ML20062N912
| ML20062N912 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 01/11/1994 |
| From: | Schrage J COMMONWEALTH EDISON CO. |
| To: | Murley T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| References | |
| GL-90-05, GL-90-5, NUDOCS 9401240202 | |
| Download: ML20062N912 (9) | |
Text
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Comm:nwxtth Edison f
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1400 Opus Place Downers Grove. filinois 60515 January 11,1994 Dr. Thomas E. Murley Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington D.C.
20555 Attn.: Document Control Desk i
Subject:
Quad Cities Station Units 1 and 2 Relief Request for Residual Heat Removal Service Water (RHRSW) Piping Localized Wall Loss (Pitting)
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References:
(1)
Teleconferences between CECO (P. Piet, et al) and the NRC -
Region til and NRR (C. Patel, et all on November 24 and 30, 1993.
(2)
P. Piet to T.Murley letter dated November 24,1993.
I (3)
Meeting between CECO (N. Chrissotimos) and the NRC (P.
Hiland) on December 8,1993.
In the Reference (1) teleconferences, Commonwealth Edison Company (CECO) provided information to the NRC staff pertaining to localized wall loss, or " pitting", on Residual Heat Removal Service Water (RHRSW) piping; the safety significance of this pitting; and the planned resolution for the pitting. CECO documented this information in Reference (2). During the November 30,1993 teleconference (Reference (1)), the NRC informed CECO that operation with the pitting would require relief from ASME Section XI l'NB-3000. Generic Letter (GL) 90-05, Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1, 2, and 3 Piping provides the basis for evaluation of the affected piping During the Reference (3) meeting, CECO provided preliminary calculations to the NRC that demonstrated compliance with GL 90-05. CECO also committed to provide a relief request for the RHRSW pitting in accordance with GL 90-05. The purpose of this letter is to provide that relief request, which includes CECO's evaluation of localized RHRSW piping wall loss in accordance with GL 90-05.
Flaw Detection On November 22, CECO visually identified several pits on the lnternal surf ace of
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the 1-1001-65D RHHSW Low Pressure (LP) Pump discharge piping. The pitting was found clustered at the bottom of a 8" x 12 dif fusing elbow, oriented in the horizontal-to-vertical position. The pits had a dark gray granular appearance and were extremely localized, with distinct edges. The two major pits appeared to be approximately 1 %" in diameter and about %" deep.
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.Dr. T.E. Murley January 11,1994 1
CECO then inspected all sister components on the other seven RHRSW LP pumps (on both Unit 1 and 2), as well as the next downstream diffusing elbow (at the High
.i Pressure (HP) pump) for similar pitting using Ultrasonic Testing (UT). These inspections i
indicated that two additional RHRSW LP Pump diffusing elbows, at the 18 and 1C l
RHRSW pumps, had similar indications of wall loss. In all cases, the pitting was located at the bottom of tha elbows. A diagram of the inspected elbows and an itemized list of l
the specific pits are provided in Attachments A and B to this letter, No significant wall loss was found within Unit 2 piping,
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Safety Significance The safety significance of the damaged elbows is considered minimal for the i
following reasons:
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1)
All pits are highly localized, with adjacent wall thickness well above the nominal value. A calculation was performed that compares the adjacent wall thickness with the required area replacement of an analogous stubbed-j in tee. The calculation indicated that sufficient metal remains to ensure structural integrity J the pressure retaining boundary.
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2)
A detailed Finite Element Analysis (FEA) of the elbow having the deepest pitting (at pump 1-1001-65B) indicated that the elbow meets code stress analysis limitations for both the as-found and the projected conditions at the end of the operating cycle (March 1994).
3)
The contained fluid is low-energy raw water from the river prior to entry i
into the RHR heat exchanger, and is thus not radiologically contaminated.
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4)
The pits are not through-wall, nor are they projected to become through-wall within the six month time-frame considered in the calculations.
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Furthermore, CECO believes that flow loss through an unlikely through-wall flaw of such size (the deepest being 1/8" in diameter) would be negligible with respect to the capacity of the system as a whole. This is based upon the existing redundancies and the excess capacity of each pump.
i Therefore, the intended safety function of the RHRSW systsm would not be I
j significantly impaired.
5)
In each case, the damage is at the bottom of the elbow. Therefore, a leakage stream would be directed toward the cubicle floor. However, the piping insulation would prevent the stream from projecting to other I
equipment.
G)
Each of the subject elbows are located within watertight cubicles, each of which is equipped with automatic sump pumping capability and automatic Control Room notification of high water level. In addition, all RHRSW pump cubicles are checked by Station operators at a minimum of once per shift.
