ML20056G286
| ML20056G286 | |
| Person / Time | |
|---|---|
| Site: | Salem, Pilgrim, Vermont Yankee, Yankee Rowe |
| Issue date: | 08/16/1993 |
| From: | Murley T Office of Nuclear Reactor Regulation |
| To: | Murray T MASSACHUSETTS, COMMONWEALTH OF |
| References | |
| IEIN-91-083, IEIN-91-83, NUDOCS 9309020362 | |
| Download: ML20056G286 (9) | |
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WASHINGTON, D.C. 20555-0001 August 16, 1993 Senator Therese Murray Massachusetts Senate State House Boston, Massachusetts 02133-1053
Dear Senator Hurray:
I am responding to your letter of June 25, 1993, in which you expressed concern about the cracks on the disks of low pressure turbines (LPT) at the Pilgrim Nuclear Power Station. With respect to the turbine rotor cracks, the NRC staff has completed an assessment of information submitted by Boston Edison, including GE test data results and analysis by Structural Integrity Associates, Inc., of the 7th stage turbine wheel, and concludes that there is no safety concern associated with normal low pressure turbine operation during l
the current fuel cycle, which is scheduled to end in April 1995.
Boston Edison has informed the NRC that it will be replacing both low pressure turbine rotors at the end of the current fuel cycle. A copy of the enclosed staff assessment (Enclosure 1) will be placed in the Local Public Document Room:
Plymouth Public Library,11 North Street, Plymouth, Massachusetts 02360.
You also mentioned turbine issues at the Yankee Rowe and Salem nuclear plants.
l It is important for you to recognize that the cracks in the disks of the Pilgrim turbines are not directly related to the events at those plants for the following reasons:
1.
The LPT at Pilgrim is manufactured by GE, whereas the turbines at Yankee Rowe and Salem are manufactured by Westinghouse. The parameters that affect the initiation and growth of cracks in the turbine disks, such as t
disk material, operating conditions, disk design, and fabrication, are different between the two turbine manufacturers. The Yankee Rowe turbine is an older turbine, which may include material that is more susceptible to stress corrosion cracking than the material in the Pilgrim turbine.
2.
The unique design of the turbine at Yankee Rowe prevented the licensee from making accurate volumetric examinations of the turbine disk.
Therefore, the licensee had never inspected the turbine since it began operation in the 1960s.
The deep cracks in the turbine disk caused the disk failure in 1980. However, the failed turbine disk was contained inside a casing, and the Yankee Rowe plant was shut down safely without i
affecting the primary system. On the other hand, Boston Edison inspected the two LPTs at Pilgrim, following the turbine manufacturer's recommendation. During each refueling outage, Boston Edison inspects the cracks in the Pilgrim disks. To date, these cracks have been much shorter than those found at Yankee Rowe.
3.
In 1991, the turbine event at Salem Unit 2 resulted from the inadvertent overspeed of the turbine and not from cracks in turbine disks.
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1 Senator Therese Mur*ay August 16, 1993 operators were conducting routine turbine tests and inadvertently caused the turbine to overspeed. The turbine disks did not fail, only the blades. The fragments of blades penetrated the casing but did not hit the reactor building or any safety systems. The plant was shut down safely, and the primary cooling system was not affected.
4.
Upon determining that the failure of the overspeed protection system at Salem was unique to the specific model of Westinghouse turbines, the NRC issued Information Notice 91-83, " Solenoid-0perated Valve Failure Resulted in Turbine Overspeed." The NRC also determined that the overspeed protection circuitry for GE turbines is not susceptible to the problem that occurred at Salem.
The staff believes that the turbine failures at Yankee Rowa and Salem do not indicate the need for concern about a failure of the LPT at Pilgrim.
If you need any additional information, please contact the Project Manager, Mr. Ronald B. Eaton, at (301) 504-3041 or by letter to the above address.
