Irregular control rod movements at Balakovo

Wednesday, 31 January 2007 Russia's Balakovo 1 underwent a sudden shutdown late on 29 January as control rods were found to be in an irregular position.

The VVER-1000 pressurised water reactor (PWR) was operating at 1030 MWe when control rods spontaneously changed position. Operators followed procedure and shut the plant down for checks at 23h15.

An event report from Rosenergoatom, the operator of all Russia's nuclear power plants, said that the loss of power to one of the circuits supplying electricity to a control system initated the event.

Control rods regulate the power of reactors by absorbing the neutrons that cause uranium-235 atoms to split in a chain reaction. In a PWR they are suspended above the reactor core in such a way that loss of power to their control mechanisms would result in the rods dropping under gravity to safe positions in the core that would cause a shutdown.

Rosenergoatom reported that the cause of the event was identified before applying for permission to restart the reactor. Balakovo 1 was reconnected to the grid just before 02h52.

Further information

WNN Nuclear Event Reports: Manual shutdown at Balakovo 1