ML20086T799

Forwards Relief Request R1-21 from ASME Code Section XI Inservice Insp Sample Expansion Requirements for Studs from Reactor Vessel Flange

ML20086T799
Person / Time
Site:	LaSalle
Issue date:	12/26/1991
From:	Richter M COMMONWEALTH EDISON CO.
To:	Murley T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation
References
NUDOCS 9201070065
Download: ML20086T799 (6)

Text

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't J Downers Grove, Illinois 60515 December 26,1991 Dr. Thomas E. Murley, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555 Attn: Document Control Desk

Subject:

LaSalle County Nuclear Power Station Unit 2 Submittal of Relief Request (RI 21) for Inservice Inspection Program NBC_D_ocket Nom 50:374

References:

(a) CECO-NRR Region lli Meeting on Reactor Vessel Head Closure Studs on July 30,1991 (held at CECO's Downers Grove office).

(b) M. Richter (CECO) letter to T. Murley (NRC),

dated October 3,1991.

(c) M. Richter (CECO) letter to T. Murley (NRC),

dated August 23,1991.

(d) dated November 8,1991.R. Barrett (NRC) letter to T. Kovach Dr. Murley:

As discussed with your staff during the Reference (a) meeting, Commonwealth Edison Com 3any (CECO) is pursuing an enhanced inspection program for the reactor vessel head c osure studs at its Boiling Water Reactor (BWR) stations.

The inspection program, which exceeds the requirements of Section XI, would include the removal of a sample of studs from the reactor vessel flange in order to perform magnetic particle examinations which would supplement the enhanced ultrasonic examinations (in-place examinations) which are currently planned, These inspections will allow CECO to make informee decisions on long-term inspection and potential replacement strategies for the studs.

To support implementation of the enhanced inspection program, code relief is requested with respect to Section XI sample expansion requirements based on the results of the magnetic particle inspections periormed on the removed studs. The attached relief request presents CECO's proposed alternate sample expansion and examination methodology (see attached relief request and flow chart) for the upcorning LaSalle Unit 2 refueling outage (January 1992). The alternate sample expansion methodology substitutes bore probe ultrasonic axaminations in place of the Section XI required magnetic particle examinations. Detected flaws will be evaluated against a maximum allowable flaw size (0.48 inch) which was determined by a conservative, bounding fracture mechanics analysis. The results of that analysis were previously submitted for your Staff review (Reference (b)).

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dr. Thomas E. Murley 2-December 26,1991 The attached relief request is only applicable for the upcoming LaSalle Unit 2 refueling outage (January 1992). This relief request proposes the same examination methodology previously submitted (Reference (c)), and approved by your Staff (Reference (d)), for the current Dresden Unit 3 refueling outage.

CECO requests verbal ap roval of this relief request to support the upcoming Unit 2 tefueling outage which is schedulec to begin on January 4,1992. CECO appreciates the Staff's attention on this matter.

Please contact this office should further information be required.

Respectfully, WE0lsQ 5

Milton H. Richter Nuclear Licensing Administrator

Attachment:

Relief Request Number RI-21 for LaSalle County Station (Unit 2) cc: A. Bert Davis, Regional Administrator-Rill B.L. Siegel, NRR Project Manager-LaSalle Senior Resident inspector-LaSalle R.A. Hermann, NRR Technical Staff J.A. Davis, NRR Technical Staff K.D. Ward, Region lil

/scl:lD995:12

ATTACHMENT RELIEF REQUEST NUMBER RI-21 FOR LASALLE COUNTY STATION (UNIT 2)

/ sci:lD995:10-11

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[31 LIEF REQUEST NUMBER ' RI21 COMPONENT 1DENTIFICATION Code Class:

1

References:

Table IWB 25001 IWB 2430 Examination Category:

BG1 Item Number:

B6.20 (in place)

B6.30 (when removed)

Descript on:

Reactor Vessel Closure Stud Examination Requirements i

CODE Rf0UIREMENT Table IWB-2500-1 requires a volumetric examination of reactor vessel closure studs if lef t in place, or a surface and volumetric examination of rea: tor vessel closure head studs when removed from the flange. Removalis not a requirement at any time, IWB-2430 requires additional examinations to be performed during the current outage if examinations performed in accordance with Table IWB-25001 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 extended in the current outage to include "the remaining number of similar components...

