Reducing the power of the Unit due to loss of control of the position of one control rod

Nuclear Facility: Unit 5, Kozloduy NPP

Event Title: Reducing the power of the Unit due to loss of control of the position of one control rod

Event description:

On 13.06.2010 at 08:00 Kozloduy NPP Unit 5 is running at 85,7 % nominal power. According to the programme for start-up and power assimilation after annual outage, actions for power increase are being performed. At the same time on the panel in the control room is observed loss of position signal for one of the control rods. A check for symmetry has been made of the reactor core by the internal reactor control system, which showed that there is no data for movement of that control rod. Immediate actions for identification and resolving the cause of the problem have been taken. The power at that time is 85-86% from the nominal. Due to failure in recovery of the signal, according to the Technical Specifications of the Unit, the operators began to reduce the power to 75%. At 15:56 the signal is restored according to an amended scheme, based on temporary technical solution. At 16:00 the power reduction has been sustained of to 75% of nominal power. At 18:00 after testing of the signal throughout the movement range of the rod the power loading of the reactor is resumed according to the programme for start-up and power assimilation after annual outage.