ML20086B335

AO 50-267/74-17A,teeth Roots of Curvic Coupling & Splitter Root on Six Pelton Turbine Buckets Discovered Cracked.Caused by Apparent Incipient Failure.Helium Circulator Speed Will Be Limited & Wheels Will Be Replaced W/Forged Wheels

ML20086B335
Person / Time
Site:	Fort Saint Vrain
Issue date:	10/22/1974
From:	Swart F, Walker R PUBLIC SERVICE CO. OF COLORADO
To:	Howard E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML20086B283	List: ML20086B280 ML20086B325 ML20086B331 ML20086B335 ML20086B339
References
AO-50-267-74-17, NUDOCS 8311170168
Download: ML20086B335 (7)

Text

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October 22,197V V E.k/,, xa

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M]D' OC/gO Mr. E. Morris Howard, Director Region IV f. ~

Directorate of RegulatoryOperations i / p United States Atomic Energy Connission g <,' ,M N i

10395 West Colfax - Room 200 s?6\ : " ' ,j ^' _'

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Dear Sir:

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Please find attached Abnomal Occurrence Report No. 50-267/74/17A, Final, concerning the helitna circulator Pc1 ton whccls.

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At the appropriate time, we may wish to request Regulatory Opera-tions to certify that the plant is ready to go to Phase 2 of the Startup 4

program as defined by LCO 4.9.1 and to proceed into Startup Tests A-6 and A-8 and then into the first part of SUr B-0 to detomine the reactor core temperature coefficient of reactivity at or about 6000F utilizing the presently installed cast Inconal Pelton whccls. During these tests <

reactor power would be limited Administratively to a maxima of 2':i re-acter rated themal power. This limit would remain in effect until new forged Inconel Pc1 ton wheel assemblies are installed.

Following this series of tests, the reactor would be shut down and the cast Inconel Pelton wheels removed, and new forged Inconel Pelton

whccls installed on the helium circulators. Following installation, each

, helium circulator will be functionally tested to 7000 rpra to assure re-

actor shutdown cooling capability.

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j Risc to 100% reactor power would follow as presently planned in the "B" series Startup Tests.

!I j - If you have any questions or comments concerning these plans,

(, please contact me as soon as possibic.

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,f ( Very truly yours,

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^ PDR ADOCK 05000267

.: d " S PDR R. F. Walker, Vice President i!

Engineerine and Planning .

Electric Department .

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o o Date: October 21, 1974 ABNORMAL OCCURRENCE

. s FORT ST. VRAIN NUCLEAR CENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO P.O. BOX 361 PLATTEVILLE, COLORADO 80651 REPORT NO. 50-267/74/17 A 4

Final i -

IDENTIFICATION OF OCCURRENCE:

i Physical examination of "A" helium circulator pelton wheel, Serial No._4, ,

indicates cracking at the roots of the teeth of the curvic coupling and at

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the root of the splitter on six (6) of the tuenty (20) pelton turbine buckets. This is considered an abnormal occurrence per Item 2.1(f) of the Technical Specifications.

CONDITIONS PRIOR TO OCCURRENCE: Steady State Power Routine Shutdown ,

Hot Shutdown Routine Load Change Cold Shutdown X Other (specify)

Refueling Shutdown Helium circulator had been re-l moved to inspect failed static Routine Startup seal bellows. Examination of '

-pelton wheel _ indicated potential problems. The circulator had been operated at various speeds over_its entire speed range (up to 10,000 rpm) using both the stenn and Pelton water turbines for a total of approximately 450 hours0.00521 days <br />0.125 hours <br />7.440476e-4 weeks <br />1.71225e-4 months <br />. This total includes operating time at both the Valmont Helium Circulator Test Facility and Fort St. Vrain. No problems were observed during operation. Cracking at the pelton wheel coupling and in the buckets was found following removal of the circulator for other reasons, i

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, The major plant parancters at the time of the event were as follows:

, Power RTR, N/A MWth ELECT, N/A IMc Secondary Coolant- Pressure N/A psig Temperature N/A 'F Flow N/A #/hr.

Primary Coolant Pressure N/A psig '

Temperature N/A 'F Core Inlet N/A 'F Core Outlet Flow N/A #/hr.

"A". helium circulator had been removed from its normal position in its PCRV bottom head penetration and had been repinced by the sparc due to probices with the static seal bellows (see Abnormal Occurrence Report #50-267/74/13 dated July 11,1974 and #50-267/74/13A dated October 3,1974). Helium cir-culator components were being examined in manufacturer's shops in San Diego, California.

DESCRIPTION OF

_0CCURRENCE: ,

Removal of "A" helium circulator from the PCRV due to static seal actuating bellows prob 1 cms afforded the opportunity to examine components of the helium circulator in detail. -

-A fluorescent penetrant (Zyglo ZL-22) examination of the " nuclear" pelton whcc1 coupling area indicated uniform cracking at the root of all the coupling teeth and revealed six (6) of twenty (20) pelton turbine buckets with crscks at the root of the splitter. None of these cracks were visible to the naked cyc.

DESCRIPTION OF OCCURRENCE: x Design Unusual Service Cond.

Including Environ.

Manufacture Installation /Const. Component Failure

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Operator X Other (specify)

Procedure Apparent' incipient failure V

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' ANALYSIS OF OCCURRENCE:

A detailed metallurgical and engineering analysis of pelton whccls Scrial No.

3 and 4 in conjunction with an experimental program has resulted in the following conclusions:

FR0!! METALLURIGICAL EXAMINATIONS (a) Based on the oxidation of the fracture surfaces, it is concluded that the cracking in the curvic coupling area on S~crial No. 3 and 4 wheels was produced during operation at the Valmont Helium Circulator Test Facility.

