ML20245K762

Safety Evaluation Accepting Ultrasonic Test Indications in N2H nozzle-to-safe End Weld During Cycle 3

ML20245K762
Person / Time
Site:	Limerick
Issue date:	05/02/1989
From:	Office of Nuclear Reactor Regulation
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ML20245K753	List: ML20245K751 ML20245K762
References
NUDOCS 8905050167
Download: ML20245K762 (5)

Text

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO ULTRASONIC TEST INDICATIONS IN THE N2H N0ZZLE-TO-SAFE END WELD LIMERICK UNIT 1 NUCLEAR GENERATING STATION PHILADELPHIA ELECTRIC COMPANY DOCKET NO. 50-352

1.0 INTRODUCTION

The staff has reviewed the licensee's submittal dated April 3, 1989 including the inspection results, metallurgical and fracture mechanics analyses, and monitoring systems to support the continued operation of the Limerick Nuclear.

Generating Station, Unit 1.

Limerick Generating Station (LGS) Unit I began its second refueling outage on January 13, 1989. During this outage, a number of normally required and augmented Inservice Inspections (ISI) of austenitic stainless steel piping welds were per-formed in accordance with ASME Code Section XI and NRC Generic Letter 88-01, "NRC Position on IGSCC In BWR Austenitic Stainless Steel Piping," dated January 25, 1988. As a result of these inspections, an indication was discovered in a reactor vessel recirculation inlet nozzle-to-safe end weld. This seven inch  !

circumferential indication appears to exhibit the vracteristics of intergranular stress corrosion cracking (IGSCC). A review of a s .struction radiograph of the weld, recently enhanced using the latest computer tecic lues, indicates the l possibility of a pre-exiting flaw (i.e., lack of weldment fusion) at the location of the indication. If this indication is in fact a crack, a pre-existing flaw would provide a plausible explanation for the measured size of the indication considering the time the component has been in service. All remaining recircula-tion inlet nozzles were inspected, and no additional reportable indications were found.  !

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j Originally, safe ends made of Inconel 600 were installed on the N2 (i.e., recir-culation inlet) nozzles in the shop. These original safe ends were subsequently )

removed and new safe ends of 316L stainless steel were installed with alloy 82

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weld metal prior to Unit 1 operation. Also, these replacement safe ends contained an alloy 182 butter. The firal materials configuration of the nozzle-to-safe end weld is shown in Figure 1. The indication is located in the alloy 182 and/or Inconel 600 materials on the nozzle side of the weld, and is also shown in Figure 1.

2.0 DISCUSSION Description of Indication A seven inch circumferential flaw indication, characteristic of intergranular stress corrosion cracking (IGSCC), was found as a result of the inspections.

This indication is located on the nozzle side of the weld, in the alloy 182 butter and/or Inconel 600, and is behind the therral sleeve. Using a wall thickness of 1.40 inches, the average depth of the indication is 0.25 inches (approximately 18% through wall), and a single indication run of 0.5 inches in length is 0.40 inches deep (approximately 29% through wall), initiating from the inside diameter and extending toward the outcide diameter. The indication is located 31.8 inches to 38.8 inches from top dead center of the nozzle, clockwise with flow.

Crack Growth Rate Assessment Assuming that the nozzle-to-safe end weld indication is a crack of the dimensions specified above, a fracture mechanics analysis was performed using conservative assumptions. The analysis concludes acceptable structural margins will exist during Cycle 3 for the N2H nozzle-to-safe end weld with the indication in the I

"as found" condition. This conclusion is based on the use of a bounding crack growth ste value that reflects a more severe environment (i.e., water conducti-vity) than the expected service environment that the nozzle-to-safe end weld in-dication will be subjected to during Cycle 3 operation.

On-Line Monitoring and Additional Inspections In order to confirm the results of the analyses described above and provide additional assurance that the structural integrity margin of the nozzle-to-safe end weld is maintained during Cycle 3 operation, a Crack Advance Verification l System (CAVS) will be installed and in operation either prior to startup from the I current Unit i refueling outage or shortly after startup. The CAVS is a computer-controlled, real time (i.e., on line) crack monitor. The CAVS utilizes a i potential drop technique to measure the crack growth which occurs in pre-cracked j fracture mechanics (i.e., compact tension) test specimens loaded in an autoclave through which Unit I reactor recirculation water will flow. Monitoring of the specimen crack growth is based on reversing direct current technology. The CAVS allows the effects of steady state and transient plant water chemistry conditions to be quantified, on-line, in terms of their effects on the growth of a sample specimen of the same material (alloy 182) as the existing N2H nozzle-to-safe end weld indication.

The Philadelphia Electric Company (PEco) has used the CAVS at Peach Bottom Atomic PowerStation(PBAPS). That experience has shown that the CAVS outpu. correlates well with actual reactor water chemistry in that there is predictable response to reactor water chemistry transients as well as steady state conditions.

Furthermore, the CAVS has been previously operated for over 9000 hours0.104 days <br />2.5 hours <br />0.0149 weeks <br />0.00342 months <br /> at PBAPS.

Finally, the licensee has improved the Unit i reactor water chemistry by imple-mentation of the Condensate Filter Demineralized Optimization Program and by following the Electric Power Research Institute (EPRI)/BWR Owners Group Chemistry Guidelines. The licensee has committed to continue to operate the plant within these guidelines.

In order to provide added assurance that the CAVS is providing representative results, the licensee has committed to perform the following inspections during the operating cycle if necessary: l l

J If after nine months of Cycle 3 operation, the CAVS specimen indicates a l crack growth greater than or equal to 0.2 inches, the nozzle-to-safe end weld indication will be inspected if the plant is shutdown for a forced outage that is planned to last for greater than two weeks.

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4 If after nine . ths of Cycle 3 operation, the CAVS specimen indicates a crack growth greater than or equal to 0.3 inches, the plant will be shutdown and an examinatio's of the nozzle-to-safe end weld indication will be performed.

Also installed is an acoustic emissions (AE) crack manitoring system for the purpose of evalueting its usefulness for monitoring crack growth.

Although not discussed in the April 3, 1989 submittal, the NRC staff would expect a marked sustained increase in valid acoustic emissions should cracking extend further than anticipated in the throughwall direction during the next operating cycle. While AE is not the method in use at this time to quantify the extent of crack growth, we request that PECO and its contractor evaluate the usefulness and accuracy of the AE results by comparing AE data to CAVS data after completion of Cycle 3. This information should be of use in assessing future applications of this experimental crack mone c*ng system.

3.0 CONCLUSION

Based upon the review of the licensee's submittal, the staff has concluded that there is reasonable assurance the facility can be safely operated during Cycle 3 with the N2H vessel nozzle-to-safe end weld in its current condition.

Supplementary information on the crack growth rates will be obtained by CAVS monitoring and acoustic emission during the fuel cycle; therefore, if crack growth were to occur more than anticipated there is reasonable assurance the I

l licensee would be aware of this unanticipated increase. The corrective action plan for ultimate disposition of the N2H indication is to be established by the licensee prior to the next refueling outage.

Principal Contributor: H. Gray and R. Clark Dated: May 2, 1989 1

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