ML20238C573
| ML20238C573 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 09/01/1987 |
| From: | Kuncl L NEBRASKA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| CNSS878246, IEB-87-001, IEB-87-1, TAC-65804, NUDOCS 8709100212 | |
| Download: ML20238C573 (11) | |
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GENERAL OFFICT Ts si Nebraska Public Power District
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September 1, 1987 l
U.S. Nuclear Regulatory Commission Attention:
Document Control Desk Washington, DC 20555
Subject:
Response to NRC Bulletia No. 87-01
Reference:
1.
NRC Bulletin No. 87-01, " Thinning of Pipe Walls in Nuclear Power Plants".
Gentlemen:
Pursuant to the requirements of Reference 1, Nebraska Public Power District (NPPD) is submitting the enclosed response on the program for monitoring the thickness of pipe walls in high energy, single-and two phase flow carbon steel piping systems at Cooper Nuclear Station (CNS).
If you have any questions regarding this submittal, please contact my office.
Sinc-
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6 L. G. K nel Vice President - Nuclear LCK:JPFirg Enclosure cc:
Regional Administrator USNRC - Region IV Resident Office Cooper Nuc1 car Station 8709100212 070901y8
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E E G E E T id E N 7/ m w r a v a m m T R I s c : g r ; 2 c m
STATE OF NEBRASKA)
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PLATTE COUNTY
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L. G. Kuncl, being first duly sworn, depcees and says that he is an authorized representative of the Nebraska Public Power District, a public corporation and political subdivision of the State of Nebraska; that he is duly authorized to submit this information on behalf of Nebraska Public Power District; and that the statements contained herein are true to the best of his knowledge and belief.
L.G./Kunc1 Sugergbedinmypresenceandsworntobeforemethis ff)k day of
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1 RESPONSE TO NRC BULLETIN NO. 87-01:
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TIIINNING OF PIPE WALLS IN NUCLEAR POWER PLANTS NEBRASKA PUBLIC POWER DISTRICT COOPER NUCLEAR STATION AUGUST 28, 1987 l
RESPONSE TO NRC BULLETIN 87-01 As requested in NRC Bulletin 87-01, the following information is provided regarding the District's current pipe wall thinning program for Cooper Nuclear Station (CNS).
A formal, comprehensive program to ' monitor erosion / corrosion is currently under development and will establish criteria to assure piping wall thicknesses do not reach the minimum allowable values.
This program when i
i implemented' will monitor the wall thickness of safety-related and non-safety related piping fabricated of carbon steel in the condensate, feedwater, steam and connected high-energy piping.
Requested Action:
1.
Identify the codes or standards to which the piping was designed and fabricated.
Response
a)
Nuclear piping systems at CNS were designed and constructed in accordance with the requirements of USAS-B31. 7,
" Nuclear Power Piping", February 1968 draft and errata for the February 1968 draft
- dated June, 1968.
This included portions of the feedwater and main steam piping, b)
CNS power piping systems (conventional steam and service piping) were designed and constructed in accordance with the requirements of USAS-B31.1 0,
" Power Piping" (1967) and ASME Section I,
" Power Boilers".
This included portions of the condensate, feedwater, extraction, and main steam piping.
Requested Action -
2.
Describe the scope and extent of your programs for ensuring that pipe wall thicknesses are not reduced below the minimum allawable thickness.
Include in the description the criteria that you have established for:
a.
selecting points at which to make thickness measurements, b.
. determining how frequently to make thickness measurements, c.
selecting the methods used to make thickness measurements, d.
and making replacement / repair decisions.
Response
2.
The existing program at CNS encompasses the following piping systems:
Feedwater Heater Drains, Extraction Steam Piping, and Main Steam Crcss-under Piping.
These systems were selected because they are high energy systems with a relatively high moisture content and are fabricated of carbon steel, 1 of 7
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a)
The inspection points have been Ldetermined 'primarily by.
analyzing 1the; piping geometry where erosion.would most likely occurf (i.e., geometry or ' configuration changes).
Using this
_ basis, ; valves,. ' elbows, tees, reducers, and piping ; 1-4 feet downstream of these fittings have been the primary locations examined in each piping' system.
b)
The frequency. of thickness measurements ' has been< based on availability of'the piping system for analysis (outages, etc.)
and on results of the previous examinations.
Generally, a system or component that had experienced no detectable erosion after umny years of service was' considered: " low. risk" and
. inspection intervals'would be limited.. Systems indicating some
' thinning would be inspected more 'of ten (i.e., every outage).
c)
Ultrasonic thickness measuring instruments have been utilized for.all examinations. The basis for this selection is the ease of operation, reliability, and repeatability such instruments 4
afford..
d)
Replacement / repair decisions have been made on an individual basis using engineering' judgement. The' main consideration for replacement / repair has been the degree of wall thinning and.
years of tarvice.
In 1983, CNS Special-Test Procedure 83-06 established guidance to indicate the' degree of wall thinning using the following guidelines:
1.
Normal erosion 20% of the erosion allowance 2.
