Laka Foundation
INES-event
INES 1

Non-closure of primary steam isolating valves on auto during total loss of power incident

TAPS unit no. 1 was operating at 156 MWe. Unit no. 2 was under refuelling outage. At 11.09 hrs, the grid frequency crashed and generator got locked out due low frequency. One diesel generator closed its breaker with 5 seconds. Another diesel generator started on auto but is frequency as shown in control room were 47 Hz and voltage 250 V respectively (frequency and voltage were normal as seen locally). it could be synchronised after 25 minutes. The third diesel generator was under maintenance. During the incident primary steam isolation valves INSO 3A and 4A were found to have not gone close as required during the total loss of power. The control room operators in an effort to close the valves gave open and close signals with the respective valve control switches. Also the air supply to the valves was closed. With these actions both the primary steam isolation valves went close. The event is rated at level 1 on the INES.
Justification for rating:
Loss of off-site power is an expected event. During the incident safety function, viz. cooling the radioactive material and controlling the reactivity were fully available. The steam line isolation valves did not close automatically but could be closed in short time by manual action from the control room. Since non-closure of PSIV's automatically on total loss of power constituted violation of station Technical Specifications, the event is rated at level 1 on INES.

Location: TARAPUR-1
Event date: Fri, 10-11-1995
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