ML20237G842
| ML20237G842 | |
| Person / Time | |
|---|---|
| Site: | Trojan File:Portland General Electric icon.png |
| Issue date: | 08/24/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20237G830 | List: |
| References | |
| TAC-65315, NUDOCS 8709020432 | |
| Download: ML20237G842 (3) | |
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UNITED STATES I
7 NUCLEAR REGULATORY COMMISSION o
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ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ON THE ULTRASONIC INSPECTION OF REACTOR COOLANT SYSTEM HOT-LEGELB0W(LOOPB)
PORTLAND GENERAL ELECTRIC COMPANY TROJAN NUCLEAR PLANT DOCKET NO. 50-344 INTRODUCTION By letter dated July 24, 1987, Portland General Electric Company (PGE) submitted 6he ultrasonic examination results of the reactor coolant system (Loop B) hot-leg elbow as required by the staff in the Safety Evaluation transmitted by letter dated June 16, 1986 (S. Varga to B. D. Withers). The staff's requirements resulted from LER 85-13, dated January 7, 1986, which stated that inoperable steam generator snubbers at Trojan may have contributed to a high stress condition of the reactor coolant system hot-leg to "B" steam generator pipe elbow.
Since in-plane flexure tests of piping elbows by Electric Power Research Institute (EPRI) have shown that cracks tend to develop on the sides of the elbows with the cracks initiating from the inside diameter (ID), the staff requested that the sides of the elbow be examined.
PGE INSPECTION RESULTS The staff's requested ultrasonic examination of the hot-leg elbow was performed by PGE between April 2 and April 12, 1987 during the 1987 refueling outage. The ultrasonic examination results were provided in Trojan Inspection Report No. IR252, " Ultrasonic Inspection of a Statically Cast Stainless Steel Elbow at the Trojan Nuclear Plant," Revision 5 Amdata. Inc., July 1, 1987 The ultrasonic examination was performed using the automated Amdata IntraSpect/98 Ultrasonic Imaging System. Seven ultrasonic indications were detected on the ID surface and mid-wall of the elbow. The indications were evaluated according to Table IWB-3514-2 of Section XI of the ASME Code in Impe11 Report No. 01-0300-1611. " Evaluation of Ultrasonic Examination Indications on RCS Loop B Hot Leg Elbow for the Trojan Nuclear Plant," Impe11 Corporation, July 1987. The Amdata and Impe11 reports were submitted as Attachments 1 and 2, respectively, to the licensee's July 24, 1987 letter.
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/ STAFF EVALUATION In general, the staff finds the ultrasonic examination performed to be accept-l able. The licensee determined that the largest indication without ultrasonic
" beam-spread correction" was one inch in length and 10% of elbow wall thickness in depth where the nominal elbow wall thickness is 3.375 inches.
The licensee further reduced the length of the indication to 0.3 iach with a
" beam-spread correction." The staff finds the " beam-spread correction" procedures inadequate in that they may underestimate the indication size, especially in cast stainless steel which has extensive ultrasonic scattering and attenuation. Thus, the staff considers -it appropriate to conservatively bound the indication size as one inch in length and 10% of elbow wall thickness. This bounding indication size exceeds the Code allowable in Table IWB-3514-2 and would require an evaluation according to IWB-3640 of Section XI of the ASME Code.
By letter dated October 31, 1986, PGE submitted a fatigue crack growth analysis for staff review relating to a fracture mechanics " leak-before-break" evaluation of the Trojan Primary loop, in Impell Report No. 01-0300-1395,
" Leak-Before-Break Evaluation of the Reactor Coolant Loop," Revision 1, Impell Corporation, October, 1986.
In this fatigue evaluation, the licensee contends that a postulated 1.5 inch long and 0.25 inch (10% nominal piping wall thickness) deep circumferential flaw at the steam generator inlet weld would have negligible growth at the end of the design life of the plant.
Although the licensee has not demonstrated that this fatigue analysis result would bound that from a similar analysis considering the size, orientation, and location of the detected indications, the staff finds that a similar postulated flaw in the elbow would have negligible growth at the end of the design life of the plant because the elbow thickness (3.375-inch nominal) is larger than the pipe thickness (2.5-inch nominal) considered in the fatigue analysis and would have lower stresses. Based on this fatigue evaluation, the staff finds that the bounding indication in the elbow would meet the requirements of IWB-3640 using loads from the Impe11 report. Thus, the staff finds that any flaw that may be associated with the detected indications would not grow to an unstable size before the next refueling outage when there will be another inspection.
CONCLUSION AND RECOMMENDATIONS Based on the information provided by PGE, we conclude that continued operation of Trojan with the detected indications is pennissible until the next refueling outage and would not significantly reduce the assurance of the plant's structural integrity or safety.
Further, PGE has committed to reexamine the same regions of the elbow volumetrically to determine if the indications have grown in size during the 1988 refueling outage.
, For the 1988 reexamination effort, we recomend that PGE develop and apply improved examination techniques to better size the indications. Further-more, the elbow thicknesses at the locations of the indications' should be measured to more accurately assess the depth of the indications.
If the size of the detected indications exceeds the Code allowable in Table IWB-3514-2, PGE clust evaluate the indications according to IWB-3640 of Section XI of the ASME Code.
Principal Contributors:
G. Johnson S. Lee i
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