01 Overview
In August 2016, the Leibstadt boiling water reactor in Switzerland was shut down after elevated radioactive contamination was detected in the coolant during routine monitoring. Investigations revealed that corrosion crud deposits on fuel rod surfaces contained higher-than-expected concentrations of cobalt-58 and cobalt-60.
02 Cause
The contamination was traced to corrosion products depositing on fuel assemblies and then becoming activated by the neutron flux. The elevated activity was partly attributed to certain stainless steel components whose corrosion behaviour was not fully accounted for in operational planning.
03 Impact
No radioactive release outside the reactor system occurred. The plant was shut down for an extended period while all 648 fuel assemblies were individually inspected. Several assemblies were replaced. The shutdown lasted approximately one year, costing significant lost generation.
04 Response
Swiss Nuclear Safety Inspectorate (ENSI) was notified and approved the inspection programme. Plant operator KKL conducted a comprehensive fuel inspection and remediation programme. ENSI confirmed the adequacy of the safety case before permitting restart in 2017.
05 Legacy
The Leibstadt incident reinforced the importance of continuous coolant monitoring as an early warning system and demonstrated the regulatory independence of Swiss nuclear oversight. The plant returned to full operation in 2017 and remains one of Switzerland's largest electricity generators.