On 12 May 1998 around 8 pm when the reactor no. 1 had been shutdown since 7 May, a water leak estimated at around 30 m3/h was detected in the reactor shutdown coolant circuit. This circuit is used when the reactor is shut down to circulate a minimum amount of water through the primary circuit to cool the fuel present in the reactor core. The system consists of two duplicate circuits. The leak was stopped by isolating one of the circuits at around 5 am on 13 May 1998. The leak resulted in water passing from the primary circuit into the sump of the reactor building.