A critical end

For six years, at the height of Britain’s expansion into nuclear energy in the 1950s and 60s, Dounreay’s experimental criticality laboratories carried out vital research.

Four cells – three for uranium and one for plutonium – were built between 1956 and 1959 and fitted with experimental reactors and rigs in a complex of buildings.

The first experiment using uranium in solution went critical on 13 August 1957. This was the first nuclear reaction to take place in Scotland. Over the next six years, a series of experiments yielded valuable information about the behaviour of uranium and plutonium.

When the experiments finished in 1963, the uranium cells (known as D1249) were cleaned out and converted to a test facility for sodium, which was demolished in 2005. The plutonium criticality research facility (D8550) was retained after 1963, in case more experiments were needed.

The criticality experiments in D8550 were carried out using the PANTHER (plutonium nitrate thermal reactor) and PUMA (plutonium moderated assembly machine) rigs in a cell formed from a 7m-high, 8m-diameter domed cylindrical steel pressure vessel. This was surrounded by a 1.5m-thick hexagonal concrete bioshield, separated from the pressure vessel by a 75mm air gap. The complex also included associated fuel storage, preparation, laboratory and control room.

At the time when the criticality experiments were conducted the scientists required no protective personal equipment. However, the final experiment carried out resulted in considerable contamination of the criticality cell by plutonium-bearing liquid.

Active operations ended in 1963, and some post-operational clean out of the facility took place shortly afterwards. This partial decommissioning was completed in 1967, and the building was then put into a ‘care and maintenance’ regime for the next 20 years. From 1987-1994, the active areas of the building outside the criticality cell underwent some basic decontamination, and care and maintenance was then re-established.

In 2000 a small UKAEA-led team comprising staff from Doosan Babcock, NDSL, Nuvia (formerly Nukem) and UKAEA (now DSRL) re-started decommissioning work on the redundant facility. Entry into the heavily-contaminated criticality cell required workers to wear airline suits.

The cell still contained the PUMA rig and a large amount of historical waste. The waste was packaged, segregated and sentenced via the appropriate waste route. The cell’s internal structure, some 300m2 of steel plate, was initially decontaminated by sponge-jetting, a technique widely used in the aviation industry. This cleaned the face of the steel cell liner, stripping off most surface contamination, without damaging the surface of the metal.

By 2002 this work had enabled the first man entry into the criticality cell for almost 40 years without respiratory radiological protection equipment.

The large and extremely heavy airtight cell door and frame was decontaminated and removed to enable the PUMA rig to be dismantled, decontaminated, size reduced and consigned as waste. The steel handling equipment and lifting frames in the cell were also size reduced and removed.

Once the cell was empty, the project team began the process of removing the 13mm thick steel pressure vessel that formed the inner face of the criticality cell. After careful consideration of all possible removal techniques, the work was carried out using hand-held oxy-gas cutting torches. The contaminated steel lining to the 3m deep pit in the floor of the cell was removed in a similar manner.

After radiological clearance surveying the cell was declassified. It was temporarily closed off in December 2006.

From April 2007 the remaining areas of the complex were systematically decommissioned on a room-by-room basis. The preliminary work

carried out in the late 1980s had included heavy scabbling of the plaster walls and painting. However, surveys indicated areas of extensive contamination on the surface of walls and floors, particularly on the painted areas. The contamination in some areas extended not only through and under the paint film into the plaster behind, but in many cases into the underlying concrete blockwork structural supporting walls themselves.

In some cases the contamination was so deep that complete runs of wall needed to be propped to support the structure above and enable the complete wall to be removed. To reduce manual handling and mitigate industrial safety hazards a Brokk semi-robotic machine was used.

The building was handed over to the site decommissioning unit’s conventional demolition experts following declassification in autumn 2008, and was finally demolished by the end of March 2009.