ML19347F536

Testimony of DG Tees Re Doherty Contention 47,turbine Missiles.Related Correspondence

ML19347F536
Person / Time
Site:	Allens Creek File:Houston Lighting and Power Company icon.png
Issue date:	05/11/1981
From:	Tees D HOUSTON LIGHTING & POWER CO.
To:
Shared Package
ML19347F516	List: ML19347F515 ML19347F520 ML19347F522 ML19347F524 ML19347F527 ML19347F529 ML19347F530 ML19347F531 ML19347F532 ML19347F534 ML19347F535 ML19347F536 ML19347F537 ML19347F539
References
NUDOCS 8105190560
Download: ML19347F536 (7)

Text

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NUCLEAR REGULATORY COMMISSION om ,

BEFORE THE ATOMIC SAFETY AND LICENSING BOARD f/ $@& W. 8 3  % s N

4 In the Matter of S 5

5 HOUSTON LIGHTING & POWER COMPTSY S Docket No. 50-466 S

6 (Allens Creek Nuclear Generating S Station, Unit 1) S 7

8 DIRECT TESTIMONY OF DAVID G. TEES ON BEHALF OF HOUSTON LIGHTING & POWER COMPANY 9 REGARDING DOHERTY CONTENTION 47/ TURBINE MISSLES 10 Q. Please state your name and address, 11 A. My name is David G. Tees and my business address 12 is 611 Walker, Houston, Texas.

13 Q. By whom are you employed and in what capacity?

14 A. I am employed by Houston Lighting & Power Company 15 as Manager of Central Mechanical Maintensnee in the Energy 16 Production Department.

17 Q. How long have you been employed by Houston Light-18 ing & Power Company?

19 A. Frcr.: June, 1966 through September, 1972 and Jan-20 uary, 1974 to the present.

21 Q. Please give a brief description of your educa-22 tional background and experience.

23 A. I received a Bachelor of Science Degree in Mechan-24 ical Engineering from Texas A&M University.in May, 1966 and 810519o @

1 a Master of Engineering Degree in Mechanical Engineering.

2 from Texas A&M University in December, 1973. I served as 3 supervisor of mechanical Maintenance Personnel performing 4 maintenance on boilers, turbines, and auxiliary equipment 5 until July, 1977. Since July, 1977 I have had responsibility 6 for scheduling and implementing all maintenance on HL&P's 7 power plants. Since 1968 I have been responsible for all of 8 HL&P's steam turbine internal inspection scheduling, includ-9 ing turbine reblading, inspections procedures, and field 10 balancing. this capacity, I am familiar with the 11 different varieties of turbine designs and the maintenance 12 requirements thereof.

13 Q. Have you read Doherty contention 47 regarding 14 turbine missics?

15 A. Yes.

16 Q. Are you familiar with the problem of turbine 17 cracking alluded to in the contention?

18 A. Yes. To the best of my knowledge, turbine wheel 19 failures have occurred in turbines manufactured from steels 20 referred to as chrome-molydenum (Cr-Mo) or an early version 21 of nickle-molydenum-vanadium (Ni-Mo-V) alloys. These wheels 22 were on older units which had a much smaller critical crack 23 size because they did not use the newer steels such as 24 Nickel-Chromium-Molydenum-Vanadium (NiCrMoV) , which is used 1 in the Allens Creek turbines.

2 Q. What do you mean by critical crack size?

3 A. Critical crack size refers to the size flaw that 4 must exist in the wheel material for a failure to occur at 5 operating conditions.

6 Q. What are the mechanisms for initiating and/or 7 growing cracks in nuclear wheels in service?

8 A. The two possible mechanisms are stress cycling and 9 stress corrosion cracking. The likelihood of initiating and/

10 or growing a crack due to stress cycling associated with 11 starts, stops, and load change is remote. Variations in 12 scress amplitude resulting from operating transients are too 13 low to produce crack initiation or significsnt crack growth.

14 The manner in which wheels are forged essentially precludes 15 the possibility of producing an internal crack-like defect 16 in the plane normal to the maximum stress (the axial-radial 17 plane). Furthermore, in additi6n to a complete visual and 18 magnetic particle inspection of all bore and external 19 surfaces, all modern nuclear wheel forgings are subjected 20 to a stringent 100% volumetric ultrasonic inspection at the time of manufacture. All modern nuclear wheels are spin 21 22 tested during manufacture at 20% overspeed, which further 23 minimizes the probability of having an undetected crack or 24 crack-like flaw with a critical size which would lead to o

1 spontaneous propagation at normal rotating speeds. Thus, 2 there is a very low probability that modern turbine wheels 3 will be placed in service with defects of sufficient size to 4 grow in service due to stress cycling. Accordingly, the 5 most probable method of in-service initiation and growth of 6 cracks is that associated with stress corrosion. This was 7 the cause of the Yankee-Mowe failure and cracking found in 8 the Zion turbine.

