Breaking up Barsebäck6 June 2018
With decommissioning of Sweden’s first commercial nuclear plant progressing well, Per Segerud explains how lessons learned from segmenting the reactor internals at Barsebäck 2 are benefiting work now underway.
In October 2017, WestInghouse ElectrIc Company completed, safely and ahead of schedule, the first reactor internals segmentation project to be performed on a commercial unit in Sweden at unit 2 of the Barsebäck nuclear power plant. The company began segmentation at unit 1 in January of this year, with the benefit of the lessons learned from the work on unit 2.
Westinghouse was contracted by Barsebäck Kraft AB in July 2015 to segment the reactor internals of both the units at Barsebäck, as the first step in the units’ decommissioning. The station had two ABB-designed BWRs rated at 615MWe each. Unit 1 shut down in 1999 and unit 2 in 2005.
Planning reactor vessel internals segmentation
Westinghouse worked with Barsebäck to plan the segmentation and packaging over the course of a year; the physical work onsite to segment and package the reactor internals at Barsebäck 2 began in August 2016.
As with other successful reactor internals segmentation work Westinghouse has conducted, meticulous and comprehensive planning was required to complete site work in the high-radiation environment of the reactor vessel.
The first year of planning was dedicated to determining the best strategic technical and process approaches from start to finish. This included conducting engineering studies, and designing and manufacturing the equipment and tools needed to segment and package the components. Work processes were planned for dismantling, moving and segmenting the components in the pool, as well as loading and removing the segmented parts from the pool, restoring the pool environment to its initial condition, and decontaminating all equipment before shipping to Barsebäck 1.
During this phase, Westinghouse conducted an optimisation study of cutting time versus number of containers, to determine the most cost-efficient cutting and packaging plans. To minimise risk during the site work, Westinghouse also planned and carried out qualification of tooling, equipment and personnel at the Westinghouse Test Facility in Västerås, Sweden.
Westinghouse and Barsebäck decided that remote underwater mechanical cutting was the best approach to segmentation. Westinghouse has successfully used this technology extensively in Sweden, Finland, Germany, Spain, Switzerland, France and the US.
Mechanical cutting creates only insoluble byproducts, such as chips and shavings, which can easily be removed with simple collection devices, and it produces very little secondary waste. The technology is safe and dose to personnel low.
Westinghouse used three-dimensional modelling as the basis for designing the tooling, finding the optimum strategy for cutting components and creating the packaging plan.
Site logistics considerations
Site logistics played a large role in planning and implementing the complex segmentation and packaging work. The reactor cavity at each Barsebäck unit is close to the storage pool for the reactor internals, which is 10m deep. That is deep enough to provide shielding when cutting even the largest internals and when loading the segmented pieces. The pieces are loaded into waste inserts in the pool prior to their removal for packaging.
The storage pool for reactor internals is flanked by the working platforms of an existing working bridge and the refuelling machine. With two working platforms, Westinghouse was able to plan parallel segmentation of different internal components.
To reduce risk to the project and personnel during the work, the company qualified new tooling and equipment, work processes and personnel at the Westinghouse Test Facility in Västerås, Sweden. Creating scenarios of the most challenging aspects of the project, Westinghouse manufactured mockups of the most geometrically complex components. Personnel tested their work processes and the newly designed tooling and equipment the company manufactured specifically for the purpose. Barsebäck’s approval was the final step before all of the tooling and equipment were shipped to the site.
To prepare the site for component cutting, a turntable was installed to hold components in the storage pool to make cutting in the crowded pool area easier. The company also installed a foreign materials exclusion barrier between the storage pool for reactor internals and the reactor pool, to prevent chips and shavings from transferring during cutting. Control rod guide tubes were removed from the reactor and placed in racks.
Above the pool, Westinghouse modified the existing refuelling machine to use as a separate cutting platform and equipped the working bridge platform with an underwater camera system monitor and controls, cutting tool manipulation and handling system, underwater lighting system support, and pneumatic and electrical connections.
On the reactor hall floor, Westinghouse installed enclosed service stands for tool maintenance, service and decontamination.
Cutting and packaging
With the qualified equipment, tooling and personnel onsite, the reactor internals storage pool prepared, and the waste packaging and handling equipment and all support systems ready, the work to segment the internals could begin.
Each component of the reactor vessel internals was extracted from the reactor using overhead cranes and placed on the turntable.
The Westinghouse-designed cutting tools are easy to manoeuvre, cut rapidly and are capable of cutting almost all thicknesses and materials. They are also very flexible, which helped maintain the schedule when obstacles had to be overcome. For example, there were occasions when the geometry of a component did not turn out to be exactly as presented on drawings. Westinghouse was able to modify cutting tools as needed onsite, without compromising the schedule.
For all of the segmentation and packaging work, operators were able to supplement their view from the working bridge with the images from the subaquatic camera system to monitor and direct component movement and cutting operations.
Westinghouse chose to segment the steam dryer first, in parallel with the control rod guide tubes. It was segmented using 18 different disc saw setups to accommodate its complex geometry. Some parts, such as the large vane banks, were cut from the steam dryer and then further segmented in a separate cutting station on the pool floor. The control rod guide tubes were cut into five pieces using a disc saw and then cut longitudinally with a punching machine to reduce the volume of waste.
The core shroud cover with its steam separators was next removed from the reactor vessel and disassembled. The nozzle pipes were cut off and a specially designed shearing tool was used to reach the core spray system. The core shroud cover was then positioned on the turntable in a stand to facilitate its segmentation.
Once the core shroud cover was completely segmented, Westinghouse adapted the turntable to hold the core grid, which was segmented with a disc saw and a shearing tool.
The last large reactor internal component was the core shroud, which was segmented with several different disc saws to cope with the geometry.
Packaging and waste handling
As the reactor internals were segmented, Westinghouse used the same handling tools to package the cut pieces in inserts that had been placed in the pool. Once the inserts were completely filled with cut pieces as per the packaging plan, they were removed from the pool using a shielded transport container and placed in waste containers on the refuelling floor. The waste containers were transported to interim storage.
The containers had been preselected according to the packaging plan. They have varying wall thicknesses and packable inner volumes (from 5.4m3 to 7.5m3) and were chosen based on the activity level of the waste to be packaged and on meeting the approved waste packaging process in Sweden.
Westinghouse verified the reactor internals’ irradiation characterisation during the project by taking samples from the internals, analysing them and comparing the results to the characterisation report provided by Barsebäck, which was an important input to the packaging plan.
Final steps: equipment removal and pool cleaning
Westinghouse removed the chips and shavings throughout the segmentation process with a final cleaning. About 95% of the chips that had accumulated at the bottom of the pool were removed with a special scoop tool. The remaining five percent were removed with an underwater suction device, restoring the pool to its initial condition.
After cleaning the tooling and equipment used at Barsbäck 2, Westinghouse packed and transported them to Barsebäck 1.
All Barsebäck 2 components were segmented. A total of 2176 cut pieces were packaged, representing about 1500m of linear cutting. By optimising the segmentation and packaging using three-dimensional modelling, Westinghouse was able to reduce the number of planned waste containers from 40 to 32.
Westinghouse is currently performing similar work on sites in France and Germany. Its continual refinements of segmentation and packaging techniques will benefit these and future decommissioning projects the company will be conducting in Slovakia and Germany.
Lessons learned from the work on both units will be useful for effective decommissioning of commercial reactors in Sweden and elsewhere in the future.
Per Segerud is Project Engineer for Westinghouse Electric Company