ML19317F080

AO-287/74-04:on 741007,flow Instrumentation Indicated RCS Flow Imbalance.Cause Unknown.Reactor Operations at Full Flow Limited to Sys Flows Not Exceeding Core Life Valve & Restricted to Power Levels Less than 80% of Full Power

ML19317F080
Person / Time
Site:	Oconee
Issue date:	10/22/1974
From:	DUKE POWER CO.
To:
Shared Package
ML19317F070	List: ML19317F069 ML19317F080
References
NUDOCS 8001080770
Download: ML19317F080 (2)

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Duke Power Company

• - Oconee Unit 3 Report No: A0-287/74-4 Report Date: October 22, 1974 Occurrence Date: October 7, 1974 Facility: Oconee Nuclear Station, Unit 3 Identification of Occurrence: Excess Reactor Coolant-System flow indicated by flow instrumentation Conditions Prior to Occurrence: Reactor at hot shutdown Description of Occurrence:

At approximately 1100 on October 7, 1974, a Reactor Coolant System flow imbalance of approximately 2.6 percent between the A and B loops was noticed on the control room flow indicators and the plant computer. The total RC System flow measured by the AP instrumentation was approximately 111 percent of design flow. The flow imbalance then began to decrease and finally became nearly zero after about 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />; however, the indicated total system flow remained relatively high (between 110 and 112 percent of design value). A calculation of the system flow, using Unit 3 perfor=ance data indicated that the system flow began to increase at 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br /> on September 14, 1974. At this time the reactor was at hot shutdown condition and the RC flow was 110.14 percent of design flow.

Designation of Apparent Cause of Occurrence:

At first, it was thought that the high flow and flow imbalance indications were due to instrumentation problems, and an investigation was initiated. It was found that (1) calibration of the instrumentation had been verified, (2) there were no leaks on the instrumentation impulse lines, (3) the instrument trans-mitters were zeroing properly when the reactor coolant pumps were stopped, and (4) the instrument lines had been blown down. Consequently, it was concluded that instrumentation was not the problem.

The Reactor Coolant System flow was measured with reduced voltage en the reactor coolant pump buses to determine whether changing taps on the reactor coolant pumps power supply transformers had any affect on reactor coolant flow. This reduction in voltage did not produce any significant change in the system flow.

On October 10, 1974, a program of calculating the core bypass flow was developed to determine whether the indicated excess flow was due to ,an open internal vent valve. The core bypass flow was calculated by measuring the loop and core AT's at power levels of 15,20,25,30 and 40 percent power. The calculated core bypass flow was within design values, and therefore the possibility of an open vent-valve was ruled out.

There is in process now a comprehensive procedure for determining whether the indicated ~ excess flow is real. . The program consists of (1) verifying loop AP transmitter readings by using a different type transmitter (2) calculating EC primary flow by obtaining precide heat balance at 40 and 75 percent power, and (3) monitoring loose parts monitor and neutron noise analysis instrumentation for 8001080 7 7 O