ML20154F553
| ML20154F553 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Haddam Neck, 05000000 |
| Issue date: | 09/09/1988 |
| From: | Mroczka E CONNECTICUT YANKEE ATOMIC POWER CO., NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| IEB-88-009, IEB-88-9, IEIN-87-044, IEIN-87-44, NUDOCS 8809200049 | |
| Download: ML20154F553 (6) | |
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i NORTHEAST UTILITIES o.n.r.i 0,,,c.
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(203) 665-$000 September 9, 1988 Docket Nos. 50-213 50-423 KDT55 Re HRC Bulletin No. 88-09 U.S. Nuclear Regulatory Commission Attn Document Control Desk Vashington, D.C.
20555
References:
(1)
C.
E. Rossi letter to All Licensees, "NRC Bulletin No. 88-09: Thimble Tube Thinning in Vestinghouse Reactors", dated July 26, 1988.
(2)
C.
E.
Rossi letter to All Licensees, "NRC Information Notice No. 87-44: Thimble Tube Thinning in Vestinghouse Reactors", dated September 15, 1987.
(3)
E.
J.
Hroczka letter to U.S.
Nuclear Regulatory Commission, "Plux Thimble Tube Vear", dated January 22, 1988.
Gentlemen Haddam Neck Plant Hillstone Nuclear Power Station, Unit No. 3 Response to % Bulletin No. 88-09 Thimble Tube Thinn4ng in Vestinghouse Reactors Reference (1) requested that Licensees who operate Vestinghouse reactors establish an inspection program to monitor thimble tube performance.
This issue had been brought to our attention by the NRC Staff initially by Reference (2).
In response to Reference (2),
Connecticut Yankee Atomic Power Company (CYAPCO) for the Haddam Neck Plant and Northeast Nuclear Energy Company (NNECO) for Hillstone Unit No. 3, had already taken actions which vere subsequently requested by Reference (1).
Hence, this report is being submitted in accordance with Reference (1) reporting requirement 3b.
The Hillstone Unit No. 3 program has already been transmitted to the NRC Staff (Reference (3)).
It is included as Attachment 2 f or your convenience.
Attachments 1 and 2 delineate the actions taken at Haddam Neck and Hillstone Unit No. 3, respectively.
8809200049 880909 gDR ADOCK0500g3
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8 U.S. Nu lear Regulotcry Commissicn A07405/Paga 2 e
September 6, 1988 Please contact us if you have any questions.
Very truly yours, CONNECTICUT YANKEE ATOMIs.'0VER COMPANY NORTHEAST NUCLEAR ENERGY COMPANY MT kt E. J./Mroczkar Senior Vice President STATE OF CONNECTICUT )
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) ss. Berlin COUtfrY OF HARTFORD
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Then personally appeared before me, E. J. Mroczka, who being duly svorn, did state that he is Senior Vice President of Connecticut Yankee Atomic Power l
l Company and Northeast Nuclear Energy Company, Licensees herein, that he is l
authorized to execute and file the foregoing information in the name and on behalf of the Licensees herein, and that the statements contained in said information are true and correct to the best of his knowledge and belief.
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My n Erpires March 31,my cct V. T. Russell, Region I Administrator l
A. B. Vang, NRC Project Manager, Haddam Neck Plant D. H. Jaffe, NRC Project Manager, Millstone Unit No. 3 I
J. T. Shedlosky, Senior Resident Inspector, Maddam Neck Plant l
l V. J. Raymond, Senior Resident Inspector, Millstone Unit Nos. 1, 2 and 3 l
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e Docket Nos. 50-213 50-423 A07405 Response to NRC Bulletin No. 88-09 Haddam Neck Plant Thimble Tube Thinning Program September 1988 J
Haddam Neck Plant Thimble Tube Thinning Program Reference (1) requires an inspection program which includes:
o the establishment, with technical justificat.'on, of an appropriate thimble tube year acceptance criterion (for example, percent throughwall loss).
This acceptance criterion should include allovances for such items as inspection methodology and vear scar geometry uncertainties.
o the establishment, with technical justification, of an appropriate inspection frequency (for example, every refinling outage),
o the establishment of an inspection methodology that is capable of adequately detecting vear of the thimble tubes (such as eddy current t(*, ting).
