ML19318A591

Forwards Response to NRC 800515 Request for Info Re Turbine Disc Integrity

ML19318A591
Person / Time
Site:	Fort Calhoun
Issue date:	06/13/1980
From:	William Jones OMAHA PUBLIC POWER DISTRICT
To:	Clark R Office of Nuclear Reactor Regulation
References
NUDOCS 8006230422
Download: ML19318A591 (6)

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Omaha Public Power District 1623 HARNEY e OMAHA. NE5RASKA 68102 a TELEPHONE 536 4000 ARE A CODE 402 June 13, 1980 Mr. Robert A. Clark, Chief U. S. Nuclear Regulatory Comission Operating Reactors Branch No. 3 Division of Licerising Washington, D. C. 20555

Reference:

Docket No. 50-285

Dear Mr. Clark:

The Omaha Public Power District received a letter from the Comission, dated May 15, 1980, requesting that certain infonn-ation be provided relative to turbine disc integrity at the Fort Calhoun Station. In response, the attached information is pro-vided.

Sincerely, e " l/f(

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g,j..,. Jones Division Manager Production Operations WCJ/KJM/BJH:jmm Attach.

cc: LeBoeuf, Lamb, Leiby & MacRae 1333 New Hampshire Avenue, N. W.

Washington, D. C. 20036 0/

3 Subscribed and sworn to before me j//

this /f-a day of -7v,vt , 1980. ADDI Lht 6 3 M/. TOSS / /

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. c SITE SPECIFIC GENERAL OUESTIONS Question 1: Provide the following information for each LP turbine:

A. Turbine type.

B. Number of hours of operation for each LP turbine at time of last turbine inspection or if not inspected, postulated to turbine inspection.

C. Number of turbine trips and overspeeds.

D. For each disc: .

1. Type of material including material specification.

2. Tensile properties data.

3. Toughness properties data including Fracture. Appearance Transition Temperature and Charpy upper steel energy temperature.

4. Keyway temperatures.

5. Critical crack size and basis for the calculation.

6. Calculated bore and keyway stress at operating design overspeed.

7. Calculated Klc data.

8. Minimum yield strength specified for each disc.

Response 1: A. Turbine type: General Electric,1800 rpm, tandem-compound, non-reheat unit with one high-pressure and two double-flow low-pressure cylinders and 38-inch last-stage buckets.

B. Number of hours: 44,375 C. Number of turbine trips: 60 Number of overspeeds: 7 (110%, tests only)

D. Answers to these questions involve data which are proprietary to the General Electric Company. Information was provided directly to the NRC during a meeting between General Electric and NRC re-presentatives on April 21, 1980. General Electric has informed the District that this information was acceptable to the Commis-sion as a reply to this question.

Question 2: . Provide details of the results of any ccmpleted inservice inspection of LP turbine rotors, including areas examined, since issuance of an operating license. For each indication detected, provide details of the location of the indication, its orientation, size, and postulated Cause.

Response 2: Magnetic particle inspections were performed on the LP turbine shrunk-on wheels during the 1975 and 1980 refueling outages. No crack-like indications were revealed.

During the 1980 outage, ultrasonic-inspection, which was performed on the tangential entry dovetails on wheels 1 through 8 (governor and generator end shaft), revealed no indications. An ultrasonic tech-nique is not presently available for inspecting wheels 9 and 10, which have a pinned finger type dovetail. In addition to the ultrasonic in-spection of the tangential entry wheel dovetails, the wheel bore re-

2 L gions of all LP wheels were inspected using an ultrasonic test developed to search for radial-axial cracks in the vicinity of wheel bore and keyway surfaces. Again, no crack-like indica-tions were revealed.

Based upon the results of their inspection, General Electric  ;

have stated that the LP rotors are satisfactory for continued op-eration.

The results of the 1980 inspection were reported to the Commission ,

in a letter dated April 23, 1980, W. C. Jones to Robert W. Reid of the Nuclear Regulatory Commission.

Question 3: Provide the nominal water chemistry conditiens for each LP turbine and describe any condenser inleakages or other significant changes in water chemistry to this point in its operating life.

Response 3: CONDENSATE AND FEEDWATER SPECIFICATIONS Conductivity Intensified Cation <1.5 u mhos/cm ,,

Dissolved Oxygen <10 ppb ~

Hydr' azine 10 - 50 ppb Ammonia si ppm Silica <20 ppb Sodium <10 ppb Copper <10 ppb Iron -

<25 ppb

-pH a. Wit Copper-Alloy comp. 8.8 - 9.2 Fort Calhoun Station has a histcry of excellent chemistry control.