Moderate erosion 40% or the erosion allowance 3.
Severe erosion - >40% of the erosion allowance Secondly, years of service adds a time element in the repair / replacement decision.
The time element factor allows engineering judgement when replacement / repair is required.
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Requested Action -
l 3.
For liquid-phase systems, state specifically whether the following factors have teen considered in establishing your criteria for selecting points at which to monitor piping thickness (Item 2a):
l-L a) piping material'(e.g., chromium content),
-b) piping configuration (e.g.,
fittings less than 10 pipe diameters apart),
c) pH of water in the system (e.g., pH less than 10),
d) system temperature (e.g., between 190 and 500*F),
e) fluid bulk velocity (e.g., greater than 10ft/s),
f) and oxygen content in the system (e.g., oxygen content less than 50 ppb).
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Response
The type of' piping material (chromium content, etc.) has been a 3.
a.
major consideration when selecting pointo to monitor piping thickness.
b.
Fiping configuration has been a. consideration when selecting pipe thickness monitoring points.
c..
Previously, the~pH of the water in the system has not'been a factor in selecting monitoring points for pipe thickness measurements..Since CNS liquid phase systems are operated in the neutral pH range, pH for these systems was not a major concern with respect to pipe wall thinning.
d.
System temperature has been a direct f actor. in selecting monitoring : points for pipe thickness measurements as systems currently being monitored are all high temperature systems (between 190*F and 500*F).
e.
The. fluid bulk velocity has been a consideration when selecting points to monitor piping thickness.
f.
Previously, oxygen content has not.been a major factor ' in determining pipe. thickness monitoring locations.
The oxygen content of feedwater at CNS is approximately 40-50 ppb.
Since an oxygen content of 40-50 ppb is not a concentration generally believed.to be conducive to corrosion (reference.EPRI Final Report on Single-Phase Erosion / Corrosion of Carbon Steel Piping, February 19, 1987) and-no occurances of single-phase erosion / corrosion have been observed in systems of high oxygen content (>30 ppb), it has not been considered a major concern.
Requested Action -
4.
Chronologically list and summarize' the results of all inspections that have been performed, which were specifically conducted for the purpose of identifying pipe wall thinning, whether or not pipe wall j
thinning was discovered, and any other inspections where pipe wall thinning was discovered even though that was not the purpose of that inspection.
a.
Briefly describe the inspection program and indicate whether it was specifically intended to measure wall thickness or whether wall thickness measurements were an incidental determination.
b.
Describe what piping was examined and how (e.g., describe the inspection measurement (s), test method, reference thickness,
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locations examined, means fer locating measurement point (s) in subsequent inspections).
c.
Report thickness measurement results and note those that were identified as unacceptable and why.
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,q d'
Describe actions already taken or planned for piping that has g
been found to'have a nonconforming wall thickness. If you'have
. performed a' failure analysis, include' the results of that J
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. analysis.
Indicate whether the actions involve repair or.
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' replacement, including any. change of materials.
Response
.The.following information is presented in. chronological order.
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-1983 a.
Special Test' Procedure (STP) 83-06 was specifically initiated to perform a comprehensive pipewall thinning inspection _ in limited L Balance of Plant (BOP) piping.. It encompassed UT measurement of piping with high velocity wet steam and piping which was suspect or had history of erosion related failures at similar facilities.
b.
Piping examined by STP 83-06 was the feedwater heater extraction steam piping.
A Krautkramer - Branson Model DM-2 portable UT device was used to take the data.
The reference thickness used was.the standard wall thickness for piping, given its pipe schedule and diameter.
Various elbows and straight runs were measured.
Metal marker ink was used to grid mark and label each point measured. All measurements were documented, evaluated and retained for future inspections, c.
Thickness measurement results revealed erosion (due to high velocity wet steam) of the carbon steel ext'. action steam piping to feedwater heaters A-5, B-5, A-4 and B-4.
Minor erosion in the remaining entraction piping was evident, however only to a degree necessitating continued monitoring, not replacement.
d.
As a. result of this inspection, approximately 20 feet of extraction piping to the B-5 heater was immediately replaced with carbon steel piping. The wet steam erosion of this piping was easily. identified and well known, therefore, no specific failure analysis was performed.
Subsequently, during the 1984/1985 outage, the carbon steel extraction steam piping to the A-5, B-5, A-4 and B-4, feedwater heaters was replaced with Cr-Mo piping.
-1984/1985 a.
Baseline wall thickness measurements of newly installed Cr-Mo extraction steam piping were taken. Crawl through inspections of portions of the main steam cross-under piping and moistere separators were conducted. These activities were specifically performed for the purpose of identifying current and future wall thinning or, in the case of moisture separators, component degradation.
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6 b.
The wall. thickness of. newly. installed Cr-Mo extraction steam piping to T feedwater heaters A-4, B-4. A-5, B-5, were UT
. measured for purpose of establishing baseline data for.the
. detection of future erosion.
A Krautkramer - Branson Model DM-2 portable 1UT device was used to take the data.. Various elbows and straight. runs were measured. Metal marker ink was used to grid mark and label each point measured.