CYAPC0 has evaluated the structural capabilities of the reactor vessel bottom mounted instrumentation (BMI) thimble tubes.
The evaluation included three possible failure mechanisms:
(a) uniform vall thinning resulting in reactor coolant leakage, (b) localized vall thinning resulting in reactor coolant leakage, and (c) uniform vall thinning resulting in tube hackle / collapse.
Since the failure mechanisms resulting in reactor coolant leakage could have an impact on the structural capabilities of the reactor coolant pressure boundary, a factor of safety of three on loads for normal operation as required by the ASME Code, was included. However, since collapse of the tube vould only impact tha operability of the instrumentation and vould not result in resetoc coolant leakage, no factor of safety was included.
Based on the results of the evaluation, it was concluded that the uniform vall thinning resulting in reactor coolant leakage was the most restrictive failure mechanism resulting in an allovable vall thinning of approximately 70% of the original tube nominal vall thickness.
The inspection methodology is eddy current testing.
This testing was done on all thimble tubes during the 1987 refueling outage.
The ' eddy current test results for the thimbles determined that tLimbles 3 and 9 had through vall indication of <20% and thimble 19 had a 30% through vall indication.
The vear in each case was located approximately 11 feet from the thimble end.
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. Therefore, the eddy current testing of the flux mapping thimbles was successful in proving that at the present time the thimbles are sound.
The three (3) indications found are well within the limits of the acceptance criterion (<70% through vall) and thus, no action is required at this time.
Given the 20 years of operation, we can conclude that wear has occurred at a maximum of 1 to 1-1/2% through vall per year.
Based on this trend, a surveillance program has been implemented to eddy current test the thimbles in three cycles to monitor thimble wear.
Such a program vill provide the degree of confidence and reliability needed to ensure Flux Mapping System operability in the
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Docket Nos. 50-213 50-423 XU7TU3 Response to NRC Bulletin No. 88-09 Millstone Nuclear Power Station, Unit No. 3 Thimble Tube Thinning Program September 1988
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L January 22,1988 Docket No. 50-423 812802 Re: Information Notice #3M4 U.S. Nuclear Regulatory Commission Attn Document Control Desk Washington, D.C. 20333 Gentlemen:
Millstone Nuclear Power Station, Unit No. 3 Flux Thimble Tube Wear in January 1937, based on INPO reports and discussion with Westinghouse on flux thimble degradation at several Westinghouse-designed nuclear plants, Northeast Nuclear Energy Company (NNECO) made a decision to perform eddy current testing of the Mllistone Unit No. 3 flux thimbles during the first refueling outage. in December.1937, under a contract with NNECo, Cramer & l.indell Engineers Inc. performed eddy current testing of the Millstone Unit No 3 flux thimbles. At the time, Millstone Unit No. 3 was shut down for its first refueling outage. The purpose of eddy current testing was to obtain base line information that could be used to track and determine wear rates over succeeding cycles.
The attached report concerning the results of eddy current testing of flux thimbles and corrective actions taken to date is being provided in response to an NRC request made during a conference call with NNECO representatives and the Staff on January 10,1933.
If you have any questions, please contact my staff directly.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY f*] / %..-
A<.
E. 3. Mroczka Senior Vice President l
D'.),b f by: W. D3omberI Vice President cc:
W. T. Russell, Region 1 Administrator i
R. I.. Ferguson', NRC Project Manager, Millstone Unit No. 3 T. J. Raymond, Senior Resident Inspector, Millstone Units No.1,2, and 3 I
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I Docket No. 50 423 i
2128Q1 Mi' Istone Unit No. 3 Reoort on the llesults of Eddy Current Testina of Flux Thimbles January, 1988
1 B12802\\Page 1 Millstone Unit No. 3 Reoort on the Results of Eddy Current Testina of Flux Thimbles Introduction In January 1987, based on INPO reports and discussion with Westinghouse on flux thimble degradation at several Westinghouse designed nuclear plants.
NNEC0 made a decision to perform eddy current testing of the Millstone Unit No. 3 flux thimbles.
In December 1987, under a contract with NNECO, Cramer &
Lindell Engineers Inc. performed eddy current testing of the Millstone Unit No. 3 flux thimbles. At the time, Millstone Unit No. 3 was shut down for its first refueling outage.
