Swiss Federal Nuclear Safety Inspectorate (Ensi) on 20 February approved the restart of the single-unit Leibstadt NPP after an extended outage, which began in August 2016.
The original outage was extended following the discovery of discoloration suggesting oxidation problems on the cladding material of some fuel rods in 47 fuel elements in the reactor core. In August 2016, operator Kernkraftwerk Leibstadt (KKL) said examinations had shown that surface oxidation had progressed so far that 32 fuel rods in 15 fuel elements needed to be replaced with zirconium rods.
Ensi said an investigation of the fuel cladding has indicated the problem was caused by a condition known as “dryout”, which occurs when the surface of fuel rods is no longer covered sufficiently by a film of water, resulting in excessive oxidation of rod cladding due to an elevated surface temperature.
To reduce the risk of dryout, Leibstadt’s thermal output will be limited to between 88% and 95% until mid-September 2017. In the event of fuel rod damage reoccurring during the scheduled operating cycle, the unit will be shut down Ensi said. Leibstadt was scheduled for restart on 17 February, but a malfunction in the exhaust system of the unit’s condenser interrupted the process.
Leibstadt is a 1220MWe boiling water reactor in commercial operation since 1984.