Crawl through inspections of portions of the main steam cross-under. piping and the moisture separators were performed. The reference thickness used for the cross-under pipe was based on original design specification.
Visual methods were utilized since the1relatively rough' interior surface was not conducive to accurate and reliable UT measurements.
Crawl - through ' inspections of the four, moisture separators revealed significant erosion of nonpressure boundary internals.
Consequently, based on engineering judgement, repairs were made on various internal components.
c.
Thickness measurements for the newly installed extraction steam piping were for baseline purposes ' and results f adicated no areas with thin walls. This was to be-expected since the pipe was new and had no previous operating history.
Because the cross-under pipe was visually inspected no thickness data was recorded.
d.
Because there was no piping found to have a nonconforming wall thickness, none was replaced. However, weld overlays of small areas of the cross-under piping in-locations judged to be of concern were made.
-1986 a.
UT measurements of feedwater heater drain piping, and carbon steel feedwater heater extraction steam piping were specifically performed for the purpose of identifying wall thinning.
b.
The piping examined was feedwater heater train
'A' drains (elbows and valve outlets), feedwater heater extraction steam piping, and the main steam cross-under piping. The inspection
'for the feedwater heater drains and the extraction steam piping was conducted using a portable UT device.
The main steam cross-under piping was visually inspected via crawl throughs.
The reference thickness for the extraction steam and heater drain piping is the nominal wall thickness for the particular schedule and diameter of pipe. The reference thickness for the cross-under piping is based on the original design specifications.
Metal marking ink was used to grid mark i
points measured on the extraction and feedwater heater dra.n piping.- The cross-under pipe was inspected visually and no specific locations were recorded.
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The ' thickness measurements results indicated 'no ~ significant
- level of erosion.
All. the ' results were determined - to. be acceptable.- Visual examination of ' the cross-under piping revealed small areas that,- based on' engineering judgement required weld overlays..
d.
Because there was no. evidence _ of. a, nonconforming wall thickness, no piping was repaired.or replaced.- However, weld overlays were - performed in - small areas of the cross-under-
. piping judged to be of concern.
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-General Results (1974.to current) a.
. Pipe wall thinning has resulted in several. small ' leaks, however, wall thickness measurements were not taken.
-b.
The leaks have been primarily in small bore carbon steel piping carrying two phase flow. The'1eaks are generally identified by
- operators on routine tours.- The most susceptible piping has
,c been steam line drainn downstream of orifices or traps and minimum flow lines downstream of throttle devices, c.
Leakage was identified at the pin hole stage with any laakage considered unacceptable.
d.
Each leak was evaluated on a case by case basis.
The high pressure turbine drains and the main steam line drains were replaced with stainless steel piping in ' 1977 and 1986, respectively.
In other cases where the piping was small bore or the system pressure was low, the affected piping was replaced without a material change.
-Each Refueling Outage a.
Thickness measurements of welds are recorded each refueling outage as part of the ASME Section XI Inservice Inspection (ISI) Program.
However, these measurements are not taken-specifically as part of an inspection intended to determine wall thinning.
b.
Thickness measurements are recorded. for Code Class I and certain Class 11 piping welds subject to the ISI Program.
These measurements have been made using ultrasonic testing devices of various models, c.
The thickness measurements have not been specifically evaluated for wall thinning.
d.
(Not applicable.)
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Requested Action -
5.
Describe any plans either for. revising the present or for developing new or additional programs for monitoring pipe wall thickness.
Response -
For the past four years, encompassing three refueling outages, the CNS program to monitor pipe wall thinning due to two-phase erosion has been treated as a special project directed by a station engineer. Due to the lack of an adequate predictive model, pipe inspection locations were typically determined by engineering judgement and past maintenance history.
Severe erosion problems were located and corrected by replacement of pipe with more erosion resistant material. Pipe we.ll thickness data has been,
collected and analyzed primarily by engineering judgement.
The District is currently developing a program to greatly enhance the existing pipe wall thickness program.
One area of improvement is,t6 e
provide a systematic approach to the existing inspection, data 7 collection, evaluation, and record retention methods.
This systematic approach wil.1 incorporate the present program into the station operations manual and Preventive Maintenance Program.
A second area of improvement directly results from the Surry-2 incident and the recent advances in pipe wall thinning predictive models.
Augmentation of the CNS program will include the use of these improved predictive models in the selection of single-and two-phase pipe inspection points.
However, engineering judgement and past-maintenance history will continue to be strongly considered when selecting pipe thickness monitoring points.
The implementation of these program improvements will be complete before the 1988 Refueling / Maintenance Outage currently scheduled to start in February 1988.
It is planned that the systems monitored by the revised program will include:
1.
Condensate 2.
Feenwater 3.
Main Steam 5.
Others as required Accordingly, the District plans on implementing the revised inspection program on the aforementioned systems during the 198o outage. This is to assure mitigation of pipe wall thinning failures that could result in
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undesirable challenges to plant safety systems.
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