An investigation is underway at the Dounreay nuclear site in northern Scotland after staff found plutonium in a manhole connected to designated ‘non-active’ drains.

On May 11, while collecting a water sample from the manhole, 40cps beta/gamma and eight cps alpha were detected. Sampling stopped immediately.

Subsequent sampling by staff with respiratory protection produced swabs with a total of 1.34KBq alpha, described as 60% Pu-239/240.

The manhole is located inside the Dounreay licensed site to the west of radioactive effluent tanks.

At the time, staff were studying the potential transfer of radioactivity in the area, in conjunction with contamination of the foreshore. One of these investigations involved taking measurements of radioactivity inside the manhole.

The results were higher than expected and laboratory analysis indicated the presence of the plutonium.

Staff who took the samples have been put on a programme of health checks and further inspection of similar manholes has been put on hold while the system of work is reassessed.

Further tests are also being carried out to verify the initial lab results.

UKAEA, the site operator, is looking into the cause of contamination and any action that might be needed. The area of ground around the effluent tanks, which is known to be contaminated, is scheduled for cleanup in later years of the decommissioning programme, at an estimated cost of £4 million.