ML19316A243

Deficiency Rept Re Design &/Or Mfg of Undervoltage Trip Assemblies for Control Rod Drive Trip Breakers.Failures Noted 720913 & 730123.Vendors Informed

ML19316A243
Person / Time
Site:	Oconee
Issue date:	02/23/1973
From:	DUKE POWER CO.
To:
Shared Package
ML19316A236	List: ML19316A241 ML19316A243
References
NUDOCS 7912030359
Download: ML19316A243 (3)

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i OCONEE UNIT 1 CRD BREAKER UNDERVOLTAGE TRIP ASSEMBLY DEFICIENCY REPORT Introduction t

Recent incidents at Oconee Unit 1 indicate two areas of deficiency in the design and/or manufacture of the undervoltage trip assemblies for the Control Rod Drive (CRD) Trip Breakers. These areas are (1) excessive heating in the undervoltage coil, and (2) insufficient tension in the trip assembly spring to open the breaker. This report sum =arizes the deficiencies found, cor-rective action taken, and an analysis of the safety i=plications.

Description of Deficiency i

l The Control Rod Drive Systen is described in the Oconee FSAR, Section 7.2 and Figures 7-6 and 7-7. A reactor trip occurs whenever power is re=oved from the rod drive cotors. The main power supply to the system is inter-rupted by opening the two AC breakers (A and 3 in Figure 7-6). Trip redundancy is assured by tripping the four DC holding power supply breakers (C and D in Figure 7-6), and.by opening the gate drive power supply con-tactors (E and F in Figure 7-6). This report concerns the two AC trip breakers, General Electric Model 224A6147 - 200 KE, A62 1, 600 a=ps, 600 volts, and the four DC trip breakers, General Electric Model 224A 3510 -

346 KV, A62 2, 225 amps, 600 volts.

On Septe=ber 13, 1972, the undervoltage coil of a DC breaker failed due to excessive heating. The coil and internal wiring near the coil were charred.

The coil failure resulted in tripping of the 20 a:p circuit breaker in the associated Reactor Protective Syste: Cabinet.

Af ter the final phase of Hot Functional Testing, on January 16, 1973, the undervoltage coil of an AC breaker failed due to encessive heating.

On Januarg 23, 1973, while =casuring holding and in-rush currents through the undervoltage coils to deter =ine cause of failu;e and proper corrective action, an instrument tcchnician noticed one DC breaker failed to trip when the undervoltage coil was de-energized.

Subsequent investigation by Oconce instrument technicians and a Cencral Electric

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i' o field engineer revealed that 18 of the 19 undervoltage asse=blies (six asse=blies for each of the three units at Oconce plus a spare for Unit 1) had coils with resistances of 12-15 ch=s. Coil resistance should be 24-26 ch=s to prevent excessive heating. Also, the tension in the undervoltage trip asse=bly spring of the DC breaker that had failed to open was approxi=ately 352 gra=s, as determined by hanging weights. The average setting for the other 18 breakers was 450 gra=s. By gradually decreasing the spring tension, it was found that the breakers consistently failed to trip at a tension of 202 gra=s.

Analysis and Corrective Action Af ter the undervoltage coil failure on Septe=ber 13, 1972, the breaker was returned to the General Electric repair shop, Savannah, Georgia, for repairs.

Since the coil ftilure resulted in the tripping of a 20 a=p circuit breaker located in a Rea'ctor Protective System Cabinet, the interconnecting wiring j f

and manual trip switch, which is in series with the 20 amp breaker and the undervoltage coil, were inspected. No da= age to the wiring or switch was found. The Babcock & Wilcox site representative was requested to evaluate existing circuit protection and also reco==end that each undervoltage coil be separately fused to protect against possible da=cge that =ight be caused by future coil failure. On Dece=ber 15, 1972, B&W authorized the addition of overcurrent protection in the trip channels for the undervoltage coils.

When the DC breaker failed to open on January 23, 1973, 35W was notified (

and the assistance of a General Electric Service Engineer was requested.

As a result of the on-site inspection by the General Electric field engineer, replace =ent undervoltage coil asse=blies will be provided by General Electric.

Six new undervoltage coil asse=blies for Unit I have already been installed i and functionally tested. The replace =ents have the sa=a part nu=bers as the original assemblies: Asse=bly Number 269C82G2 and Coil Nu=ber 75031G26.

Resistances in the replace =ent coils for Unit 1 were =casured to be 24-25 ohns. The trip asse=bly spring tensions were set betweet. 420-490 gra=s to diva a dropout voltage of 60-65 volts. After setting the tension, the adjustment was safety wired to prevent shifting of the setpoint.

Safety Inolications .

In the event of the failure of an undervoltage coil due to an overcurrent

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condition and excessive heating, the Reactor Protective System and the Control i Rod Drive System will continue to perform all protective and control functions.

An overcurrent condition will de-energize the undervoltage coil by:

(1) tripping the 20 a=p control circuit breaker:

(2) opening the undervoltage coil by excessive heating; (3) opening the associated 5 amp fuse installed as a result of this incident.

The de-energi::ed undervoltage coil will trip the associated breaker. System redundancy prevents the failure of a single breaker to cause a single rod or rod group to drop. The Trip Fault Detector will alert the operator of the breaker trip by clarm and indicator. [FSAR 7.2.2.2.1 and FSAR 7.2.2.3.4(5))

The failure of a single CRD breaker to open on a Reactor Protective System trip signal would not prevent the insertion of the control red assenblies.

In the event that one of the main power AC breakers failed to open, independent trip signals would open the DC power supply breakers for the four safety rod assenblics and interrupt the gating power supply necessary to hold the regulating  :

rod assemblies. A trip signal to the main power AC breakers would interrupt ,

l all CRD power and cause the insertion of all rods even though a DC holding j power supply breaker failed to trip. [FSAR 7.2.3.3.11 System redundancy and series trip devices assure that the plant can be safely l l

controlled and shut down in the event of either an undervoltage coil failure i or failure of a breaker to trip.

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