Origins
Dounreay was built in the 1950s to support the UK's development of nuclear power. Its design included chemical plants that could recycle used nuclear fuel by undergoing a process of disassembly and chemical separation known as reprocessing. This allowed valuable uranium and plutonium to be recycled as new fuel.
The fuel sent to these plants for reprocessing between 1958 and their shutdown in 1996 came from the Dounreay Fast Reactor, the Prototype Fast Reactor, the Dounreay Materials Test Reactor and other research reactors elsewhere in Britain and abroad.
The used fuel from materials test reactors was dismantled underwater in ponds before being transferred to a dissolver vessel for chemical separation. The underwater dismantling techniques included milling, crushing and cropping. These generated fragments known as swarf that consisted of metal cladding and occasionally some of the underlying fuel.
The swarf accumulated in the bottom of the ponds. Periodically, the water was drained to allow the pond to be cleaned out. The bulk of the accumulated swarf was removed from the bottom of the pond and transported across the site to the shaft and silo, the historic disposal and storage facilities authorised to receive such material under the Radioactive Substances Act.
Some swarf was discharged from the pond with the drained water. This was routed to settling tanks that were designed for removal of solids before discharging liquid radioactive effluent to sea. Hundreds of thousands of particles probably entered the drains connected to this system but the number of particles that was released to the sea from the settling tanks is not known. Other particles were generated when used fuel ignited in a dissolver. Some of these also entered the active drainage system.
The evidence today points to the release of particles occurring primarily in the late 1960s, but further releases into the active drainage system before filtration was fitted at the source facilities in 1984 cannot be ruled out. Particles could have been held up in the effluent system and may have been released after this.
A full account of the origin of particles at Dounreay can be found in the Third Report of Dounreay Particles Advisory Group.
Many particles came from fuel of the type irradiated in research reactors such as the Dounreay Materials Test Reactor, seen here under construction in the 1950s
The underwater milling machine in the D1204 fuel pond used to disassemble MTR-type fuel elements
Decommissioning the old fuel pond at the Dounreay Materials Test Reactor
The old effluent tanks where low level liquid waste from all parts of the site, including the fuel plants, was collected for sampling prior to sea discharge
Excavating part of the low-active drain in 2006
