RISK OF nuclear explosion within the radioactive contents of a deep waste shaft at Scotland's Dounreay nuclear facility could threaten the massive £355M, 25 year clean up operation, it emerged last week.
The UK Atomic Energy Authority said that enough weapons-grade uranium and plutonium has been dumped into the shaft over the last 40 years to form a critical mass large enough to create a significant risk of explosion.
Government approval for the first stage of the operation to clear the shaft was given at the end of last month (NCE 2 April). But the UKAEA has admitted that inadequate recording meant its inventory of about 16,000 items of radioactive scrap in the shaft is 'very detailed, but incomplete'.
Environmental restoration senior manager Sandy McWhirter said: 'UKAEA can't discount criticality (achieving critical mass for explosion) as a factor.' Current estimates show between 98kg and 1,187kg of enriched uranium and 2.2kg to 4kg of plutonium in the shaft, almost entirely in the form of small particles.
Only 25kg of enriched uranium is needed for criticality, or less than 4kg of plutonium. Environmental campaigners fear that the disturbance of the shaft's contents during recovery operations could concentrate enough of the currently dispersed uranium and plutonium particles to create a serious risk of explosion.
They point to the 1977 accident when sodium dumped into the shaft apparently reacted violently with water to create an explosion powerful enough to blow off the concrete shaft cap. But McWhirter said UKAEA was confident that a safe removal technique - probably involving ground freezing - could be developed before recovery operations proper began in around five years time.