Headquarters Daily Report NOVEMBER 14, 1997 *************************************************************************** REPORT NEGATIVE NO INPUT ATTACHED INPUT RECEIVED RECEIVED HEADQUARTERS û REGION I û REGION II û REGION III û REGION IV û PRIORITY ATTENTION REQUIRED MORNING REPORT - REGION IV NOV. 14, 1997 Licensee/Facility: Notification: Houston Lighting & Power Co. MR Number: 4-97-0087 South Texas 1 Date: 11/14/97 Wadsworth,Texas RESIDENT INSPECTORS Dockets: 50-498 PWR/W-4-LP Subject: UNIT 1 REACTOR TRIP Reportable Event Number: 33245 Discussion: On November 10, 1997, at 5:01 PM (CST), the overspeed protection circuitry for the Unit 1 main turbine/generator actuated, and then cleared. It actuated and cleared a second time, and eight seconds later an automatic reactor trip occurred. All control rods fully inserted into the core, and all plant systems functioned as designed. The overspeed protection circuitry signal caused the turbine governor valves and intercept valves to close momentarily and then open. Because the main turbine governor valves open significantly faster than the intercept valves, main steam pressure began to increase in the moisture separator reheaters and the relief valves lifted. The opening and closing of the governor valves resulted in a secondary transient that caused steam generator levels to oscillate. The oscillating steam generator levels resulted in oscillations in the feed flow and resulted in deviations between steam and feed flows. The deviations were reflected in the primary system as an increase in average reactor coolant temperature and control rods began to automatically insert into the reactor core to compensate for the increased temperature. Approximately ten seconds after the initial overspeed protection circuitry signal occurred an over-temperature/delta-temperature actuation in the reactor protection system actuated as a result of the secondary and primary plant transients, causing the reactor to trip. The licensee identified the cause of the inadvertent overspeed protection circuitry signal as a failure of a TRIAC card (diode type relay) in the main turbine overspeed protection circuitry. In 1976, Westinghouse recommended replacement of these TRIAC cards with mercury type relays. The licensee accomplished a modification to do this, however, they failed to remove the TRIAC cards which remained installed in parallel with the mercury relays. Unit 1 was restarted on November 12, but power ascension is on hold pending repairs to leaking moisture separator reheater relief valves. Regional Action: The resident inspectors responded to the site and were in the control room during plant stabilization and initial investigation of the cause of the reactor trip. Contact: Joseph I. Tapia (817)860-8243 Ronald A. Kopriva (817)860-8104 _