COVER STORY | SWEDEN
Coarse control rods Ågesta – data ● Number: 27 units ● Length: 5.1m with the absorber in the non-deployed position
● Absorbers control length: 3m ● Largest diameter: 182mm ● Weight: approximately 400kg ● Absorber composition: inner and outer tube made of
stainless steel and a mid-layer comprised of silver, indium and cadmium.
● Dominating nuclides: The isotopes which provide the highest contributions to dose rate are Ag-108 and Co-60. ■
“Since there were no ‘ready to use’ concepts we had to design a method and several tools ourselves,” says project manager Linda Ekstrand, Vattenfall’s project leader for the control rod segmentation at Ågesta.
V The control rod management challenges Ågesta contained 30 control rods in all, 27 coarse control rods and three fine adjustment control rods. The coarse control rods are 5m long and weigh approximately 400kg each. They have a 3m long extension containing the neutron absorber, in which the highest levels of dominant persistent radionuclides are found. The absorber is of sandwich construction, with inner and outer tubes made of stainless steel and a mid-layer comprising silver, indium and cadmium. The isotopes which provide the highest contributions to dose rate are Ag-108 and Co-60. Additional investigations were made during preparations
for decommissioning, including measurement of dose rates from the control rods. These indicated dose rates one order of magnitude higher than indicated previously, ie a maximum surface dose rate exceeding 1000mSv/h and up to 30mSv/h at a range of 1m. There were no suitable ‘ready to use’ concepts for on-site management, transport, or segmentation and conditioning of the Ågesta control rods. A feasibility study was initiated in 2018 to investigate
possible options for managing the control rods. The result was a comparative analysis between the main options identified — off-site segmentation or in-situ segmentation. In early 2019, Vattenfall’s Business Unit Nuclear Decommissioning (BU.ND) decided to proceed with in-situ segmentation.
Developing an in-situ segmentation option In order to minimise the volume of waste that must be deposited in the Swedish repository for long-lived radioactive waste, activated parts had to be separated from the rest of the control rods. The absorber part was segmented in-situ with remote controlled equipment, to enable manual handling of the remaining part. The equipment developed to handle the movement, segmentation and packing of the control rods consists largely of modules built up using proven technology. The aim was a simple and robust methodology and reliable equipment. The modules were to some extent modified before being mounted on site. It was essential to address ALARA in the design and planning stages. Manual handling of the control rods would be avoided as much as possible, and the segmentation equipment was placed and arranged so that other operations in the plant would be affected as little as possible. The equipment was designed and painted to minimise the potential for secondary surface contamination and so that it could be easily cleaned. Robotic control cabinets and scissor hydraulic units were placed outside the cutting working space in order to facilitate repair or restoration. “We performed the work deep down in the facility, far
away from us and other ongoing work, with additional shielding coming from the several floor layers in between the equipment and other personnel. The process was remote controlled with cameras and computers from a station in the reactor hall,” says Ekstrand. The solution for moving the control rods was a 6m
radiation protection tube attached to a specially-made lifting yoke, suspended from the plant’s main traverse. This was combined with a 1t telfer, which was used to lift the rods up into the shielded tube. During lifting, a docking ring was designed to be placed on the position with the control rod, which stabilised the heavy radiation protection tube and ensured that the telfer could be hooked into the
Above left: Control rod in cutting position Photo credit: John Guthed Above right: Cutting station showing enclosure to stop spreading of contamination Photo credit: John Guthed 34 | February 2022 |
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