9 Q. Where does the problem occur in the turbine?

10 A. For the generation of the large turbine wheel 11 missles that are of concern, the most likely region for the 12 initiation and growth of stress corrosion is the bore re-13 gion. The nominal rotational and shrink stresces are maxi-14 mum at the bore. The presence of the axial keyway at this 15 location introduces a region of stress concentration. The 16 material properties are generally somewhat lower in the 17 keyway region (the critical crack size is smaller). In 18 other regions of a nuclear turbine wheel, such as the 19 Periphery, stresses are considerably lower, so that crack 20 growth rates would be slower. Critical crack sizes would be 21 much greater, so that cracks would be, detectable during 22 periodic surface inspections before reaching critical size.

23 Q. What design changes have been nade to address this 24 problem?

4 s

1 The design of the keyways used to attach the

,A.

2 wheels to the shaft is extremely important in determining 3 the likelihood of the initiation of stress corrosion cracks 4 in the bore region of th'e turbine wheel. Studies in the 5 United Kingdom, following the turbine failure at the Hinkley

$ Point "A" Power Station in 1969, showed that keyways with a 7 quasi-rectangular shape exhibited a strong resistance to 8 stress corrosion cracking, when operated in essentially the 9 same manner as the wheels with other keyway designs. No 10 keyway stress corrosion cracks were found in wheels with 11 quasi-rectangular keyways. The ACNGS turbine has the quasi-

12 rectangular keyway design.

13 0 Is it possible to design the turbines to totally 14 preclude the possibility of stress corrosion cracking?

15 A. No. That is the reason that we have a very 16 thorough testing and inspection program. In addition, the 17 turbine is aligned to reduce damage from missles in the 18 unlikely event they occur.

19 0 Would you describe the maintenance and inspection 20 procedure?

21 A. Although many design and operational techniques 22 have been used to minimize the possibility of stress corro-23 sion cracking, and even though no verified incidents of ,

24 stress corrosion cracking have occurred in GE nuclear turbine

9 1 wheels, test equipment has been developed to detect stress 2 b corrosion cracking wel) before cracks cou'd grow to the size 3 necessary to cause wheel failure. For peripheral portions 4 of the wheel, the critical crack size is very large, ap-5 proachna the axial thickness of the wheel. Through the 6  ! periodic use of surface inspection techniques, cracks on the 7 periphery would be detected before they could grow to crit-8 ical size.

9 The bore and keyway regions of the wheels cannot be 10 tested by surface or magnetic particle tea: techniques 11 without removing them from the shaft. Therefore, an ultra-12 sonic test has been developed to detect cracks in these 13 regions with the wheels in place on the shaft. Tests are 14 made by pre;ecting an ultrasonic beam at various angles to 15 the bore from the surfaces of the turbine wheel. The ultra-16 sonic beams are projected and received by transducers placed 17 on the wheel surfaces. Radial and/or axial defects will be 18 detected by reflections of '.5e Seam that occur when a defect 19 is encountered. By vary.ng the transducer positions, essen-20 tially the entire length of the wheel bore can be tested.

21 To cbtain maximum coverage, each whee,1 may receive as many 22 as 40 individual scans.

23 0 IIas the use of newer steels f acilitated the ab-24 ility to detect cracks?

1 A. Through the use of the new NiCrMoV steeis in 2 manufacturing the turbine wheels, the si;c of the flaw 3 necessary to cause wheel failure has greatly increased, 4 compared to the critical crack sizes of any of the.cheels 5 that have burst. Critical crack sizes in modern turbine 6 wheels are thus many times larger than the detectable sizes.

7 Q. Would you please summerize your test.imony?

8 A. In summary, the thorough pre-service inspections, 9 designs which result in low stresses and thus permit use of 10 lower strength materials, and the in-service inspection 11 program, all combine to produce a very low probability of 12 failurc of General Electric modern turbine wheels. Added 13 protection is achieved by alignment of the turbine to reduce 14 the probability of impacts in the unlikely event that c 15 missle ever occurs.

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