ML20198D138
| ML20198D138 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 05/07/1992 |
| From: | Shields J COMMONWEALTH EDISON CO. |
| To: | Murley T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9205180141 | |
| Download: ML20198D138 (6) | |
Text
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s Commonwesith Edison m
1400 Opus Place V
- Downers Grove, Illinois 60515 May 7, 1992 Dr. Thomas E. Murley, Director Office of Nuclear Reactor Regolation U.S. Nuclear Regulatory Commission Washington, D.C.
20555 Attn:
Document Control Desk
Subject:
LaSalle County Station Units 1 and 2 In-Service Inspection Program Submittal of Relief Request hi-24 HRC Docket Nos. 50-3113nL50-374
References:
(a)
M. Richter (CECO) letter to T. Murley (NRC), dated October 3, 1991; Structural Margin Evaluation for Reactor Pressure Vessel Head Studs.
(b)
M. Richter (CECO) letter to T. Murley (NRC), dated December 26, 1991; Relief Request RI-21 for Unit 2 Reactor Vessel Head Closure Studs.
Dr Murley:
Commonwealth Edison (CECO) is oursuing an enhant.ed inspection program for the reactor vessel head closure studs at its Bolling Hater Reactors.
This inspection program will allow CECO to ske informed decisions on long-term inspection and potential replacenient strategies for the studs.
To support implementation of.the enhanced inspection program, code relief is requested with respect to ASME Section XI sample expansion 1
requirements based on the results of the magnetic particle inspections performed on the removed studs.
The attached relief request, RI-24, prest nts CECc's proposed alterrote sample expansion and exat ation methodology.
The attached relief request is applicable to btth Unit' I and 2, and it is requested that the relief extend through the remainder of the first 10 year Inspettion Interval, which will be completed after each Unit's sixth refueling outage.
Commonaealth Edison requests approval of this relief request to support the upcoming Unit I refueling outage which is scheduled to begin on September 26, 1992.
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ZTRAIN/139-1
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On; Thomas E Hurley, Director May 7, 1992 PageL2 Please contact this office should further information be requ'. red.
Respectfully, Q MM JoAnt M. Shields Nuclear Licensing Administrator
Attachment:
-Relief Request RI-24 for LaSalle County Station
~
A. Bert Davis, RegionalLAdministrator-RIII cc:
B.L. Siegel,3 NRR Project Manager-LaSalle 0.- Hills,_ Senior Resident-Inspector-LaSalle R.A.-Hermann, NRR Technical Staff J,.A.-Davis,.NRR Technical Staff K.D. Hard,-Region III h
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L ZTRAIN/139-2
.g RELIEF REQUEST NO. RI-24 70R LASALLE COUNTY STATION UNITS 1 &2 CQHPQNENT_IDENTIFICATICN Code Class:
1
References:
Table IWB-2500-1 Paragraph IWB-2430 Examination Category:
B-G-1 Item Number: B6.20 (In Place)
B6.30 (When Removed)
==
Description:==
Reactor Vessel Closure Stud Exanination Requirementi C, ODE REOUIREMENT
-LaSalle County Station is committed to the 1980 Edition, Wintc:: 1980 Addenda of ASME Section XI.
Table IWB-2500-1_ requires a volumetric examination of Reactor Vessel Closure Studs if left in placo, or a surface and volumetric examination of Reactor Vessel-closure Studs when removed from the flange.
Removal is not a requirement at any time.
IWB-2430 requires that additional examinations be performed during the current outage if examinations performed in accordance with Table IWB-2500-1, reveal indications exceeding the acceptance standards of Table IWB-3410-1.
If-indications exceeding the acceptance standards-of Table IWB-3410-1 are found as a result of the additional examinations, IWB-2430 requires examinations to be further extunded in the current outage'to include "the remaining number of similar components within the same examination category...."
-BASIS FOR RELIEF Commonwealth Edison Company (CECO) discovered stress corrosion cracking (SCC). in two reactor vessel closure studs at.Dresden Unit-2 in late 1988.
CECO is currently analyzing the stud material microstructure and mechanical pro proactive program perties.
CECO is also pursuing a of enhanced stud-inspections which exceed the requirements cf Section XI and the recommendations of General Electric Nuclear Energy-(GE)
- Rapid Information-Communication Services Inft: J.ation Letter (RICSIL) 055, Revision l',
Supplement 1,
" Reactor 1
Pressure Vessel Head Stud Cracking," March 26, 1992.
The CECO-program is also intended to include some of the additional-recommendations of Regulatory Guide
- 1. 6 5. -
GE RICSIL 055 recommends that enhanced end shot UT be performed on "at least five RPV head studs either during the next refueling outage or at the next available opportunity."
However, for the remainder of the first 10 year ISI Inspection Interval which encompasses the next two scheduled refueling outages (fifth and sixth) for LaSalle County Station Units 1 &
2, CECO-plans to perform enhanced end shot LT of all RPV closure head studs (68 in Unft 1, 76 in Unit 2).