All 58 thimbles were inspected, at a minimum, from the lower half of the fuel assembly, back to the seal table. Several thimbles with apparent higher wall loss signals were withdrawn several inches and retestec with the wear indication moved from the wearing surface.
This analysis revealed potential wall loss of some degree in 21 of the 58 flux thimbles.
wear of only a couple percent, up to approximately 40%.This degradation ranged from outside diam The other 31 thim detectable indications of potential wall loss. As a result of the indications found, six tubes were axially repositioned approximately 1 1/4 inches to move the wear scar away from the point of wear.
In addition, one thimble was capped off to allow the measurement of wear after two cycles of operation with the wear scar in the same location.
Discussion The eddy current method used in the testing of the Millstone Unit No. 3 thimbles utilized 2 frequencies to detect potential wall loss on both the inside diameter (10) and 00 of the thimbles.
Each of these two signals was analyzed separately, and also mixed together, using a vector addition tech-nique to effectively cancel out si<inals from core components, thereby leaving only the potential wear induced s< gnal.
The two individual signals and the mixed signal were analyzed with both a phase angle anal the signal phase angle is used to determine potential wear, ysis, whereby and an amplitude analysis, whereby the signal amplitude is used as a measure of wall loss.
Since the 60 khz frecuency is more sensitive to 00 damage than the 200 khz signal the results obtained from the analysis of its signal and the mix signal, were given more weight than the results of the 200 khz signal.
Furthermore, the amplitude of the signal had to be considered when reviewing the results.
A smaller amplitude signal will result in a much less accurate analysis of the signal than a larger amplitude signal.
For example, a large phase angle on a small amplitude signal could show up in the phase angle analysis as a very large wear scar, when, in fact, there is little or no wear.
Therefore, it is critical to consider the amplitude of the signal and the results of the amplitude analysis when evaluating results.
A sumary of the results of the eddy surrent testing of the Millstone Unit No. 3 flux thimbles was received from Cramer & Lindell Engineers Inc. on December 29, 1987. Of the 58 thimbles, 21 showed indication of some potential wall loss at the lower core plate.
Potential wall loss on these thimbles ranged from approximately 13% to approximately 40%.
Two thimbles showed some indication of potential wear between the lower core plate and the core support
e 812202\\Page 2 forging, ranging from 13% to 14%. Three thimbles showed some signs of wear at the core su vall loss. pport forging. This ranged from 11% wall loss to approximately 37%
Seven thimbles were found to have potential wall loss ranging from 11% to 18% at the upper tie plate. Nine thimbles showed signs of wear ranging from !!% to 23% at the lower tie plate.
Thirty-one thimbles showed no signs of detectable wall loss.
This wear is believed to be the result of flow induced vibration of the flux thimbles, and is particularly noticeable at the to) of the lower core plate.
This phenomena has t,een observed at several otier Westinghouse plants, as documented in NRC Information Notice #87 44.
Wear in these plants has ranged from a few percent to through wall defects.
Corrective Actions As a result of the defects found during the eddy r.urrent testing, NNECO has instituted several courses of corrective action.
Tb2 results of the eddy current testing were forwarded to Westinghouse for their review and recommen-dations.
Westinghouse recommended that all thimble tubes with indication of potential wall loss exceeding 30% be either axially repositioned to move the wear scar away from the point of wear or canped off.
!r interest of censerva-tism, NNECO extended this to include all tiimbles with p.ential wall loss in excess of 25%.
Six thimbles were retracted 1 1/4" +1/32 0", the thimble cut at the seal table, and the high pressure leal remade.
Each of these thimbles was eddy current tested again to verify trat the wear tear had moved a minimum of 3/a".
This testing showed that each cf the wear scars was moved approxi.
mately l',
and that each wear scar was reroved from the location in the vessel where wear was occurring.
In addition, one thimble with approximately 38%
wear at the lower core plate was left 13 its current axial position, and capped at the seal table.
This thimble will remain in this position during the next cycle of operation.
At the end of cycle two, eddy current testing will again be performed on all 58 flux thimbles to determine whether thimbles continue to wear, and, if so, at what rate.
In addition to repositioning the thimbles, Westinghouse has performed an analysis to determine whether the structural Integrity of the six repositioned thimbles can be maintained for another cycle of operation.