Sincerely, Thomas E. Murley, Director Office of Nuclear Reactor Regulation
Enclosure:
Pilgrim Turbine Evaluation w/2 attachtnents
Senator Therese Murray August 16, 1993 operators were conducting routine turbine tests and inadvertently caused the turbine to overspeed. The turbine disks did not fail, only the bl ades. The fragments of blades penetrated the casing but did not hit the reactor building or any safety systems. The plant was shut down safely, and the primary cooling system was not affected.
4.
Upon determining that the failure of the overspeed protection system at Salem was unique to the specific model of Westinghouse turbines, the NRC issued Information Notice 91-83, " Solenoid-0perated Valve Failure Resulted in Turbine Overspeed." The NRC also determined that the overspeed protection circuitry for GE turbines is not susceptible to the problem that occurred at Salem.
3 The staff believes that the turbine failures at Yankee Rowe and Salem do not indicate the need for concern about a failure of the LPT at Pilgrim.
If you need any additional information, please contact the Project Manager, Mr. Ronald B. Eaton, at (301) 504-3041 or by letter to the above address.
Sincerely, Original signed by:
Thomas E. Murley, Director Office of Nuclear Reactor Regulation
Enclosure:
Pilgrim Turbine Evaluation w/2 attachments i
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PILGRIM UNIT 1: ASSESSMENT OF LOW PRESSURE TURBINE ANALYSIS Le ng the refueling outage in April 1993, General Electric (GE) inspected the rotor in low pressure turbine "A" (LPA) at Pilgrim Unit I and found flaw indications in disks 4, 5, 6, and 7.
GE recomended that the licensee either remove the seventh stage disk on the generator side (disk 7GA) or warm the LPA rotor before starting the turbine. The licensee later retained Structural Integrity Associates, Inc. (SIA) to evaluate flaw indications in disk 7GA.
On May 12, 1993, the licensee submitted the SIA analysis (Reference 1) to the NRC project manager, who requested that the NRC Materials and Chemical Engineering Branch (EMCB) review the SIA analysis to determine:
(1) any gross error in the SIA analysis and (2) any effect on plant safety.
Pilgrim Unit I has two low pressure turbines, LPA and LPB. with shrunk-on disks.
The flaw indications of the 7GA disk are located in both the hub and web. Although the ft,arth and fifth stage disks have more and larger flaws than the 7GA disk has, GE determined that the 7GA disk is the limiting disk based on operating conditions, the fracture toughness of the disk, and the consequences of a disk failure.
t SIA performed parametric studies to determine effects of the fracture appearance transition temperature (FATT), fracture toughness variability, pre-warming, crack growth rate, and stress intensity factors. The EMCB staff compared key parameters used in both the GE and SIA analyses to our estimates (see Attachment 1).
Parameters used in the GE analysis were extracted from the SIA analysis because GE's analysis was not available at the time of this assessment.
For the 7GA disk, GE reported one indication of 3.556 m [0.14 in) in the hub and an indication in the web which GE could not accurately size.
For that indication, GE assumed a crack size of 6.35 m [0.25 in] based on flaw indications from other power plants' inspection data and laboratory data. The staff believes that the initial crack size of 6.35 m (0.25 in] is conservative but could not quantify the uncertainty associated with the assumed size.
GE used a fracture mechanics model of an edge crack in an infinite plate having constant loading.
GE's model is conservative because it is more compliant than the actual geometry, whic,does not consider the radial decreaseh is a radial keyway. Moreover, its constant loading in hoop stress with increasing distance from the bore. SIA's model is a hole i
in an infinite plate with attenuated loading along the crack. The staff assumed a model of a thick wall cylinder with attenuated loading.
GE used a crack growth rate of 1.52 m [0.06] inch each year, which was the median value from a statistical study correlating the average crack growth rate with the wheel operating temperature from turbine inspection data of both BWR and PWR plants. SIA used 0.416 m [0.0164 in), 0.51 m [0.02 in), and 1.52 mm [0.06 in] each year in its studies. The staff calculated a crack
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growth rate of 0.51 m [0.02 inch] each year from previous inspection data of the LPA rotor. The staff believes that the actual crack growth rate may be between 0.51m [0.02 in) and 1.52 m [0.06 in) each year. However, GE's data indicate the upper bound growth rate (2 standard deviations) at an operating temperature of 78 *C [172 *F] could be as high as 2.03 mm [0.08 in] each year.
