ML20056C472
| ML20056C472 | |
| Person / Time | |
|---|---|
| Site: | Pilgrim |
| Issue date: | 06/17/1993 |
| From: | Strosnider J Office of Nuclear Reactor Regulation |
| To: | Butler W Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9306240210 | |
| Download: ML20056C472 (6) | |
Text
06/22/93 07:43
+1 301 504 3716 NRC PUB AFFAIRS NO.317 F002/005
'b p* *
- ouq UNITED STATES t
Y 19 NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C, 20066 0001 9 3 22 r00.52
{
[
% l, 8
JilN 171993 MEMORANDUM FOR:
Walter R.
Butler, Project Director Project Directorate I-3 Division of Reactor Projects I/II FROMt Jack R.
Strosnider, Chief Materials and Chemical Engineering Branch Division of Engineering SUBJECTt PILGRIM UNIT 1: ASSESSMENT OF LOW PRESSURE TURBINE ANALYSIS During the refueling' outage in April 1993, General Electric (GE) inspected the rotor in the low pressure turbine (LPA) at Pilgrim Unit 1 and found flaw indications in disk numbers 4, 5,
6, and 7.
GE recommended that the licensee either remove the 7th stage disk on the generator side (7GA) or pre-warm the LPA rotor before startup.
Subsequently, the licensee retained Structural Integrity Associates, Inc.
(SIA) to evaluate flaw indications in the 7GA disk.
On May 12, 1993, the licenses provided to the NRC project manager the SIA analysis (Reference 1). The project manager requested that the Materials and Chemical Engineering Branch (EMcB) perform a review of the SIA analysis to determine 1) whether there is any gross error in the SIA analysis and 2) the potential impact to plant safety.
Pilgrim Unit i has two low pressure turbines, LPA and LPB, with shrunk-on disks.
The turbine deck is orientated unfavorably with respect to the reactor building.
The flaw indications of the 7GA disk are located in both the hub and web.
Although the 4th and 5th stage disks have more and larger flaws than that in-the 7GA disk, GE determined that the 7CA disk is the limiting disk based on operating conditions, fracture toughness of the disk, and potential consequences of the disk failure.
SIA performed parametric studies to determine effects of fracture appearance transition temperature (FATT),
fracture toughness variability, pre-warming, crack growth rate and stress intensity models.
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.
contact:
John Tsao, DE/EMCB, i
504-2702
)
}[T,I
ltTCBF
} $062 HOSP 9
to 9
06/22/93 07:43
+1 301 504 371614PC PUB AFFAIRS I40.317 P003/006 2
For the 7GA disk, GE reported one indication of 0.14 inch in the hub and an indication in the web which GE could not accurately size.
For that indication, GE assumed a crack size of 0.25 inch based on flaw indications from other power plants' inspection data and laboratory data.
The staff believes that the initial crack i
inch is conservative but the staff could not quantify size of 0.25 the uncertainty ammociated with the assumed size at this time.
GE used a fracture mechanics model of an edge crack in an infinite GE's model is conservative because plate having constant loading.than the actual geometry which is a radial it is more compliant crack emanating from the keyway.
Moreover, its constant loading does not consider the radial decrease in hoop stress with increasing distance from the bore.
SIA's model is a hole in an The staff infinite plate with attenuated loading along the crack.
assumed a model of a thick wall cylinder with attenuated loading.
GE used 0.06 inch per year for the crack growth rate which was the taken from a statistical study of the average crack median value growth rate vs. wheel operating temperature from turbine inspection SIA used 0.0164, 0.02, and 0.06 data of both DWR and PWR plants.The staff calculated a crack growth inch per year in its studies.
rate of 0.02 inch per year from previous inspection data of the LPA rotor.
The staff believes that the actual crack growth rate may lie between 0.02 and 0.06 inch per year.
However, according to the upper bound growth rate (2 standard deviations) at GE's data, an operating temperature of 172*F could be as high as 0.08 inch pel.
year.
i The critical stress intensity (Kge) is an indicator of fracture toughness of the disk material.
The lower the Kc used in the j
fracture mechanics analysis the more conservative the results will j
GE's analysis used a lower bound value of 105 kniv'in which was be.
taken from the graph of critical stress intensity vs.
excess (Test Temperature - FATT).
The staff judges that the temperature value of 105 KsiVin is conservative.