This analysis modelev, the eddy current irespection indication geometry as defined by Cramer & t.indell Enginoers.
The geometry for the 60% tube wall loss condition over a lengtl, of J/2" was investigated.
This analysis showed a primary membrane stress re,ulting from exposure of the thimble to normal reactor coolant system pressure to be hess than that allowed by the ASME Boiler and Pressure Vessel Code.
Therefore, Westinghouse conclud-ed that the thimble tubes with 60% material wear loss over a length of 1/2" is structurally adequate for the applied external pressure of 2250 psig.
Furthermore, the repositioning of the six thimbles and the capping of one has been reviewed for impact on the ability of the plaM to perform Technical Specifications required flux maps.
No impact results from repositioning or capping the thimbles involved.
In addition to the corrective actions discussed above, NNECO is taking, or considering, several long term actions. Among the long term corrective action items available for consideration are several design changes.
Thesa include the addition of isolation valves to our flux thimblet, or possible addition of sleeves within the flux thimble guide tubes in the lower vessel internals.
Other possible long term action is to remcVe one or more ef the worn thimbles for hot cell examinatica for comparison of actual wear to eddy current results, and determination of the exact wear mechanism.
I B12802\\Page 3 Justification for Continued Operation This justification for continued operation addresses the ability of Millstone Unit No. 3 to start up and operate safely for Cycle 2 with the measured thimble wear, and with six thimbles axially repositioned approximately 1 1/4 4
inches to move the wear scar way from the point of wear and with one thimble 1
capped.
Impact on the Design Basis Accidcnts There are two arsas which must be reviewed for potential impact on the design i
basisaccident(DBA's).
o the impact of thimble thinning on the RCS pressure coundary, and o
the effect of retracting the thir.bles on the ability to determine 4
l the power distributions using the incore instrumentation.
I The primary pressure boundary integrity has been verified in the Westinghouse l
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analysis for conditions up to 60% wall loss on the thimble exterior. Assuming a linear wear rate from cycle 1 to cycle 2 and 5% measurement acetracy, thimble tubes with less than 25% thinning do not need to be retracted.
Tubes j
with greater than 25% wear have been retracted or capped to protect the i
pressure boundary.
This provides reasonable assurance that no uncapped tube j
will have greater than'60% wall loss at the end of cycle 2 oporation ((<25% +
1 5%)
- 2 cycles < 60%).
The capped thimble may experience more wear but will
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not endanger the pressure boundary since it is capped.
Repositioning the thimble tubes will not result in either an increased wear rate at the currently worn region or at the new spot where wear is expected.
For these reasons, the probability of a loss of pressure boundary integrity is not impacted by this change.
i The ability to obtain measurements at the top of the active fuel region using the incore instrumentation is not impaired by retracting the instrument thimble tubes less than 2".
Retracting the thimbles 2" will still leave the l
top of the thimble above the top of the active fuel.
Incore measurements of l
the upper 15% of the fuel are disregarded per technical specifications. Thus, retraction of the thimbles by less than 2" will not impact the ability to obtain the required information in the upper reoion of the core.
The capped l
thimble is not needed to perform the Technical specification surveillance and is not identified in the Technical Specification.
Thus, capping the thimble I
will not adversely inpact the ability to ver'.y that
- e. ore parameters are f
within their limits.
Creating a New Unanalyzed Event
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l-Repositioning of the thimble has no impact on safety related components and does not introduce any new failure modes.
Thus, repositioning of the thimble does not create a new unanalyzed event.
Since the capped thimble meets all the appropriate codes and standards applicable to the RCS pressure boundary, it will assure that additional wearing of the capped thin.ble dus not cau*e a new unanalyzed event.
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Margin of Safety The applicable technical specification liTits will still be able to be deter-mined and met with the reconfigured incore inetrumer,tation.
The capping and repositioning of the thimbles ensures that th.re.. ** no reduction in the margin of safety for the RCS pressure boundary due u
. be thinning.
Thus, the margin to safety is not adversely affected.
Conclusions It is concluded that Millstone Unit No. 3 can be operated safely for cycle 2 l
with the thinned incore instrument thimble tubes and that the public health and safety is act endangered.
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