Workers at the UK’s former fast reactor test facility at Dounreay in Scotland - which operated from 1955 to 1994 - have completed the first in a series of campaigns to seal the waste containers into place in the two near-surface low-level waste (LLW) disposal vaults at the site.
There are two nuclear sites at Lower Dounreay - the Nuclear Power Development Establishment site owned by the Nuclear Decommissioning Authority (NDA), and the adjacent Ministry of Defence Vulcan Naval Reactor Test Establishment. The two sites hosted five nuclear reactors, three formerly owned and operated by the UKAEA and two by the Ministry of Defence.
As part of the waste disposal process in the low-level waste vault, the spaces between the containers are being filled with grout. The team undertook a series of trials to confirm that the grout would readily flow between the containers and also tested the membranes that will be used to seal the grout shutters. A first campaign of grouting has now been completed within the vault with 16 waste packages sealed into their final positions. Further grouting campaigns of increasing size are planned.
In parallel with this work, the external space around the outside of the walls of the vault has been backfilled with aggregate. In the first phase of this work. Enough aggregate to fill an Olympic-sized swimming pool has been used to fill up to 2.5m high around the structure. This is only 2% of the total volume of material that will be needed to backfill, cap and close the two vaults once they are full.
“This work is a strategically important demonstration that we can carry out this crucial part of the disposal process, which will eventually enable us to close the vault,” said Project Manager Graeme Morgan. Senior Engineer Angus Mackay added: “We’ve had a fantastic level of support from so many people across the site and the quality of the workmanship has been superb. It’s great to see the disposal process moving forward.”
The work was completed by a team from Dounreay Site Restoration Ltd along with local contractors Arch Henderson, John Gunn & Sons, JGC Engineering and Hugh Simpson Contractors. The Dounreay site applied to the Highland Council for planning permission to build six LLW disposal vaults in 2006, and the permission was granted in April 2009. Graham Construction won a tender to design and build the facility, and construction began in November 2011.
Construction of phase one of the facility, comprising two vaults, was completed in 2014. One of the vaults will be used for disposal of LLW arising from previous operations at Dounreay, while the other will accommodate waste generated by decommissioning operations at the site. Each vault covers approximately 1.8 hectares and is 11m from top to bottom. Each vault consists of a reinforced concrete floor slab founded on bedrock with reinforced concrete walls, and sheltered by a portal frame cover building. Drainage and pumping systems keep the vaults dry during waste emplacement. Commissioning of the associated encapsulation plant began in 2015 when a container packed with LLW was filled with grout.
LLW comprises debris such as metal, plastics and rags that have been contaminated during the clean-out of facilities where radioactive materials were handled. By volume, LLW represents more than 80% of all the radioactive waste generated during Dounreay's decommissioning, but in terms of radiological hazard it represents less than 0.1%.
The LLW vaults will be used for the disposal of up to a maximum of 175,000m3 of solid LLW, which is expected to be generated during the decommissioning of the Dounreay site, in addition to waste that will be retrieved from a series of historical LLW pits there.
Image: Interior of the decommissioning waste vault at Dounreay (coourtesy of gov.uk)