Krško NPP Experience with RTD Bypass Elimination

Abstract

This paper describes Krško experience and problems that had to be resolved with RTD Bypass Elimination project (RTDBE). Following RTD bypass manifold isolation valve leak in 2008, Krško decided to perform RTDBE modification on reactor coolant system narrow range temperature measurement system. The installation was performed during Outage 2013. Soon after the plant returned to power, newly configured measurement channels showed that OPΔT reactor trip was oversensitive to spikes caused by auxiliary relay operation in the cabinets nearby. The solution was to reconfigure OPΔT trip filtering constants to filter out short-term spikes in the signal. After almost full operating cycle of trouble-free operation, RTD failures started occurring on reactor coolant system cold leg, which was caused by the high frequency vibrations (3-5 kHz) induced by reactor coolant pumps. To resolve RTD failures, Krško ordered re-design of the RTDs to add robustness and specific qualification in high-frequency vibration operating environment. Improved RTDs were installed in Outage 2016 and were operating one full cycle with minor deviations.