ML19317E070

Abnormal Occurrence AO-269/74-05:on 740301,reactor Protective Sys Power/Imbalance/Flow Channel Setpoint Found in Error.Caused by Flow Voltage Input to Function Generation Being Higher than Intended

ML19317E070
Person / Time
Site:	Oconee
Issue date:	03/12/1974
From:	DUKE POWER CO.
To:
Shared Package
ML19317E069	List: ML19317E068 ML19317E070
References
NUDOCS 7912120837
Download: ML19317E070 (3)

Text

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DUKE POWER COMPANY OCONEE NUCLEAR STATION Report No.: A0-269/74-5 Report Date: March 12, 1974 Occurrence Date: March I, 1974 Facility: Oconee Unit 1 identification of Occurrence: Reactor protective system power / imbalance /

flow channel calibration error Conditions Prior to Occurrence: Power operation including 100 percent full power Description of Occurrence:

During a review of field operations by the Babcock t, Wilcox Company, it was discovered that the reactor protective system power / imbalance / flow channel setpoint was in error. On March 1, 1974, B&W notified the Oconee Nuclear Station superintendent of this error and recommended that calibration pre-cedures for the power / imbalance / flow trip setpoints be based on four-pump reactor coolant flow at 100 percent full power instead of design flow.

Regulatory Operations, Region ll, was informed of this error in the reactor protective system setpoint on March 2, 1974.

On March 6, 1974, calibration procedures for the reactor protective system power / imbalance / flow channels were revised. Using these revised procedures, the gain of the summing amplifier was reset to compensate for greater than design flow, and thu:: established the correct trip setpoint.

Designation of Apparent Cause of Occurrence:

The maximum power-to-flow ratio permitted by the technical specification is 1.08 for Oconee Unit 1. The power-to-flow ratio is actually set at 1.06 to account for electronic drift and tolerances. Therefore, a trip should occur when the indicated flux is greater than or equal to 1.06 times flow, less a correction for imbalance. Figure 1 is a simplified schematic of the flow channel of the reactor protective system terminating in the power /

imbalance / flow trip bistable.

Voltages V0A and V0B are proportional to the differential pressure sensed across the flow tubes in loops A and B respectively. Voltages VIA and VlB are proportional to the actual flow, where 0 volts corresponds to O pounds per hour flow and 10 volts corresponds to 90 x 106 pounds per hour flow in each loop. The resistor networks simply divide both VIA and VlB by 2 in order q91212 0 N

that the total input to the summing amplifier (V2A + V2B) is 0 to 10 volts.

The output of the summing amplifier, V 3, depends on the setting of the gain Ky. The flux channel is calibrated for .08 volts per i percent power.

The gain factor (K]) was established with the following procedure:

1. The flux voltage for 100 percent flow that would require tripping is 1.06 times the 100 percent flux voltage of 8.48 s. (on the imbalance range where there is no correction). ,

2. Using a reference input to the summing emplifier that corresponds to the design flow rate (131.32 x 106 lb/hr. 7.29 v.) the gain factor (K]) is calculated that will yield the tripping voltage of 8.48 v. Thus, K1 =

1.16.

i The actual system flow rate is 108.6 percent of the design flow rate as documented by the Oconee Unit i Startup Report, November 16, 1973 Thus, the flow voltage iriput to the function generation is higher than intended and results in power / imbalance setpoints that are artificially high.

1 i Analysis of Occurrence:

1 The setpoints resulting from the incorrect flow calibration procedure would be less restrictive than the technical specification setpoints; however, the technical specifications are based on the design flow rate (131 32 x 10 6 lb/hr) with no credit being taken for the increased flow in the system.

Thus, limits would not necessarily have been exceeded even if a trip would have occurred at a setpoint higher than allowed by the technical specifi-cation.

Additionally, the LOCA power / imbalance alarm is more restrictive than the technical specification reactor protective system power / imbalance setpoint.

The unit has operated within this alarm region, and therefore, no reactor protective system setpoints were exceeded. It is concluded that this oc-currence did not affect the safe operation of the unit or the health and i safety of the public.

, Corrective Action:

l The field calibration procedure for the summing amplifier has been changed such that the gain factor (KI) is set by utilizing actual system flow.

Thus, the normal flow voltage for four pump operation input to the function generator and subsequently to the bistable is the required 8.48 volts. This is the proper base from which the imbalance correction should be made, and yields setpoints which are conservative and verified by analysis.

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