The enhanced end shot UT technique developed by CECO utilizes a 3/4" to 1" diameter transducer with a frequency of 3.5 MHz or 5 MHz; the sensitivity of the examination is maximized by setting the background noise level at about-5% full screen height.
This technique reliably detects a 0.3" deep saw cut notch from the top end of a reactor vessel stud.
Any indications found with the enhanced end shot UT technique will be sized with bore probe UT.
The bore probe UT technique developed by CECO reliably detects a 0.1" deep saw cut notch.
At each refueling outage CECO also plans to remove, if practicable, approximately 1/6 of the total number of studs (12 in Unit 1, 13 in Unit 2) from the flange of the LaSalle Reactor Pressure vessel for a wet fluorescent MT.
Studs which are normally removed each outage to allow for'the installation of the Cattle Chute will be excluded from the sample because they are
'not exposed to the aqueous-environment likely to cause pitting.
Cracking is believed to occur when pitted studs are tensioned while still exposed to' water at the end of a refueling outage.
There are several reasons for removing a sample of studs during the remaining two refueling outages in the first 10 year ISI Inspection Interval for surface examination:
To provide dats on incipient stud cracking.
To allow for additional metallurgical evaluation of cracking mechanisms and potential embrittlement phenomena, if cracked l
studs'are found and replaced.
To provide a correlation between enhanced end shot UT, bore probe UT, and MT results,.if 2
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cracked studs are found.
This information is necessary to make informed decisions on.long-term inspection / replacement strategies.
Code structural margins will be assured through the enhanced end shot UT of_all studs, and bore probe UT sizing of all cracked studs.
Enhanced end shot and bore probe UT results will be evaluated in accordance with " Fracture Mechanics Based Structural Margin Evaluation for Commonwealth Edison BWR Reactor Vessel Head Studs," GE Nuclear Energy Report GE-NE-523 0991, DRF 137-0010,, September 1991_(submitted with a M.H. Richter (CECO) letter to T.E. Murley (NRC) dated October 3, 1991).
The GE structural margin cvaluation is based on conservative fracture mechanics mett.odology and actual fracture toughness testing of material from one of the low-toughness Dresden Unit II studs.
If the end shot is found to be nonconservative, then an expanded sample with the morv sensitive bore probe will be performed.
This approach will assure that Code structural margins are maintained with out expanding the MT sample.
Results of the enhanced end shot UT, bore probe UT, and MT will be compared in order to benchmark the minimum detection limit of the enhanced end shot UT technique.
The minimum detection limit of the enhanced end shot UT technique.will_be judge > cgeinst a conservative, bounding maximum allownele flaw size (established by the GE structural margin evaluation) which would be acceptable in all studs at the same time.
If the minimum flaw detection limit of the enhanced end shot UI is found to be greater than the maximum allowable k
flaw size,' additional bore probe UT examinations will be performed in lieu of the section XI required MT sample expansion.
Expanding the MT sample if unacceptable surface indications are found would greatly increase the
. critical path time and manrem burden during the outage.
And, as other utilities have found, it may be impossible to remove the desired sample of studs, without damage, within the time constrains of a refueling outage.
It is estimated that complete removal of all studs, assuming no stuck studs, would take 8 additional critical path days and expend 7 additional manrem.
The proposed program is highly proactive, in that Section XI only requires a normal sensitivity end shot 3
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a e
UT to be performed in place, and RICSIL 055 only recommends enhanced end shot UT of at least five studs.
In accordance with Section XI, structural margin vould still be assured by the enhanced end shot and bore probe UT.
Yet much essential information could be gained by surface examination of a limited sample of studs.
For these reasons, CECO requests relief from the MT sample expansion requirements of Section XI IWB-2430 for the remainder of the first 10 year ISI Inspection Interval for both LaSalle County Station Units 1 & 2.
PROPOSED ALTERNATE EXAMINATIE In lieu of the Code Requirement, at each refueling outage conducted in the applicable time period for LaSalla County Station Units 1 & 2 each LaSalle County Station stud will be examined in place using enhanced end shot UT. Any flaws detected with the enhanced end shot UT will be sized using bore probe UT.
If an MT exhmination of a sample _of studs reveals indications which are found by bore probe UT to exceed the maximum allowable flaw size, and were not detected by the enhanced end shot UT, then sample expansion will proceed using bore probe UT in lieu of the Section XI required MT sample expansion.
APPLICABLE TIME-PERIOD This relief is requested for each refueling outage for LaSalle County Station Units 1 & 2, beginning with the fifth refueling outage for Unit 1 which is scheduled to l
begin September 26, 1992.
It is also requested that i
the-relief extend through the remainder of the first 10 l
year Inspection Interval for each Unit (1 & 2) which l
will be completed after that Unit's sixth refueling outage.
The sixth refueling outage for LaSalle County Station Unit 1 is scheduled to end in May of 1994.
The sixth refueling outage for LaSalle County Station Unit l
2 is scheduled to end in May of 1995.
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