within the same examination category....*

BASIS FOR REllEF '

Commonwealth Edison Company (CECO) discovered extensive stress corrosion cracking (SCC) in two reactor vessel closure studs at Dresden Unit 2 in iate 1988. CECO is currently analyzing the stud material microstructure and mechanical properties. CECO is also pursuing a proactive program of enhanced stud inspections which exceed the requirements of Section XI and the recommendations of General Electric Nuclear Energy (GE) Rapid Information Communicatica Services information Letter (r,1CSIL) 055, Revision 1, ' Reactor Pressure Vassel Head Stud Cracking," September 30,1991. The CECO program is also intended to include some of the additional recommtndations of Regulatory Guide 1.65.

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, at the next scheduled LaSalle Unit 2 refueling outage (L2R04, currently scheduled to begin in January of 1992),

CECO plans to perform enhanced end shot UT of all 76 RPV closure head studs. The enhanced end shot UT technique developed by CECO uses a 3/4" to *i" diameter transducer with a frequency of 3.5

. or 5 MHz; e.e s9nsitivity of the examination is maximized by setting the background noise level at about 5% tuli screen height. This technique reliably detects a 0.3" deep saw cut notch from the top end of a reactor vessel stud. Any indications fotnd 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.

CECO also plans to remove, if practicable,13 studs (approximately 1/6 of the total number of studs)-

from the flange of the LaSalle Unit 2 vessel for a wet fluorescent MT, Cattle chute studs will be excluded from the sample because they are normally withdrawn from the flange and kept dry until the end of the outage; for this reason 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.

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There ere ssverd r:asons for removing a sample of studs during L2R04 and puforming a surfaco examination:

To provide data e incipient stud cracking.

To allow for additional meta"urgical evaluation of cracking mechanisms and potential embrittlement phenomena, if cracked studs are found and replaced.

To provide a correlation between enhanced end shot UT, bore UT, and MT results, if cracked studs are found.

This information is necessary to make informed decisions on long-term inspection and potential replacement strategies.

Code structural margins will be assured thru the enhanced end shot UT of all studs, and bore probe UT sizing of all cracked studs. Enhanced end shot UT and bore probe UT results will be evaluated in accordance with ' Fracture Mechanics Based Structural Margin Evaluation for Commonwealth Edison BWR Reactor Pressu e Vessel Head Studs," GE Nuclear Energy Report GE-NE 523-93-0991, DRF 137-0010, September 1991 (submitted with an M.H. Richter (CECO) letter to T.E. Murley (NRC) dated October 3,1991). The GE structural margin evaluation is based on conservative fracture mechenics d

methodology and actual fracture toughness testing of material from one of the low-toughness Dresden Unit 2 studs. If the end shot UT is found to be nonconservative, then an expanded sample with the more sensitive bore probe UT will be performed in accordance with the methodology described in the attached flow chart. This approsch will assure that Code structural margins are maintained without 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 judged against a conservative, bounding maximum allowsble flaw size (established by the GE structural margin evaluation) which would be acceptable in all 76 studs at the same time (referred to as MAXAF on the attached flow chart). If the minimum flaw detection limit of the enhanced end shot U1 is found to be greater than the MAXAF, additiorial bore probe 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 burdeti during L2R04. And, as other utilities have found, it may be impussible to remove the desired sample of studs, without damage, within the time constraints of a refueling outage. It is estimated that complete removal of all 76 studs, assuming no stuck studs, would take 8 add;tional critical path oays and expend 6.5 ac'ditional manrem.

The proposed program is highly proactive, in that Section XI only requires a normal sensitivity end shot UT to be performed in place, and RICSIL 055 only recommends enhanced end shot UT of at least five studs. In accordance with Section XL structural margin would 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 XiIWB 2430 for the L2R04 refueling outage.

PROPOSED ALTERNATE EXAMINATION During L2R04, each LaSalle Unit 2 stud will be examined in place using enhanced end shot UT. Any flaws detected with enhanced end shot UT will be sized using bore probe UT.

If MT of a sample of studs reveals indications which are found by bore probe UT to exceed the MAXAF, 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-requireo MT sample expansion (see attached flow chart).

APPLICABLE TIME PERIOD Relief is requested for LaSalle Unit 2 during refueling outage L2R04, currently scheduled to begin in January of 1992.

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