One of the biggest engineering jobs at Dounreay is nearing completion.
The finishing touches are being put in place to the facility that will receive almost a thousand breeder elements from the site’s experimental fast reactor.
This is the breeder containment building where each element will be cut up, washed, examined, and packaged so that it can treated as waste or re-usable material.
Safely removing each element from within the reactor vessel is a cornerstone of the site clean-up programme.
Dounreay’s experimental fast breeder reactor was developed in the 1950s to show how a fast reactor could create as much new fuel as it consumed.
Positioned around the core fuel were 1872 breeder elements, each made up of 14 cylindrical slugs of natural uranium wrapped in stainless steel cladding.
Natural uranium had no practical use by itself but, when exposed to the neutrons flowing from a fast reactor, would turn into plutonium, which could then be used for fuel in other reactors. This is where the term breeder comes from.
The Dounreay Fast Reactor shut down in 1977 and work started to remove the core fuel and breeder material. This work stopped in the early 1980s as a result of a logjam in the nearby fuel plants and the discovery that some of the breeders could not be easily extracted.
What remains inside the reactor vessel today is an experimental fuel assembly, two elements in the inner breeder zone and 975 elements in the outer breeder zone.
These items are currently immersed in a sodium-potassium alloy known as NaK that was used to transfer the heat from the reactor core to create steam for electricity generation.
The precise condition of the remaining elements cannot be verified until the liquid metal levels have been lowered.
A £15 million treatment plant for the destruction of the liquid metal is in its active commissioning phase. Once full operation commences, it is expected to take two years to destroy all 57 tonnes.
In parallel, approximately £50 million is being invested in the facilities plant needed to remove and process the breeder left in the reactor vessel.
Specialist equipment has been arriving at Dounreay from engineering plants in England and France for this next phase of the decommissioning.
Inside the sphere itself, remotely operated cutting equipment is being installed. This will reach down inside the reactor vessel and remove each element, using a combination of 16 specialist tools. Each element is 2.5 metres long and 33 mm in diameter.
The breeder elements will be lifted into a flask and transported via a transfer tube to a process cell in the containment building nearing completion at the base of the sphere. All this work is undertaken within a nitrogen atmosphere to prevent the ingress of air, as NaK reacts in contact with air.
Within the containment building the process cell is divided into two compartments. The first compartment is used for dismantling and washing the breeder elements prior to transfer in the second compartment which is used for packaging and transferring the breeder to storage and transport containers.
Reinforced concrete walls provide shielding to protect the workers from radiation while this work is carried out. Within the walls are lead glass windows which allow the operators to safely view operations within the cell. Operations are undertaken using long extension arms known as master/slave manipulators that reach inside the cell.
The containment building has a floorspace of 1000sq m, is 13m in height, 30 metres wide and 44 metres long. Construction started in 2004.
Commissioning the facility is expected to be completed early in 2009 and the facility handed over to the operators. It is expected to receive its first delivery of material from the reactor in 2011 once the bulk NaK has been destroyed, and will take an estimated two to three years to process all breeder material.
This phase of DFR decommissioning is being carried out by an alliance of companies – BNS Nuclear Services, Framatome, SGN and Weir, Strachan & and Henshaw – for Dounreay Site Restoration Ltd on behalf of the Nuclear Decommissioning Authority.
When all the breeder material and liquid metal have been removed from the reactor, work can begin to cut up and dismantle the entire system. This is due for completion by 2025, by when the total amount spent on decommissioning DFR is expected to be £240 million.
There are two options for managing the breeder once it has been packaged safely – to send it for conditioning as intermediate-level waste or treatment through a reprocessing plant. A review of these options is being carried out in conjunction with the NDA.