During operation at rated power, at 3.31 p.m. on September 29, 1992 the plant was automatically shutdown by "Reactor Water Level Low" signal, and the high pressure coolant injection system (HPCI) was activated.
Successive investigation revealed that:
Before the reactor automatic shutdown took place, the plant had been under operation with the high pressure condensate pumps (HPCPs) 'A' and 'B' and with the turbine driven reactor feedwater pumps (TDRFWPs) 'A' and 'B', while the power supply panel for the standby HPCP 'C' being activated. The spurious signal brought the operating HPCP 'A' to automatic stop in accordance with requirement that the number of operating HPCPs was limited to two; which further brought an automiatc stop of the reamaining HPCPs 'B' due to low delivery pressure. Then the TDRFWP 'B' was brought to automatic stop due to the condition of the number of operating RFWPs. Finally that the total loss of the three HPCPs brought the remaining TDFWP 'A' to an automatic stop, thus lowering the reactor water level and bringing the reactor to automatic shutdown, resulting in an automatic activation of HPCI. There was no release of radioactive materials outside of the reactor facility, nor any unplanned radiation exposure received by employees.
Both, criterion 1 and criterion 2 are not relevant for the rating of this event. This event is rated by the defence-in-depth criterion. There are two "expected" initiators, which are "loss of feedwater" and "reactor scram" with "full" safety functions, and box A1 on Table III.2 gives the basic rating of level 0, as provisional rating.
Event date: Tue, 29-09-1992