ML20086R380

Forwards Relief Request CR-18 from ASME Code,Section XI Inservice Insp Program Based on Results of Magnetic Particle Insps of Studs from Reactor Vessel Flange

ML20086R380
Person / Time
Site:	Quad Cities
Issue date:	12/24/1991
From:	Richter M COMMONWEALTH EDISON CO.
To:	Murley T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation
References
NUDOCS 9201020024
Download: ML20086R380 (6)

Text

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Commonwoalth Ediscn 2f 1400 opus Place v

Downers Grove. Illinois 6051$

December 23,1991 Dr. Thomas E. Murley, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555 i

Attn: Document Control Desk

Subject:

Quad Cities Nuclear Power Station Unit 2 Submittalof Relief Request (CR 18) for Inservice Inspection Program NB0_DocketNou50:205

References:

(a) CECO NRR Reglon lli Meeting on Reactor Vessel Head Closure Studs on July)30,1991 (held at CECO's Downers Grove office 1

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

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dated Octo 3er 3,1991.

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-(c) dated August 23,1991.M. Richter (CECO) letter to T. Mj (d) dated November 8,-1991.R. Barrett (NRC) letter to T. Kov Dr. Murley; i

As discussed with your staff during the Reference a) meeting, for the reactor vessel head c osure(CECO)is aursuing an enh(anced inspection program Commonwealth Edison Com 3any studs at lLs Bolling 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

- wiii allow CECO to make informed 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 parilcle inspections performed on the removed studs. The attached relief request presents CECO's proposed alternate sample expansion and

- Quad Cities Unit 2 refuelin(g outage (January 1992). The alternate sample expa methodology substitutes bore proas ultrasonic examinations in place of the Section XI required magnetic particle examinations. Detected flaws will be evaluated against a maximum al owable flaw size (0.67 inch) which was determined by a conservative, j

boundlog 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 23,1991 l

refuellng outage (January 1992) quest is only applicable for the upcoming Quad The attached relief re

. 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 3roval of this relief request to support the upcoming Unit 2 refueling outage which is schedulec' to begin on January 1,1992. CECO appreclates the j

Staff's attention on this matter.

Please contact this oilice should further information be required.

Respectfully, f)]Es Milton H. Richter Nuclear Licensing Administrator

Attachment:

Rollef Request Number CR 18 for Quad Cities Station (Unit 2) cc: A. Bert Davis, Regional Administrator Rill L.N. Olshan, NRR Project Manager Ouad Cities T.E. Taylor, Senior Resident inspector Ouad Cities R.A. Hermann, NRR Technical Staff J, A. Davis, NRR Technical Staff K.D. Ward, Region ill

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1 ATTACHMENT RELIEF REQUEST NUMBER CR-18 FOR QUAD C'ITIES STATION (UNIT 2)

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, REllEF REQUEST NUMBER CR.18 COMPONENT IDENTIFIC ATION Code Class:

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References:

Table IWB 25001 lWB 2430 Examination Category:

BG1 Item Number:

B0.20 (in place)

D0.30 (when removed) i

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Description:==

Reactor Vessel Closure Stud Examination Requirements CODE REQUIREMENT Table IWB 25001 requires a volumetric examination of reactor vessel closure studs if lef t in place, or a surface and volumetric examination of reactor vessel closure head studs when removed from the flange. Removalis not a requirement at any time.

lWB 2430 requires additional examinations to 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 34101. If indications exceeding the acceptance standards of Table IWB-34101 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 late 1988. CECO is currently analyzing the stud matsdal 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 Communication Services information Letter (RICSIL) 055, Revision 1, " Reactor Pressure Vessel Head Stud Crac'"ng," September 30,1991. The CECO program is also intend)d to include t sme of the additional recommendations of Regulatory Guide 1.65.

GE RICSIL 055 recom nends 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 Quad Cities Unit 2 refueling outage (02R11, currently scheduled to begin in January of 1992), CECO plans to perform enhanced end shot UT of all 92 RPV closure head studs. The enhanced end shot UT technique developed by CECO uses a 3/4* to 1" diameter transducer with a frequency of 3.5 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 sired 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,' 16 studs (approximately 1/6 of the total number of studs) l from the flange of the Ouad Cities 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 cre several rocsons for removing a sample of studs during 02R11 and performing a surfcce e,xamination:

To provide data on incipient stud cracking.

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

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

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

Code structural margins will be assured thru the nhanced 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 Pressure 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 mechanics 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 at*3ched flow chart. This approach 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 allowable flaw tire (established by the GE structural margin evaluation) which would be acceptable in all 92 studs at the same time (referred to as MAXAF on the attached flow chart). If the minimum flaw detectior 'imit of the enhanced end shot UT ;s found to be greater than the MAXAF, additional bore probe exaininations will be performed in lieu cf the Section XI required MT sample expansion.

Expanding the MT r. ample if unacceptable surf ace indications are found would greatly increase the critical path time and manrem burden during 02R11. And, as other utilities have found, it may be impossible to remove the desired sample of studs, without damage, within the tirno *:onstraints of a refueling outage. It is estimated that complete removal of all 92 studs, assuming no siuck studs, would take 10 additional critkal path days and expend 8 additional 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 Sectinn XI, structural margin would still be assured by the enhanced end shot and bore probe UT. Yet much essentialinformation could be gained by surface examination of a limited sample of studs. For these reasons, CECO requests relief from the MT sample expansion l

requirements of Section XIIWB 2430 for the 02R11 refueling outage.

PROPOSED ALTERNATE EXAMINATION l

During 02R11, each Unit 2 stud will be examined in place using enhanced end shot UT. Any flaws l

detected with enhanced end shot UT will be sired using bore probe UT.

If v " of a sample of studs reveals indications which are found by bore probe UT to exceed the tv

.AF, 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 (see attached flow chart).

APPLICABLE TIME PERIOD I

Relief is requested for Quad Cities Unit 2 during refueling outage 02R11, currently scheduled to begin p

in January of 1992.

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