• (b) Some limited propogation of the Curvic cracking may have occurred during operation of Scrial No. 4 whccl at Fort St. Vrain.

(c) Due to the lack of heavy oxidation of the fracture surfaces and since bucket cracking was confined to Serial No. 4 wheel which had also been operated at Fort St. Vrain, in addition to operation at Valmont, it is assumed the bucket cracking took place at Fort St. Vrain.

(d) All the cracking on both wheels appeared similar in character and was concluded to be due to high cycle fatigue.

(e) The material and physical properties of the pelton whcc1 castings obtained from the heat treatment were considered to be within specifi-cations limits for this particular casting configuration.

• Wheel Serial No. 3 was operated only at the Valmont Helium Circulator Test Facility. Wheci No. 4 was operated at both Valmont and Fort St. Vrain.

i FROM THE PREVIOUS OPERATING HIFTORY AND EXPERIMENTAL STRESS AND ANAD*TICAL ANALYSI3:

1. Incipient cracks in the Buckets j

(a) The cause of the cracks is high-speed (> 8000 rpm) water operation.

(b) At 7000 rpm, cracks will not be initiated.

, (c) At 7000 rpm, buckets with incipient cracks have essentially an l infinite life. (no crack propogation)

2. Localized cracks in the Curvic Coupling Area (a) The cracks were initiated by a vibratory stress, most probably cmanating from the steam turbine disc.

(b) Steam conditiens at Valmont (pressure, temperature, and flow) required higher than design pressure ratios across the turbine nozzles (choked flow condition) to adequately test the circulator gas side performance. It is thought these conditions were conducive to exciting the turbine disc at the amplitude sufficient to generate the damaging alternating stress component.

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(c) The total stress experience at the Curvic coupling, although sufficient to initiate cracks in the 718 pelton wheel, is within the strength capability of the wrought 422 stainless material of the steam wheel, i (d) The cracks will be confined to the immediate vicinity of the Curvic coupling and will not impair the capability of the unit

to function satisfactorily.

, SAFETY ASPECTS:

To evaluate safety aspects of the Fort St. Vrain operation with reduced rpm

] (7000 rpm vs 10,500 rpm) of the Pelton whccis, the following approach was adopted:

(1) From a list of events requiring operation of the circulators j

on Pelton wheel drives, the events imposing the most severe requirements were

, selected. (2) These events were re-analyzed assuming restricted operating i

conditions of the Pelton wheel drives. (3) The re-analysis was then evaluated i

with respect to cafety limits and AEC accident-dose guidelines. As the result l of re-analysis of the safety aspects of plant operation, the following was i

concluded: (1) The safe Shutdown Earthquake or Maximum Tornado events are in-consequential, i.e., no release of radioactive material occurs, and no damage which could increase the potcatial for release takes place. (2) The Rapid Depressurization event does not result in component or fuel damage which would increase the release of fission products beyond the icvel already reviewed by the AEC via the original FSAR analysis (10,500 rpm) uith one circulator operat-ion limited to 7000 rpm.

On the basis of the technical investigations, the following conclusions have

been reached

) (a) No new limitations are required for steam operation of the helium

circulators.

(b) The forged pelton wheels can be safely operated for postulated j cmcrgency conditions by imposing a lower limit on maximum pelton i whccl speed. (7000 rpm vs 10,500 rpm)

(c) The plant safety is not. compromised at 105% power conditions with an

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I imposed Pelten wheel speed limit of'7000~ rpm.

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• These conclusions are based on specific facts and evidence obtained from (1) review of the operating history, -(2) metallurigical investigations and

, analyses, (3) experimental results, (4) fracture mechanics analysis, and (5) plant safety analyces associated with reduced pelton whcci maximum speed. ,

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• See attached report GA-A13175 for details.

CORRECTIVE y ACTION: '

• i i We are taking the following corrective actions:

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1. Limiting the1 helium circulator speed to 7000' rpm when operating on 4 the pelton turbine..

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2. Replacing the present cast inconel 718 pelton wheels with forged inconel 718 pelton wheels.

3. Including in our Technical Specifications an in-service surveillance program for the helium circulator pelton wheels. This is to be discussed and finalized with AEC-DOL.

Reducing the helium circulator operating speed to 7000 rpm, when driven by.

the Pelton wheel, will assure a reduction of stress Icvels in the buckets that will not produce or propogate cracks. Also, the use of forged inconel will increase the bucket resistance to fatigue cracking.

The use of forged inconel 718 to replace the east inconci 718 material will markedly increase the strength of the pelton whccl assembly. The metalluricical and mechanical properties of the forged inconel 718 are more similar to the forged 422 stainless material of the steam turbine whcci which has to date in-dicated no signs of stress at the curvic coupling.

A review of the postulated accident conditions indicates the most severe condit- j fons requiring helium circulator operation on pelton turbines are associated with the Rapid Depressurization accident. Reanalysis of this accident indicates that although peak temperatures during the transient are higher with the circulator operating at 7000 rpm as compared to those if the circulator is operating at 10,500 rpm, no ccmponent safety limits are exceeded and the risk to the health and safety of the public is not increased. *

• For inore details, see report GA-A13175.

i FAILURE DATA /SIMU.AR REPORTED OCCURRE"CES: I 1 l

Sec reports

October, 1973 l To: Dr. Donald Knuth Directorate of Regulatory Operations Deficiency Report, '

Fort St. Vrain Uclium Circulators August, 1972 To: Mr. F.E. Krucsi Directorate of Regulatory Operations j Deficiency Report j Fort St. Vrain Helium Circulators

_PROCRAhMATIC IMPACT: '

10 - 12 week delay.

_ CODE IMP!.CT:

None / ,

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s /h _ p ** DW '

rederic E. Svart Superintendent Nuclear Production

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