ML20070B976
| ML20070B976 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 06/17/1994 |
| From: | Schrage J COMMONWEALTH EDISON CO. |
| To: | Russell W NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9407010155 | |
| Download: ML20070B976 (8) | |
Text
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N Commonwealth Edison i/ 1400 Opus Place Donners Grove, !!hnois 60515 June 17,1994 Mr. William T. Russell, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington D.C.
20555 Attn.' Document Control Desk
Subject:
Quad Cities Station Unit 1 Flaw Evaluation for Core Spray Crack Indication NRC DonkeLNo 50-25_4
Reference:
Teleconference between USNRC (C. Patel, R. Hermann, et al) and CECO (J. Schrage, R. Walsh, et al) on June 13,1994 Mr. Russell, During the current Quad Cities Station Unit 1 refuel outage (Q1R13), CECO identified a crack indication in the 'B' Core Spray header, in the referenced teleconference, the NRC (NRR and Region Ill representatives) indicated that the flaw evaluation for this crack indication should be submitted to the Staff. The Attachment to this letter provides this evaluation as the basis for acceptability of operating with the indication through one operating cycle. This will allow CECO to investigate remote repair options which are necessary to avoid significant personnel exposure associated with this project.
The flaw evaluation includes a structural analysis, a crack leakage estimate, a LOCA analysis impact, and a loose parts analysis. Based upon these analyses, CECO has concluded that Quad Cities Station Unit 1 can safely operate during the next operating cycle (01C14) with the existing crack in the 'B' Core Spray header.
if there are any further questions or comments, please contact John L. Schrage at 708-663-7283.
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Mr. W. T. Russell June 17,1994 Very truly yours,
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1 John L.
chrage Nuclear Licensing Administrator Attachment cc:
J. Martin, Regional Administrator - Region 111 C. Miller, Senior Resident inspector - Quad Cities Station C. Patel, Project Manager - NRR Office of Nuclear Facility Safety - IDNS l
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ATTACHMENT
'B' CORE SPRAY HEADER FLAW EVALUATION INTRODUCTION The in-vessel visual inspection performed during the current Unit 1 refueling outage (Q1R13) identified a crack indication in the 'B' Core Spray header where the piping and the junction (tee) box meet, as shown in Figure 1. The function of the junction box and the attached distribution pipes is to direct Core Spray flow from the vessel inlet nozzle through the j
annulus area to the spargers inside the shroud. The integrity of this safety-related piping system must be maintained to ensure that Core Spray flow can be delivered to the core.
Design and fabrication activities for a mechanical clamp were pursued for installation (local installation by divers) during Q1R13, however, CECO subsequently determined that this local repair effort would be dose prohibitive (a minimum dose projection of 12 rem). As a result, this evaluation provides the basis for the acceptability of operating with this crack through one operating cycle, affording the opportunity to investigate remote repair options for implementation during the next refueling outage (01R14).
DISCRIPTION i
The crack in the 'B' Core Spray header is located where the piping and junction box (south side of junction box) meet in the heat affected zone of the weld. The crack is visually estimated to be 7 inches in length, covering approximately 120 of the pipe circumference (using the top of the Core Spray pipe as a reference of 0 azimuth, the crack was located from approximately 30 azimuth to 150 azimuth). Figure 2 provides a schematic of the
'B' Core Spray header, detailing the crack location as well as the location of the closest restraints. Two standard bolted brackets are located +/- 60" from the junction box, and two additional clamps are welded +/- 10 from the junction box. The clamps closest to the junction box (+/- 10 ) provide additional vertical structural support of the Core Spray piping.
The crack is most likely caused by intergrannular stress corrosion cracking (IGSCC) based on the following:
the pipe materialis Type 304 high carbon stainless steel; the crack is in the heat affected zone of the junction box weld; there is a high potential for the presence of welding residual stress andbending stress (due to installation configuration);
the coolant in this area is highly oxidizing and hydrogen water chemistry is not effective in this region; the characteristics of the crack are s:milar to those typically demonstrated by IGSCC and similar cracking, caused by IGSCC, has been experienced at other BWR facilities (reference I.E.Bulletin 80-13).
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2-l EVALUATION 1
An evaluation of the significance of the Core Spray pipe crack was performed which included the following elements:
a structural analysis; a crack leakage estimate; a LOCA analysis impact; and a lost parts analysis.
