Slovenia’s Krsko nuclear power plant is implementing a raft of measures to deal with damaged nuclear fuel assemblies. The work is expected to extend the current refuelling outage by about a week, with longer-term measures taking ‘at least a year’ according to operator NEK.

The fuel damage occurred on 8 October, when a 50cm length of fuel rod broke off during transfer of a fuel assembly to the spent fuel pool. A fuel rod is metal tube, about one centimetre in diameter that contains stacked pellets of uranium. If such a rod is not adequately sealed or is damaged, radioactive substances can leak into the water of the primary circuit.

NEK said the event fell under the category of an operational anomaly, which is below the threshold for classification by the International Nuclear Event Scale.

Pictured: Part of the NEK board of directors in what appears to be the Krsko main control room.

Subsequent investigations revealed mechanical damage in other peripheral fuel assemblies, all at the same height in the upper part of the reactor core. NEK determined that the damage was caused during operation as a result of cross flow of the coolant through the gaps between the vertical baffle plates – a phenomenon called baffle jetting.

The Slovenian nuclear regulator ÚJD said that there are "several possible causes" for the fuel damage, and that it is "very difficult to identify a single one". It named friction, foreign material damage or a potential manufacturing error as possible causes.

Solutions

NEK said that all fuel assemblies have now been tested and that five damaged fuel assemblies (of a full core of 121) are now ‘permanently disqualified’ from further service. The remaining fuel assemblies will be transferred back into the reactor core.

Foreign bodies have also been removed from the reactor vessel, spent fuel pool and the transport channel, NEK said.

As a protective measure, the four fuel elements to be installed in positions exposed to cross flow coolant or vibration will be mechanically stabilized, NEK said. Three of the damaged fuel elements were located in these positions in the core.

"The reinforcements will be made ​​by the installation of seven steel rods, which will replace the exposed fuel rods of each of the four fuel assemblies," NEK said.

The operator has repeated the design of the reactor core and associated safety evaluation for the use of the steel stabilizer bars.

NEK plans to carry out a proven modification to the structures of the reactor to prevent occurrence of the phenomenon in future. Preparation of this long-term solution will take a ‘minimum of one year and is slated for completion during the next outage,’ NEK said.

According to ÚJD, the problem with the fuel rods has been known since the summer of 2012, when it began to increase the level of radioactivity in the primary circuit. However, since the concentration of radioactivity was far below [around 3% of] the permissible limits for safe operation, NEK did not need to take immediate action, and waited until the current outage.