Condenser leaks have been few and of very short duration, and as a result, chemistry parameters hava seldom been out of specification.

Question 4: If your plant has not been inspected, describe your proposed schedule and approach to ensure that turbine cracking does not exist in your turbine.

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Response ~ 4:- Not Applicable Question 5: If your plant has been inspected and plans to return or 'has re-turned-to power with cracks or other defects, provide your pro-posed schedule for the next turbine inspection and the basis for this inspection schedule, including postulated defect growth rate.

Response 5: Not Applicable  :

Question.6: Indicate whether an analysis and evaluation regarding turbine missiles have been performed for your plant and provided to the staff. If such an analysis and evaluation has been performed and reported, please provide appropriate references to the avail-able documentation. In the event that such studies have not been made, consideration should be given to scheduling such an action.

Response 6: Such an analysis has been performed. The results are discussed l in Subsection 5.8.2 and Section 14.8 of the Fort Calhoun FSAR.

GENERIC QUESTIONS Question 1: Describe what quality control and inspection procedures are used for the disc bore and keyway areas.

Response 1: After the rough machined wheel / disc forging has been tempered, ma-terial is removed from surface locations to measure mechanical properties. The forging is then subjected to a 100% volumetric ultrasonic inspection. If the test results meet stringent accep-tance standards, the forging is released for final machining.

During final machining, attention is continually paid to the fin-ish, contour and dimensions of every surface. For instance, the keyway depth, width, location, radii, and surface finish for every wheel are checked for conformance to drawings. Quality control personnel assure that tolerances are maintained. Any deviation from accepted tolerances are reported to engineering for disposi-tion.-

.0nly coolants and lubricants approved by engineering are used in the manufacturing and assembly process. These coolants and lubri-cants have undergone extensive laboratory corrosion testing to en-

sure their ' acceptability prior to their approval for use in manu-facturing. Periodic sampling is done on all such fluids to verify that their chemistry is within acceptable limits.

4 Pesponse 1: After finish machining, each wheel is thoroughly cleaned and (Continued) given a magnietic particle inspection of all surfaces. If acceptable, the buckets are assembled and the wheel is statiscally balanced. After assembly on the shaft, each wheel is inspected and measurements are made to assure its proper location. A high speed balance is performed on the assembled rotor which is then spun to 20% overspeed. Finally, after a magnetic particle in-spection of the buckets, the rotor is cleaned to prepare for ship- .

ment.

Question 2: Provide details of the General Electric repair / replacement proce-dures for faulty discs.

Response 2: Stress corrosion cracks have not been observed to date in nuclear wheels manufactured by General Electric, and GE does not anticipate that removal or replacement of wheels will be required because of this phenomenon. The water erosion which has been observed in the keyways of wheels on several non-reheat machines is being studied intensively. General Electric currently believes that the erosion process is self-limiting and should not require the replacement of any wheels.

Question 3: What immediate and long term actions are being taken by General Electric to minimize future " water cutting" problems with turbine discs? What actions are being recommended to utilities to mini-mize " water cutting" of discs?

Response 3: No immediate actions are required to minimize water erosion because of the apparent self-limiting nature of the phenomenon. However, if future inspections sho'; an unexpected progression of the water erosion, appropriate op A ating restrictions and/or modifications will be implemented.

, Question 4: Describe fabrication and heat trratment sequence for discs, including thermal exposure during shrinking operations.

Response 4: The wheel / disc forgings are heat treated in the rough machined con-dition. The heat treatment consists of soaking at a temperature above the upper critical temperature with the time and temperature sufficient to ensure complete austenitization throughout the forging, ~

followed by a quench in cold, vigorously circ 6 ated water for a suf-ficient time to ensure complete transformation throughout the section.

The forgings are. heated uniformly to a tempering temperature below the lower critical temperature and held for a sufficient time to soften to the desired tensile range. After temparing, the forgings are still-air cooled to room temperature.

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5 j Response 4: . After final machining, the wheels (discs) are uniformly heated in

-(Continued) an electric furnace to a temperature below the embrittling range, but sufficiently high to increase the wheel diameter enough to assemble on the shaft with the required shrink fit.

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