Plutonium criticality cell - D8550

The plutonium criticality laboratory, code-named D8550, was built in the 1950s to carry out criticality experiments on plutonium-bearing materials as D1249 had insufficient shielding for the work.

The building housed a heavily-shielded cell, 7 metres high by 8 metres wide, in which experiments were carried out on solid and liquid-bearing plutonium. The criticality cell, known as cell 4, housed the two plutonium experimental rigs known as PUMA and PANTHER.

The experiments ceased in September 1963.  Early attempts to clean up contaminated areas of the plutonium building resulted in a number of incidents that, looking back, seem to have made the problem worse. In 1967, the facility was deemed redundant and a decision taken to seal up what was left inside.

In 1999, following the decision to close down Dounreay, one of the facilities identified for early decommissioning was the plutonium criticality laboratory. The facility was still heavily contaminated, so workers who entered the building to begin dismantling the remaining test rigs and clean up the contamination all around had to wear full airline suits to protect them from any airborne radioactivity.

The heavily-contaminated criticality cell was lined with 300 sq m of steel plate that had to be decontaminated and cut up. Large steel handling equipment and lifting frames were taken apart and the heavy airtight door to the cell was turned into waste. A13mm thick steel pressure vessel, which enveloped the cell, had to cut up, all of it by workers wearing uncomfortable but essential protective equipment.

By 2002, parts of the building could be entered without respiratory equipment for the first time in 40 years.

In 2008, the whole building was declared safe from the hazard of plutonium and it became possible to walk through it without respiratory protection. It had taken staff some 25,000 individual entries wearing respirators or fully protective airline suits since 2000 to decommission D8550. In spite of these difficulties, there have been no major safety incidents during the decommissioning work since 2000, a notable achievement.

 The laboratory complex was demolished in the spring of 2009, bringing to a close the story of Dounreay’s important contribution to nuclear criticality research.

 

D8550 technical history 

Click here to view video and scroll to watch a five-minute film about decommissioning D8550

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Image:

Construction of the criticality cell

Image: Some of the debris that needed to be cleared from the building, photographed during one of the earliest entries prior to the start of decommissioning

Some of the debris that needed to be cleared from the building, photographed during one of the earliest entries prior to the start of decommissioning

Image: Radioactive contamination is identified on the cell door after a full survey of the building prior to decommissioning

Radioactive contamination is identified on the door of the plutonium cell after a full survey of the building prior to decommissioning

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 Preparing to crane the ventilation stack from the building

Image: Plutonium criticality cell pit removal

Plutonium criticality cell pit removal

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The bulldozer demolishing the complex of buildings surrounding the hexagonal criticality cell