Discovery of damaged fuel rods during core unloading and fuel inspections

In October 2013 the refuelling outage started in the Krško NPP. During transfer of fuel assemblies from the reactor to the spent fuel pool a 50 cm long segment of a fuel rod was found at the bottom of fuel transfer channel. Visual inspection of all fuel assemblies unloaded from reactor core showed open defects in 9 fuel rods cladding of 4 fuel assemblies. These 9 fuel rods were all broken with some parts of cladding and fuel pellets missing. The cause of such extensive fuel cladding failure is secondary degradation by zirconium hydride. The primary cause of fuel rods leakage is attributed to baffle jetting that can occur at these fuel assemblies’ locations at the core baffle plate. Open fuel cladding defects were diagnosed from measurements of I-134 activity of the reactor coolant since July 2012, two months after the previous refuelling outage. The release of fission products from the damaged fuel occurred during the NPP operation. During refuelling outage in October and November 2013 there were no additional radioactive releases from the damaged fuel rods. There was no additional exposure to the workers during the fuel transportation.
Extensive inspection of fuel assemblies and the core baffle plate was carried out to determine the cause of fuel defects and to assure the integrity of the fuel assemblies to be reloaded into the new reactor core. Fuel reconstitution campaign was carried out to replace several fuel rods at exposed locations on the core baffle with stainless steel dummy rods that can resist to the baffle jetting. The root cause analysis of the fuel damage including the proposed corrective actions will be prepared by the fuel supplier and by the operator after the outage.

Location: KRSKO
Event date: Tue, 08-10-2013
Nuclear event report
Legenda & explanation