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.Dr. T.E. Murley January 11,1994 Flaw Evaluation i
in an effort to fully ensure the continued safe operation and regulatory compliance of the three affected Unit 1 elbows, CECO performed a full flaw evaluation. Initially, CECO performed a Code Required Minimum Wall Thickness (T.) calculation based on design temperature and pressure, considering primary and secondary stresses. The calculations indicated that the minimum required thickness was 0.198". - Based on a linear l
wear rate, the projected minimum acceptable thickness for a conservative period of six months was 0.204".
I After determining that the subject elbows had severallocalized regions of thickness less than 0.204", CECO utilized the guidance provided in GL 90-05 for flaw evaluation via the " wall thinning" approach. CECO performed a " local thinning" evaluation.
according to subsubarticle 3620 of the ASME Code Case N-480 (a flaw sizing and proximity calculation). By use of ASME Code Case N-480 Article 3600, as directed by I
Generic Letter 90-05, CECO determined that the subject elbows are acceptable for use until the next refueling outage (scheduled to begin in March 1994), at which time CECO will repair the elbows. CECO's use of GL 90-05 for flaw evaluation in low-energy Code Class 3 piping is referred to in Enclosure 2 of Generic Letter 91-18, Resolution of i
Degraded and Nonconforming Conditions and Operability.
t Augmented inspection CECO has implemented the following actions to address the localized wall loss of RHRSW LP piping:
1)
All sister train components on the other RHRSW LP pumps were inspected for similar wall loss.
2)
The next downstream similar fitting was inspected for similar damage.
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3)
Nuclear Work Requests were initiated and included in the workscope for the next refueling outage (March 1994). During that outage, CECO will repair the affected elbows.
4)
An investigation into the root cause of the pitting was completed. The cause of the damage was determined to be cavitation erosion.
5)
Three of the eight RHRSW LP pumps have been recently modified with new l
impellers. Operating experience at Quad Cities Station indicates that the modified impellers have drastically reduced the amount of cavitation-induced vibration, which in turn will reduce the potential for future cavitation erosion of the piping. The remaining five pumps are scheduled j
for the same modification, to be performed during 1994.
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4 Dr. T.E. Murley January 11,1994 i
' The following is a list of actions that will be implemented in order to minimize cavitation i
erosion in the future:
'i 1)
A study will be performed at Quad Cities Station to determine if other safety related systems and select balance-of-plant systems are susceptible i
to similar cavitation conditions. The following parameters will be considered: 1) bulk fluid velocity,2) similar piping geometry,3) operating pressure,4) operating temperature, 5) presence of excessive noise.
2)
The susceptible locations from the above study will be added to the Erosion / Corrosion reinspection database, i
i Relief Request j
Based upon the evaluation performed in accordance with GL 90-05 (as described
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above), CECO requests relief from performing a flaw evaluation for flaws detected in portions of Code Class 3 piping in accordance with ASME Section XI IWB 3000. This relief request is provided as Attachment C to this letter.
If there are any further questions, please contact John L. Schrage at 708-663-i Very truly yours,.
O f
ohn L.
hrage Nuclear Licensing Administrator i
l Attachments: A, B and C l
cc:
J. Martin, Regional Administrator-Rill T. Taylor, Senior Resident inspector-Quad Cities C. Patel, Project Manager-NRR Office of Nuclear Facility Safety - IDNS I
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ATTACHMENT B
- Table A
SUMMARY
OF LOCATIONS INSPECTED BY UT COMPONENT LINE NUMBER INSPECTION RESULTS -
U-1D RHRSW LP PUMP DIFFUSER ELBOW 1 - 1003 A-12" Thinned--See Table B U-1D RHRSW HP PUMP DIFFUSER ELBOW 1 -1004 A-12" No damage found i
U-1C RHRSW LP PUMP DIFFUSER ELBOW 1-1003B 12" Thinned--See Table C U-1B RHRSW LP PUMP DIFFUSER ELBOW 1-1003C-12" Thinned--See Table D U-1 A RHRSW LP PUMP DIFFUSER ELBOW 1-1003D-12" No damage found U-2A RHRSW LP PUMP DIFFUSER ELBOW 1 -1003 A-12" No damage found U-2B RHRSW LP PUMP DIFFUSER ELBOW 1-1003B-12" No damage found
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U-2C RHRSW LP PUMP DIFFUSER ELBOW 1-1003C-12" No damage found U-2D RHRSW LP PUMP DIFFUSER ELBOW 1-1003D-12" No damage found i
Table B PITS FOUND INSIDE ELBOW ADJACENT TO RHRSW LP PUMP 1-1001-65D l
PIT #
MAX DIAMETER' MIN THICKNESS' BASE THICKNESS' 1
1.858" 0.159" O.429" 2
1.511" 0.164" O.439" 3
0.665" 0.194" 0.424" i
l Table C PITS FOUND INSIDE ELBOW ADJACENT TO RHRSW LP PUMP 1 1001-65C l
PIT #
MAX DIAMETER' MIN THICKNESS BASE THICKNESS' 2
1 0.250" 0.158" O.410" 2
0.250" 0.150" 0.434" 3
0.125" 0.140" 0.436" 4
0.125" O.115" O.430" 5
0.188" O.120" O.430" 6
0.188" 0.130" O.455"
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ATTACHMENT B (continued)
TABLE D PITS FOUND INSIDE ELBOW ADJACENT TO RHRSW LP PUMP 1-1001-65B 2
PIT #
MAX DIAMETER' MIN THICKNESS BASE THICKNESS
+
1 0.125" 0.110" 0.434" i
2 0.125" 0.135" 0.430" 3
0.125" 0.170" O.440" 4
0.188" 0.155" 0.434" 5
0.125" 0.110" 0.440" 6
0.125" 0.180" O.440" I
7 0.125" 0.125" 0.430" 8
0125" 0.125" 0.434" 9
0.125" 0.165" 0.444" 10 0.125" 0.130" 0.434" 11 0.125" O.155" O.420" i
'For larger pits, naaximum diameter was conservatively calculated as the maximum diagonal of the bounding rectangle.