The critical stress intensity (K ) is an indicator of fracture toughness of ic the disk material. The lower the K used in the fracture mechanics analysis themoreconservativetheresultswUlbe. GE used a lower bound value of 115 MPalm [105 ksi/in] which was taken from the graph of critical stress intensity vs. excess temperature (test temperature - FATT). The staff finds that the value of 115 MPalm [105 ksi/in] is conservative.
GE and SIA calculated the critical crack sizes (depths) of 8.64 m [0.34 in) and 13.72 mm [0.M in), respectively. SIA conservatively assumed that the crack length is the length of the keyway bore. SIA indicated that if the crack aspect ratio it known, the critical rack size may be larger than 13.72 mm (0.54 in). SIA's calculation resm.- '
a critical crack size of about 11.43 m [0.45 in] for the thick wall c,
- er model.
Using the above parameters, the staff estimated a factor of safety for flaw size ranging from 1.21 to 3.6 based on the ratio between the crack length at end of the current fuel cycle in April 1995 to that of the critical crack size of the cylinder model (see attachment). The factor of safety for stress intensity (K ranges from 1.1 to 1.89 which was estimated by taking square root of the,)afety factor for flaw size.
s The NRC desires that the turbine disk failure probability be IE-5 each year or lower for an unfavorably orientated turbine. GE's analysis is based on a turbine disk failure probability of IE-5 failure per year. SIA did not perform a probabilistic fracture mechanics analysis.
Using engineering judgment, the staff estimated that the turbine disk failure probability for the LPA turbine is between IL-5 and IE-4 per year. The NRC would permit a turbine in this condition to remain in service until the next scheduled outage, at which time the licensee should ensure they meet the turbine disk failure probability to the IE-5 each year criterion (Attachment 2, Ref. 2).
Upon assessing the information available, the staff found no safety concern for normal operation of the LPA turbine to the end of the current fuel cycle, although the SIA analysis is less conservative than the GE analysis. The staff intends to perform a confirmatory review of the GE analysis and its methodology.
The Boston Edison Company has informed the NRC that it will be replacing both low pressure turbines during the next refueling outage, which is expected to be April 1995.
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ATTACHMENT 1 PILGRIM TURBINE EVALUATION Initial Crack growth KIC lower bound Critical Time to crack (m/yr[in/yr])
(MPadm[ksidin])
crack depth failure size (m[in])
(years)
Analysis (m [in]*)
GE 6.35[0.25]
1.52[0.06],
115[105]
8.64[0.34]
1.5 SIA 6.35[0.25]
1.52[0.06]
115[105) 13.72[0.54]
4.8 0.51[0.02]
14.5 i
NRC 6.35[0.25]
1.52[0.06]
115[105]
11.43[0.45]
4 0.51[0.02]
12
- Actual measured sizes range from 3.05 m [0.12in] to 3.56 m (0.141n]
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APPLIED KI MODEL
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FACTORS OF SAFETY ON FLAW SIZE / STRESS INTENSITY FACTOR (BASED ON NRC ASSUMPTIONS)
Crack growth Factor of Factor of rate per year safety at safety at m [in]
startup for flaw size normal operation for i
(at 24*C [75 *F) flaw size (at 78*C [172 *F])
1.52 [0.06]
1.21 2.82 0.51 [0.02]
1.55 3.60 l
Crack growth Factor of Factor of rate per year safety at startup safety at normal operation m [in]
for stress intensity for stress intensity factor factor (at 24 *C [75 *F])
(at 78 *C [172 *F])
l 3.52 [0.06]
1.10 1.68
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0.51 [0.02) 1.24 1.89 l
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ATTACHMENT 2
References:
1.