StructuralIntegrity A structural analysis was performed in order to verify that the integrity of the Core Spray piping (with the crack) will be maintained for all conditions of operation over the next operating cycle.
A crack arrest evaluation was performed considering normal operating load stresses, weld residual stresses, and pipe restraint stresses. Since the applied normal loading is predominantly displacement controlled, the stresses relax as the crack grows, and the compliance (or flexibility) of the pipe increases. Studies have shown that when the crack reaches 180 of the circumference, the compliance is increased sufficiently to relieve almost all of the displacement controlled stresses.
Consequently, the crack growth is expected to be negligible, or at virtual arrest, prior to reaching 180.
Although the crack can be expected to arrest prior to reaching 180, an evaluation (utilizing a limit-load approach) was performed to determine the maximum allowable circumferential through-wall flaw size for the Core Spray pipe. The loads considered in this evaluation were deadweight, seismic, downcomer flow impingement, pressure and system flow. Stresses due to water hammer loads were considered insignificant and neglected given: the ' keep-fill' feature of the system; the opening time of the -
injection valve (approximately 10 seconds); and the fact that the header piping is filled with water during system actuation (non-condensibles are vented through the vent hole on the top of the header junction box). The results of this evaluation confirm that the Core Spray pipe can tolerate a through-wall crack of up to 240 (at the crack location) without incipient f ailure. This crack length is greater than the maximum estimated crack length projected at the end of the next operating cycle (8.4 inches or approximately 150 of the pipe circumference, see ' Crack Leakage Estimate' discussion), therefore, CECO concluded that the structual integrity of the piping with a crack will be maintained for all conditions of operation.
Attachment (cont.)
l Crack Leakage Estimate Since there is no direct measurement of leakage from the crack during the operation of the Core Spray System, an estimated crack leakage (supported by a crack growth evaluation) was pursued.
The crack growth evaluation was performed assuming IGSCC for crack propagation.
Based upon a conservative crack growth rate of 5x10 inches / hour, CECO estimated that the current crack size would grow less than 1.4 inches during the next 18 month operating cycle. The total crack length at the end of the next operating cycle is estimated to be apprcximately 8.4 inches (or approximately 150 of the pipe circumference).
For the determination of the maximum leakage estimate through the crack, a 180 (9.97 inches) through-wall crack configuration was used. This configuration was considered to be the upper bound based on the crack arrest results, and is also conservative to the estimated crack length after one additional cycle of operation (8.4 inches). A crack width of 0.01 inch was conservatively assumed based on the results of Linear Elastic Fracture Mechanics (LEFM) methods which showed the crack opening to be <0.01 inch under the applied loads (deadweight, reismic, downcomer flow impingement, pressure, and system flow). Using the aforementioned assumptions, the leak rate was determined to be 28 gpm.
LOCA Analysis Impact An assessment was performed to estimate the LOCA Analysis impact based on a 28 gpm leak rate from the Core Spray header crack. It was estimated that the bounding peak cladding temperature (PCT) increase resulting from the Core Spray leakage is 15 degrees F (based on the core cooling delay caused by the leakage from the Core Spray header). This estimated increase in PCT is insignificant based on the margin in the current LOCA analysis.
Lost Parts Analysis Based on the structural analysis, lost parts (looso pieces) in the reactor from the Core Spray header are not expected. Nevertheless, CECO performed an evaluation of the possible consequences of a potentialloose piece. Two different types of loose pieces were postulated: a section of core spray pipe; and a small piece of the core spray pipe. CECO concluded that:
the probability of unacceptable flow blockage of a fuel assembly or unacceptable control rod interference due to lost parts is negligible; the potential for corrosion or other chemical reactions with reactor materials does not exist because the piping materialis designed for in-vessel use; and
Attachment (cont.) -
the loose pieces are not expected to cause damage to the other reactor pressure vessel internals.
CONCLUSION Based on the evaluation performed, CECO has concluded that Quad Cities Unit 1 can safely operate during the next operating cycle (Cycle 14) with the existing crack in the 'B' Core Spray header. Additionally, CECO has determined that no unreviewed safety question exists. Remote repair options are currently being investigated for implementation during the next Unit 1 refueling outage (Q1R14).
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F FIGURE 1 LOCATION OF CORE SPRAY CRACK J
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FIGURE 2 TOP VIEW OF 'B' CORE SPRAY llEADER Jtinction [ tee}
box clamps [2]
cra k brackets [2]