GE and SIA calculated a critical crack size (depth) of 0.34 inch and 0.54 inch, respectively.
SIA conservatively assumed that the SIA indicated that crack length is the length of the keyway bore.
if the crack aspect ratio is known, the critical crack size may be larger than 0.54 inch.
Based on SIA's calculation, the critical inch.
crack size for the thick wall cylinder model is about 0.45 on the above parameters, the staff estimated a factor of Based based on the ratio safety on flaw size ranging from 1.21 to 3.6 between the crack length at end of the current fuel cycle in April The 1995 to that of the critical crack size of the cylinder model.
f actor of safety on stress intensity (Kr) ranges from 1.1 to 1.99 which was estimated by taking square root of the safety factor on flaw size.
06/22g 07:44
+1 301 504 3716 NPC PUB AFFAIRS NO.317 P004/006 3
requires that the turbine missile failure probability be The NRC lE-S per year or lower for an unf avorably orientated turbine.
GE's is based on a turbine missile generation probability of analysis IE-5 failure per year.
SIA did not perform a probabilistic fractura mechanics analysis.
Based on engineering judgement, the staff estimated that the missile failure probability for the LPA For this condition, the turbine is between 1E-5 and 1E-4 per year.
NRC permits the turbine to be kept in service until the next at which time the licensee should take action to scheduled outage, reduce the failure probability to the 1E-S per year criterion (Ref.
2).
The licensee has indicated that they are considering the replacement of both LPA and LPB rotors during the next scheduled refueling outage, which is expected to be April 1995.
Based on an assessment of the information available, the staff concludes that there is no safety concern for normal operation of LPA turbine to the end of the current fuel cycle. The SIA the conservatisms used analysis appears to reduce some of the obvious in the GE analysis.
Jack R.
Strosnider, Chief Materials and Chemical Engineering Branch Division of Engineering
Enclosure:
- Pilgrim Turbine Evaluation : References cc: B.
D. Liaw R.
Eaton S.
Sheng
06/22293 07:44
+1 301 504 3716 tRC PUB AFFAIRS NO.317 P005/006 4
4 ATTACMM5MT 1 6/8/93 FILGRIM TURRIME EYALUATIDM PARAMETER 93 AZA 352 INITIAL CRACK sInB
.25"
.25"
.25" (NOTE: ACTUAL MEASURED SIZES.12" TO.14")
APPLIED KI MoDEL n
CRACK GROWTM 0.06"/YR 0.06"/YR O'.06"/YR O.02"/YR 0.02"/YR KIC, kaiVin 105 105 105 LO* DER BOUND LOWER BOUND LOWER BOUND CRITICAL CRACK
.34"
.54"
.4S" DEPTM TIME TO FAILURE 1.5 YRS 4.8 YRS 4.0 YRS
- 14. 5 YRS -
12 YRS rAcroRs or sarBTT oM Flaw sInz/sTRuss INTausITY.rAcroR (ansmD oE MRc Assup TIOus)
CRACK GROWTH STARTUP NORMAL OPERATION
. RATE PER YEAR (9 75 dog F)
(6 172 dag.F) 0.06" 1.21/1.10 2.82/1.68 3.60/1.89 0.02" 1.55/1.24 l
06/22/93 07:45
+1 301 504 3716 I4PC PUB AFFAIRS t40. 317 P006/006 o
5 ATTACHMENT 2 References 1 1.
May 12, 1993 letter from D.
Rosario and P.
Riccardella of Structural Integrity Associates to J. Gersty of Boston Edison,
Subject:
Evaluation of the Pilgrim Unit 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.
.06/22/93 07:42
+1'301 504 3716 NRC PUB AFFAIRS NO.317 P001/006 0
ge 8600 l
- z
\\'
/
UNITED STATES
-n 3. ' 'll' 1
NUCLEAR REGULATORY COMMISSION
,f
'o, s,,,* w [p WASHINGTON. D.C. 20555 Office of Public Affairs b 7 'I ! $ 3 DATE
'N o 4 W A DN t C0$loCb To:
FAX 11 UMBER:
301bli31N3 VERIFY:
72"I3 14 UMBER OF PAGES:
PLUS COVER SHEET COMMENTS:
h8C out k $ T CO GS ** N I
FROM U.S. 11uclear Regulatory Commission office of Public Affaira 301/504-2240 Fax Number 301/9043716