Dounreay, Scotland

The Dounreay Nuclear Power Development Establishment was inaugurated in Scotland by the United Kingdom Atomic Energy Authority (UKAEA) in 1955. In 1977, there was an explosion in a waste disposal shaft that is considered the second worst nuclear disaster in Britain.[1] However, the major source of nuclear contamination at Dounreay has turned out to be its long-term discharge of liquid containing fragments of the radioactive fuel rods.

Waste Shaft Explosion

In the mid-fifties, the Dounreay engineering team drilled a vertical shaft 65.4 meters [211 feet] deep, with an average diameter of 4.6 meters [15 feet] to aid in the construction of the effluent discharge tunnel. A short horizontal tunnel was dug from the vertical shaft and met up with the in-process effluent discharge tunnel.

The Dounreay waste shaft in operation

The Dounreay waste shaft in operation

The shaft was used to remove broken-up rock and as the main pumping route to keep the tunnel dry during excavation. After the discharge tunnel was completed, the tunnel connecting it to the vertical shaft was plugged with concrete and Dounreay began using the vertical shaft for nuclear waste disposal. In 1959, the shaft was licensed for intermediate-level nuclear waste disposal, so this marks the beginning of its official use.[2] In the picture to the right, you can see waste being lowered through the hole in the middle of the plug at the top of the shaft.

On May 10, 1977 there was an explosion in the shaft. It was fairly impressive:

papers released under the Freedom of Information Act …. show the shaft’s concrete plug, weighing seven tonnes, was blown three to four metres [10 to 13 feet] into the air and thrown against a security fence, while a steel plate, nearly 1.5 metres [5 feet] in diameter, was blasted 12 metres [39 feet].

Debris was projected over the boundary fence on to the sea shore, lead sheeting was thrown over the security fence and two six-metre scaffolding poles were found outside the fence, one 40 metres [130 feet] away on the beach. The windows of the control room were also shattered and asbestos weather shields surrounding the shaft and a 20ft length of the nearby security fence were extensively damaged. About 50 spots of ground contamination were found to the north of the shaft and pieces of asbestos were discovered up to 75 metres [245 feet] away.[3]

The aftermath of the Dounreay shaft explosion

After the explosion. In the rear, you can see the square concrete plug (which the man was kneeling on in the above picture) and in the foreground the shattered concrete around the hole where the plug was.

What caused the explosion? A build-up of hydrogen gas under the cap of the shaft. The coolant for the Dounreay reactors was a mixture of sodium and potassium that is liquid at room temperature. This mixture was disposed of by dumping it down the waste shaft, which is a questionable idea since the waste shaft was mostly filled with sea water. Those who remember their high school chemistry will recall that both sodium and potassium react with water, producing lye, hydrogen gas, and a great deal of heat – potassium in fact usually explodes when submerged in water.

The dumping of wastes from the nuclear facility into this waste shaft was unsupervised and undocumented. And handled according to the whims of those disposing it:

There was unsupervised fly-tipping into the shaft and workers firing rifles into it to sink polythene bags floating on water, with no regard to the shaft’s hazardous contents.[4]

The shooting is interesting: in other words, some waste was dumped in plastic bags to minimize its contact with the water; it reacted in small amounts anyway, generating gas that caused the bags to float. The Dounreay workers shot the bags, causing them to sink again, and giving the sea water free access to their contents.

We don’t know what has transpired beneath the surface of the water, but in 1977 the hydrogen gas from these reactions had built up enough under the cap of the shaft that igniting it (a spark from pumping equipment is officially blamed) touched off the explosion.

Removing the waste that remains in the shaft is considered the most difficult decommissioning job at the Dounreay site, because at 65 meters deep it is the deepest nuclear decommissioning attempt in the world,[5] and because the shaft’s precise contents are unknown.[6]

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[2] “Disposals to shaft” , Dounreay Site Restoration Ltd.

[3] “Dounreay chiefs played down major blast at plant”, The Scotsman, July 13, 2005.

[4] “No-one knows what is left in the Dounreay waste shaft”, The Herald, January 25 2007.

[5] “Dounreay prepares for world’s deepest nuclear clean-up”, John O’Groat Journal and Caithness Courier, July 9, 2012.

[6] “No-one knows what is left in the Dounreay waste shaft”, The Herald, January 25 2007.

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