Information about the fuel degradation in Krško plant
Krško NPP, which is in the refueling outage since the beginning of October, is experiencing problems with the fuel. As the issue has received also substantial media coverage here in Slovenia with this information we want to inform general public bout actual facts.
The plant has operated in its 26th, 18 month long fuel cycle since May 2012. First indication of the leaking fuel appeared very early in the 53th day after the start-up. The primary coolant activity gradually increased during the cycle, showing possible three fuel rods leakages at the end of the cycle. During the cycle two unplanned reactor shutdowns occurred which have probably contributed to the development of fuel pin failures. As the activity of the primary water was far below the technical specification limit (at the end of the cycle it has reached 2.95% of the limit), the plant did not have to stop for midcycle refueling.
After the removal of the fuel from the core several broken fuel pins were found in three fuel elements and additional few fuel elements have been recognized to have tight leaks. During the transfer of the fuel from the reactor vessel to the spent fuel pool about half a meter long pin even broke and was found at the bottom of the transfer channel. All fuel elements with broken pins were positioned at the outer edge of the core next to the core baffle.
Westinghouse, who is the fuel supplier and main refueling service provider, has made preliminary analysis recognizing the influence of the baffle jetting as the main cause of troubles (See attachment). Therefore as the immediate corrective measure number of outer pins of the fuel elements at exposed locations are being replaced by dummy steel pipes just to avoid potential repeated damage. As a long term correction during the next refueling outage most probably the baffle plate arrangement will be converted from so called down-flow to up-flow thereby reducing the pressure difference and possibility for jetting.
None of the damaged fuel elements will be returned into the core. All the fuel elements were checked by In Mast Sipping methodology and visually inspected, while most suspected elements also by ultrasonic methods.
This is not the first time the Krško NPP experienced fuel leakage problems in its operation history. There were serious problems in mid nineties, while in recent years amount of leakages was relatively small. Two fuel cycles ago, however, there were even no leakages detected.
Slovenian Nuclear Safety Administration is tightly following the situation to be sure that the operator is doing everything that can be done. We want to be sure that the next fuel cycle is starting without suspicious fuel elements, that immediate corrective actions are in place and that straightforward and adequate action plan is prepared in case the activity of the primary coolant starts to increase during the next fuel cycle.
The formal root cause analysis will be completed during next 2 months. It will contain final leakage mechanism determination for all Cycle 26 leaking fuel assemblies, root causes, corrective actions and plans to follow up corrective actions.
Currently it seems that the refueling outage will be prolonged for ten days and the plant plans to restart in the middle of November.