The blast in 1977 caused extensive damage and propelled material up to 75 metres away from the shaft, which was used as a dump.
But papers released under the Freedom of Information Act show that a 93-word press release issued on the day described it as a "Minor Incident at the Solid Waste Facility Dounreay".
It said that a chemical reaction, probably involving 2.5kg of sodium, occurred, adding: "The energy generated displaced the concrete cover and some insignificant spots of activity were deposited close to the facility. No injury occurred, damage was minor and the public was not involved."
However, the papers show the shaft's concrete plug, weighing seven tonnes, was blown three to four metres into the air and thrown against a security fence, while a steel plate, nearly 1.5 metres in diameter, was blasted 12 metres.
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 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 away.
Accounts from staff at the time show that the blast, at 12:20am on 10 May, 1977, was heard all over the site, with one worker saying the shaft was left "in ruins" and a white "plume" was seen moving towards the sea. The cause of the explosion was said to be hydrogen collecting in the air space above the water in the shaft.
Lorraine Mann, convener of Scotland Against Nuclear Dumping, said: "
It shows how [Dounreay] have had a propaganda machine in operation all these years, trying to lie to people. In this example it's not even being economical with the truth, it's blatant lies."
The contents of the shaft, which was used until 1977,
include quantities of plutonium and uranium.
Mrs Mann said the explosion could be linked to radioactive particles which have appeared on beaches and the sea-bed near Dounreay, as it could have opened up fractures in the rock and allowed material to escape.
A spokesman for Dounreay said there was a gap between the industry's understanding of radiological issues and the public's perception of them. "I don't think it is a description the industry would have used today." He said a connection between the explosion and particles had not been conclusively ruled out, but was recognised to be very unlikely.