Neil Cannon, 36, was removing steelwork inside a pile chimney at the Sellafield site in Cumbria as part of decommissioning works there when he fell around 95m and suffered fatal injuries.
At the time of the accident, Mr Cannon was trying to remove a girder from the ledge. The girder tipped upwards, sliced through Mr Cannon's safety lanyard, and caused him to fall through the gap between the ledge and the working platform.
HSE principal inspector Mark Cottriall said: “Ongoing monitoring of the work in progress should have identified the changes and the increased risk. If this had been done it is almost certain that a way could have been found to do the work without leaving the safety of the platform.”
Sellafield was fined £150,000 for a breach of the Health and Safety at Work Act and was also ordered to pay £50,500 in costs.
Sellafield was the client and principal contractor while P C Richardson was the decommissioning and demolition sub-contractor.