01 Overview
In March 2002, inspectors at the Davis-Besse Nuclear Power Station in Ohio discovered that boric acid corrosion had eaten through nearly the entire thickness of the carbon steel reactor pressure vessel head. Only a thin stainless steel liner - about 3/8 inches thick - remained to contain reactor coolant at high pressure.
02 Cause
Boric acid had been leaking from control rod drive mechanism nozzles for years. The acid corroded the carbon steel of the reactor vessel head, creating a cavity roughly 30 cm wide and 15 cm deep. The corrosion went undetected because previous inspections were inadequate and regulators had granted the plant an inspection deferral.
03 Impact
If the thin liner had failed, it would have caused a loss-of-coolant accident similar to Three Mile Island. The hole was the largest ever found in a reactor vessel head. Plant staff had been aware of boric acid deposits but failed to adequately investigate or report the issue.
04 Response
The plant was shut down immediately upon discovery. The reactor vessel head was replaced. The NRC issued a $5.45 million fine - the largest ever at the time - to FirstEnergy. Criminal charges were filed against a plant supervisor for falsifying safety records.
05 Legacy
Davis-Besse led to major NRC regulatory changes requiring more rigorous inspection programmes for reactor vessel heads across the US fleet. It reinforced the critical importance of nuclear safety culture and the willingness to report problems without fear of retaliation.