ML20058D248
| ML20058D248 | |
| Person / Time | |
|---|---|
| Site: | Grand Gulf  |
| Issue date: | 11/23/1993 |
| From: | Hutchinson C ENTERGY OPERATIONS, INC. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 93-005, 93-5, GNRO-93-00143, GNRO-93-143, NUDOCS 9312030108 | |
| Download: ML20058D248 (4) | |
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Po t Gwn MS 3940 M ini 437 2KO C. R. Hutchinson November 23, 1993 ggy*
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r U.S. Nuclear Regulatory Commission Mail Station P1-137 Washington, D.C. 20555 Attention:
' Document Control Desk
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1
Subject:
Grand Gulf Nuclear Station Unit 1 l
Docket No. 50-416 License No. NPF-29 Special Report 93-005 l
Recordable Indications identified During Turbine Disc Inspection l
GNRO-93/00143 i
i Gentlemen:
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GGNS Operating License Condition 2.C.(26) requires that the keyways and bores of f
the low pressure turbine discs be ultrasonically inspected. The inspections are performed in order to check for cracks during a 50,000 hour0 days <br />0 hours <br />0 weeks <br />0 months <br /> operational cycle. -The license condition further requires that all unacceptable indications and their dispositions be reported prior to startup for the next cycle of operation.
On October 19,1993 during an inspection by Siemens Power Corporation (SPC) of "C" low pressure (LP) turbine rotor, several recordable indications were identified in disc No. 4. Although the recordable indications do not adversely impact safety or operability l
nor do they restrict operation of the unit, we are submitting this report to be consistent with past reporting practices.
l RESULTS OF INSPECTION i
During ultrasonic inspection of "C" LP turbine, indications were detected from 0 to 360 l
degrees at the steam inlet side of disc No. 4 generator end (GE) and turbine end (TE).
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GNRO-93/00143 Page 2 of 5 Out of approximately 175 total indications found, only eleven were recordable. The recordable indications were found on the inlet side of disc No. 4 GE. None of the indications were located in the keyway area or shrink-fit area of the disc bore. The i
deepest indication was approximate;y 6 mm. To account for uncertainties, 3 mm were added to this value in order to estimate the allowable service time.
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FRACTURE MECHANICS ANALYSIS Based on SPC's evaluation and industry experience, stress corrosion cracking mechanism is considered to be the reason for the observed indications in the disc bore region of shrunk on discs. The cracks grew due to fatigue and/or stress corrosion phenomena. The stress corrosion crack (SCC) growth occurs under steady stress and aggressive environments, whereas fatigue crack growth occurs due to cyclic stresses as a result of start-stop cycles.
The SCC growth rate is a function of the temperature, material yield strength and the purity of tne intet steam. The propagation of SCC is essentially independent of the stress level. Therefore, SCC growth is basically a linear function of operating time.
l Assuming a 9 mm indication, two different methods were used to determine the additional service life of disc No. 4. The additional service life was calculated to be in excess of 159,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> by the method used during the licensing of the plant. Using a more recent method which is based on the operating experience of SPC supplied turbines, the additional service life was caiculated to be in excess of 327,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
The result of these two calculations are based on a 9 mm crack growing to 1/2 of the l
critical flaw size. These times are in addition to actual turbine accumulated run time.
l Since the calculated service times in both cases exceed the inspection frequency of 50,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br /> and the indications do not affect the structural integrity of the rotor l
assembly, the existing condition is acceptable without restriction for continued
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operation.
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November 23, 1993 GNRO-93/00143 Page 3 of 5 i
TURBINE OPERABILITY Based on the subsequent evaluation of the identified indications, it was determined that the identified indications on disc No. 4 did not affect previous analysis for disc No. 2.
i Disc No. 2 is still considered the critical bounding disc. The inspection did not identify any other indications on the remaining discs of the "C" LP turbine. Therefore the indications identified during the above inspection do not adversely impact the integrity j
of the turbine rotor assembly, nor do they compromise the safe operation of the turbine-generator.
Yours truly,
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November 23, 1993 l
GNRO-93/00143 Page 4 of 5 cc:
Mr. R. H. Bernhard Mr. H. W. Keiser l
Mr. R. B. McGehee Mr. N. S. Reynolds Mr. H. L. Thomas Mr. Stewart D. Ebneter Regional Administrator U.S. Nuclear Regulatory Commission Region ll 101 Marietta St., N.W., Suite 2900 Atlanta, Georgia 30323 Mr. P. W. O'Connor (w/2)
Office of Nuclear Reactor Regulation l
U.S. Nuclear Regulatory Commission i
Mail Stop 13H3 Washington, D.C. 20555 I
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