing an adequate travel path for a package whose total weight approaches 300t requires revisiting the design of a structure that was built 40 years ago. At that time, the materials used were typical of the state of the industry. The original design loads on the 2500psi to 3000psi concrete and 40,000psi reinforcing steel did not consider movement of components as heavy as this RV. From high, localised jacking loads to moving loads affecting larger areas, the BRP MCR project loads the structure in ways such that existing slabs require clearly defined load paths with the addition of local shoring to ensure a safe load movement. The installation of local shoring has now been completed.
Containment sphere opening
To enable the RVTS to be delivered and removed from the containment, an opening of sufficient size has been engineered and cut in the containment sphere at the existing equipment hatch location. The opening allows access to a loading dock and then to site access roads. It was necessary to provide steel reinforcing to withstand seismic and tornado loads around the opening.
Because of the overall length and weight of the RVTS package, the existing loading dock crane was not adequate to handle moving the package from the slide beam system on the dock to the bed of the highway transport and was removed as part of the containment opening modification work. This will be accomplished using a hydraulic gantry system which will lift the package, allow it to be rotated 90º to align with the highway transport, and then lowered onto the tie down support skid.
In order to lift the RV from the reactor cavity, a crane of approximately 125t capacity is required. The existing EOT crane was rated at a capacity of 75t. The required upgrade has been provided by Consumers Energy and incorporates a temporary column/bridge/single-failure-proof trolley configuration that can be removed and reused after work inside the BRP containment is complete.
The crane will be used to lift the RV from the cavity, move it west to the shipping package location and lower it into the upended shipping package.
Pedestal section removal
When the RV was initially installed, it was transported into the containment on a sled and upended before the reactor cavity was completed. Now, to remove the RV from the reactor cavity, the vessel must be lifted to clear the top of the cavity at an elevation of 7.7m. The upgraded EOT crane configuration did not remove the existing EOT gantry legs or bridge. Therefore, no additional headroom has been provided to permit lifting the RV clear of the deck.
To allow sufficient clearance to remove the RV, a trench was cut into the reactor cavity structure. The trench was cut and sectioned using diamond wire cutting technology and sections were placed into containers for removal. This slot also facilitates handling other large components for removal such as the steam drum. This work was completed by BNFL in June 2001.
Concrete block removal
Where the pipework accesses the RV cavity on the southern side of the RV, concrete blocks surround the pipes. These were successfully removed and stacked for disposal by the waste contractor during spring 2001.
There are 61 pipe penetrations into the RV of differing diameters. Each one will be cut and capped for RV shipping. A hydraulically powered split ring cutter was used to cut and prepare the upper nozzles. The remaining lower nozzles will be completed prior to RV removal in 2003. Should these lower nozzles be inaccessible to the split ring cutter, alternative means of cutting will be employed such as wire saws or thermal cutting.
In order to maintain clearance for placing the RV into the RVTS package, all nozzles through the sides of the RV will be cut such that they fall within the outer diameter of the vessel. The control rod drive (CRD) tube stubs at the bottom of the vessel have been trimmed to within 6cm of the bottom of the RV shell's lowest point to ensure a proper fit into the shipping container.
Twelve brackets that are attached to the exterior vessel shell support the vessel. Twenty-four 5cm-diameter hanger rods attached to these brackets transmit the vessel weight to supports anchored in the supporting concrete. Six of the hanger rods that are attached to large steelwork inserts are classed as main restraints. The remaining 18 hanger rods are secondary suspension rods. They and their support brackets will be removed while the reactor is supported on the main steelwork embedment hanger supports and not from the crane. Once the RVTS lifting plate is installed and the crane attached to take the weight of the RV, the remaining supports will be removed. Nut splitters will probably be used to rupture the nuts that fasten the RV to the brackets, although additional equipment will be available if it is required.
The shipping package consists of the container with an integral transport cradle and turning trunnions and will be delivered to the containment sphere by the road transporter. The RVTS container will be slid into the sphere on the hydraulic slide rail system previously mentioned. The container will be positioned with its head directly under the EOT crane. A pair of A-frames will then be attached to the container trunnions. By means of the EOT crane and the A-frames taking the container weight, the container will be manoeuvred into a vertical position, ready to receive the RV.
After vessel preparation, including draining the RV water inventory and nozzle cutting and capping, the RV will be lifted from the reactor cavity structure, moved across the slab, and lowered into the upended RVTS container. To remove the RV, the lifting plate will be attached to the vessel flange via the head studs. The EOT crane will then be attached to the lifting plate and the RV lifted out of the cavity, traversed west and lowered into the container. Using an automated welding machine the plate will then be full penetration welded to the container to become part of the package containment boundary. The lifting lug attached to the top plate will then be removed and low density cellular concrete (LDCC) will be placed into the RV through the bolt openings. The LDCC density for filling the RV internal void is 30pcf (minimum) to 36pcf (maximum). This will fix loose contamination on the RV's interior surface and the included internals, and will establish the RV and internals as an integral component.
Once the RV has been grouted internally the top plate lifting lug will be reinstalled. The container will then be lowered into the horizontal plane by means of the EOT crane and A-frames and will be held in this position by the A-frames ready for placing LDCC (50-60pcf) into the void between container and vessel through four injection ports. After curing of the RV interior LDCC, the top lifting lug will be removed and the holes in the top plate plugged and welded. After curing of the external void LDCC, the injection ports will be plugged and seal welded, providing a containment boundary for the package. Once the sealing of the package is complete, the unit will be moved using the slide beam system through the containment access opening to the external loading dock. From the dock, it will be transferred to the highway transporter. The package will then be taken by road to a rail site where it will be transferred by rail to Barnwell. The package will finally be transferred to Duratek for burial.