May 12, 1993, letter from D. Rosario and P. Riccardella of Structural Integrity Associates to J. Gerety of Boston Edison,
Subject:
Evaluation of the Pilgrim thit 1 Low Pressure Turbine Rotor 7th Stage Shrunk-on Disk.
2.
NUREG-1048, Safety Evaluation Report related to the Operation of Hope Creek Generating Station, Supplement No. 6, July 1986.
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DUE: 07/14/93 EDO CONTROL: 0009077 DOC DT: 06/2S/93 FINAL REPLY:
SCn. Therese Murray H20sachusetts State Senate TO:
Jcmes Joyner, RI FOR SIGNATURE OF:
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Murley DESC:
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CONCERNS RE CRACKED TURBINE AT PILGRIM NUCLEAR Taylor PLANT Spiezek Thompson I
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Ygt 4617i722 1330 mtATHCa*E PuesC SArgTv June 25, 1993 James H. Joyner, Chief Nuclear Regulatory Commission Region 1 475 Allendale Road King of Prussia, PA 19406-1415
Dear Mr. Joyner:
I am alarmed at the serious situation that exists at the Pilgrim Nuclear Plant, in Plymouth, Massachusetts.
Recent reports by the manufacturers state that there is a cracked turbine in the non-nuclear part of the power plant.
It is imperative that the necessary repairs take place immediately, in order to avoid a possible accident.
To say that General Electric's report is too pessimistic, considering the heavily populated location of the plant, is completely irresponsible, and shows no concern for the public at large.
The NRC itself hao acknowledged that the risk of turbine breakdown in the next year is higher than their technical guidelinen recommend.
The personal and environmental consequences are far too great to ignore f.uch warnings.
It is essential that the plant be shut down so that the necessary repairs can take place.
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both the The NRC cannot be overly cautious given the facts.
At fnow-closed Yankee Rowe reactor, and a Salem HJ plant, cracks caused turbine failures and reactor damage.
It seems ill i
advised to simply wait for scheduled maintenance shutdown, when a possible accident could be avoided by making necessary repairs now.
I do not think this situation can be assessed by a simple cost benefit analysis.
It seems clear to me that while there is some doubt about the possibility or extent of armage due to a breaking apart of the turbine, there can no doubt about the results of a break in the reactor wall.
Please act in a ra5ponsible manner, to O h n n n vr.
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Pugete( SaetTv June 25, 1993 J a rue s H. Joyncr, Chief Nuclear Regulatory Commission Region 1 475 Allendale Road King of Prussia, PA 19406-1415
Dear Mr. Joyner:
I am alarmed at the serious situation that exists at the Pilgrin Nuc1 car Plant, in Plymouth, Massachusetts.
Recent reports by the manufacturers state that there in a cracked turbine in the non-nuclear part or the pove ** plant.
It is imperative that the necessary repairs tara v'
immediately, in order to avoid a possible accident.
To say that General Electric's report is too pessimistic, considering the heavily populated location of the plant, is em:r.m completely irresponsible, and shows no concern for the public at large.
The NRC itself has acknowledged chat the risk of tur bine breakdown in the next year is highe2 than their technical guidelines recommend. The personal and environmental consequences are far too great to ignore such warnings.
It is essential that the plant be shut down so that the necessary repairs can take place.
The NHC cannot be everly cautious given the facts.
At both the now-closed Yankee Rowe reactor, and a Salem NJ plant, similar cracks caused turbine failures and reactor damage.
It seems ill j
advised to simply wait for scheduled maintenance shutdown, when a j
possible accicent could be avoided by making necessary repairs now.
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1 Simple cost 3 do not think th1S situation Can be DSSessed by a benefit analysis.
It seems clear to me that while there is sope doubt about the possibility or extent of damage due to a breaking apart of the turbine, there can no doubt about the results of a break in the reactor waII.
Please act in a responsible manner, to protect the citizens of Massachusetts.
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