The clean-up could take 20 years

The work has an estimated price tag of �355m and is expected to take up to 20 years.

The UKAEA described the task as "challenging" and said it wanted to build up a team of suitable companies.

Plans for the clean-up were announced in 1998 by the UK Government, which said waste retrieval was the best option for a shaft which no longer met "acceptable" disposal standards.

It was created during the construction of a sub-sea pipeline in the 1950s and used as a makeshift dump until the 1977 explosion.

Treatment plant

However, its use was phased out from the early 1970s onwards after a silo was introduced as a more acceptable form of storage.

The clean-up operation will involve both the shaft and the silo, which hold a combined total of around 1,400 cubic metres of waste.

The UKAEA said the clean-up project would also involve the construction of a waste treatment plant and an above-ground store.

Work started in February on drilling new boreholes around Dounreay in preparation for the emptying of the waste shaft.