' Minimum thickness was the lowest measured reading within the specific pit.
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' Base thickness was taken as the maximum base metal thickness to adjacent to the pit to ensure a conservative wear rate.
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ATTACHMENT C RELIEF REQUEST l
C_OMEONENLIRENIlEICAllRN Code Class:
3
References:
==
Description:==
Residual Heat Removal Service Water (RHRSW) Piping experiencing regions of I.D. pitting below Code required wall thickness.
l Component identifiers:
Diffusing elbows adjacent to discharge of RHRSW Low i
Pressure (LP) pumps 1-1001-658,1-1001-65C, and 1-1001-65D.
COREEEQUlBEMENIS 1
1 10 CFR 50.55a(g) requires nuclear power f acility piping and components to meet the applicable requirements of Section XI of the ASME Boiler and Pressure Vessel Code (the Code). IWB 3000 of Section XI of the ASME Code specifies acceptance standards for crack-like flaws found in Code piping. However, the Code does not address the evaluation and acceptance of flaws caused by internal wall thinning mechanisms such as l
erosion / corrosion or cavitation erosion. According to Generic Letter (GL) 90-05, certain j
piping flaws may be deemed acceptable by a " wall thinning" ovaluation specified in ASME Code Case N-480.
COREEEllEE.EEQUISI i
Relief is requested from performing a flaw evaluation and acceptance for flaws detected in portions of Code Class 3 piping in accordance with ASME Section XI Article IWB 3000.
BASIS FOR RELIEE Flaws subject to evaluation in accordance with IWB 3000 are applicable to linear, planar and laminar crack-like flaws. Wa:1 thinning is not addressed within Section XI.
Const;quently, GL 91 18 allows the licensee to use the acceptance criteria specified in GL 90-05 for wall thinning mechanisms within Code Class 3 low-energy piping. GL 90-05 states that although ASME Code Case N-480 addresses wall thinning as a result of erosion / corrosion (flow-accelerated corrosion), the acceptance standards therein are extended to all wall thinning mechanisms.
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ATTACHMENT C RELIEF REQUEST (continued)
- of Generic Letter 91-18 states that the licensee may treat the affected system as operable until relief is obtained from the NRC, provided the acceptance criteria in GL 90-05 are met. The Station performed an evaluation uf all flaws using the technique specified in Code Case N-480 Article 3600 and accepted all subject elbows for continued use.
PBQEO_ SED _ALIEflNAIE EBOYJS10NS The Station has inspected all sister components on both Units 1 and 2. The three affected elbows will be repaired during the next refuel outage, scheduled for March of 1994. Based on a root cause analysis (RCA), the cause of the pitting was cavitation erosion induced by the adjacent pumps' discharges. Therefore, the damage is determined I
to be isolated to the high velocity regions of the LP pump discharges. The Station is in the process of modifying the LP pump irnpellers for reduction of cavitation induced vibration. The said modifications are scheduled to be completed for all LP pumps on both units within the year 1994. The impeller modification is expected to alleviate the cavitation erosion. However, the subject elbows will continue to be monitored via the Station Erosion / Corrosion (E/C) Program.
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APfilCABLEllMELEEfilOD Relief is requested until the next refuel outage, scheduled to begin in March of 1994, at which time, the subject components will be brought into compliance with the Code requirements.
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