On 31 January 2000, a transfer failed following an inverter failure (primary cause) this led to a loss of power of a 220 VAC uninterrupted monophasic safety bus bar (class II).
Several channels of the safety systems were then tripped (shutdown system #1, shutdown system #2, containment system, and emergency core cooling system) and the turbine control system including turbine steam bypass were lost.
Due to loss of the turbine control, the turbine tripped and led the relief valve to actuation and for short period the safety valves (MSSV) also acted because of the failure of steam bypass. Turbine trip produced an automatic power reduction and high-pressure steam caused a ramp reduction power (set back 0.5%/sec.). The reactor was tripped manually at 12% of power. During the event, the back up computer (DCCY) was lost together with five monitors. Due to a relay dormant fail (fail safe) the emergency core cooling system is initiated but actual injection is impaired because of high pressure of the primary system (any way some mixing between heavy water and light water was detected).
The reactor reached cool shut down 10 hrs. after the initiation of the event.
Justification of the Rating:
This event was rated level 1 in accordance with section III-2.5.2 using the 1992 edition of the INES manual, and the section III-2.9 was taken into account because there was no actual challenge to the safety system.
Event date: Mon, 31-01-2000