ML20199K771
| ML20199K771 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 01/21/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20199K753 | List: |
| References | |
| NUDOCS 9901270021 | |
| Download: ML20199K771 (5) | |
Text
_.
pME:o
,te t
UNITED STATES g
j j
NUCLEAR RECULATORY COMMISSION o
2 WASHINGTON, D.C. 30686-0001 s,...../
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE INSERVICE INSPECTION PROGRAM REQUEST FOR RELIEF FROM ASMF CODE REPAIR REQUIREMENTS FOR ASME CODE CLASS 3 PIPING STP NUCLEAR OPERATING COMPANY SOUTH TEXAS PROJECT. UNIT.2 DOCKl:T NO. 50-499
1.0 INTRODUCTION
10 CFR 50.55a(g) requires nuclear power facility piping and components to meet the applicable l
requirements of Section XI of the American Society of Mechanical Engineers (ASME) Boiler l
and Pressure Vessel Code (hereafter referred to as the Code).Section XI of the Code specifies Code-acceptable repair methods for flaws that exceed Code acceptance limits in piping that is in-service. A Code repair is required to restore the structural integrity of ilawed Code piping, independent cf the operational mode of the plant when the flaw is detected.
Those repairs not in compliance with Section XI of the Code are non-Code repairs. However, the implementation of required Code (weld) repairs to ASME Code Class 1,2 or 3 systems is often impractical for nuclear licensees since the repairs normally require an isolation of the l
system requiring the repair, and often a shutdown of the nuclear power plant.
l Alternatives to Code requirements may be used by nuclear licensees when authorized by the l~
Commission if the proposed alternatives to the requirements are such that they are shown to l
provide an acceptable level of quality and safety in lieu of the Code requirements [10 CFR 50.55a(a)(3)(i)), or if compliance with the Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety [10 CFR 50.55a(a)(3)(ii)).
l A licensee may also submit requests for relief from certain Code requirements when a licensee has determined that conformance with certain Code requirements is impractical for its facility l
(10 CFR 50.55a(g)(5)(iii)). Pursuant to 10 CFR 50.55a(g)(6)(i), the Commission will evaluate determinations of impracticality and may grant relief and may impose alternative requirements as it determines is authorized by law and will not endanger life or property or the common defense and security and is otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
Generic Letter (GL) 90-05, entitled " Guidance for Performing Temporary Non-Code Repair of ASME Code Class 1,2 and 3 Piping," and dated June 15,1990, provides guidance for the staff in evaluating relief requests submitted by licensees for temporary non-Code repairs of Code 9901270021 990121 ENCLOSURE PDR ADOCK 05000499 G
i t
i l
l Class 3 piping. For the purpose of this generic letter, impracticality is defined to exist if the flaw detected during plant operation is in a section of Class 3 piping that cannot be isolated for l
completing a Code repair within the time period permitted by the limiting condition for operation I
of the affected system as specified in the plant technical specifications, and performance of Code repair necessitates a plant shutdown.
l
2.0 BACKGROUND
l During plant operation, the licensee discovered a small discolored area at the bottom of cast l
flange at orifice #FO6956 on 3-inch line EW-2410 in the Essential Cooling Water System j
(ECWS) at the South Texas Project. The ECWS is a low pressure system with normal operating pressures of approximately 50 psig and the design pressure is 120 psig. The flaw l
was located in Unit 2 piping flange and the flawed pipe flange material was identified to be cast l
alurninum-bronze (ASME SB 271 CA 952 material). The discolored area was located at the l
bottom side of the flange. The discoloration was removed with a wire brush, but it returned within a few weeks which indicated that dealloying is taking place in the aluminum-bronze cast fitting. By letter dated December 22,1997, the licensee requested relief from the ASME Code,Section XI repair or replacement requirements under the provisions of 10 CFR 50.55a(g). The licensee based its request for relief on the results of a flaw evaluation that was performed by the licensee in accordance with the guidelines and acceptance criteria contained in GL 90-05.
3.0 LICENSEE'S REllEF REQUEST 3.1 Comoonents for Which Relief is Reauested ASME Code Class 3 essential cooling water system piping (one 3-inch cast pipe flange).
3.2 Section XI Edition for South Texas Project. Unit 2 1983 Edition including Summer 1983 Addenda of the ASME Code,Section XI.
3.3 ASME Section XI Code Reauirement The ASME Code Section XI requires that repairs or replacements of ASME Code Class components be performed in accordance with rules found in Articles IWA-4000 or IWA-7000, respectiva!y. These rules serve to provide an acceptable means of restoring the structural integrity of a degraded Code Class system back to the original design requirements.
3.4 Conte-nt of the Relief Reauest Relief is sought from performing a repair or replacement of one 3-inch cast pipe flange (Unit 2 l
line EW-2410 at orifice #FO6956) per the requirements of Article IWA-4000 or IWA-7000, l
respectively. Relief is being sought until the next South Texas, Unit 2 refueling outage which was scheduled to take place during the latter part of 1998.
3.5 Basis for Relief l
Request for relief has been submitted by the licensee under 10 CFR 50.55a(g)(i) (presumably, the licensee meant 10 CFR 50.55a(g)(6)(i)), and alternatives to the Code requirements have a
d
L 3
been proposed by the licensee. The licensee has evaluated the flaw in accordance with the guidance provided in GL 90-05. Based upon the evaluation, the licensee established that the discovered flaw satisfies the criteria for non-code repair as described in GL 90-05 and performing permanent repair in accordance with the ASME Code prior to an extended outage or refueling as permitted by the limiting condition for operation may not be practical due to the potential for fit-up problems during repair. The licensee also stated that it prefers to perform the l
Code repairs under controlled conditions during a scheduled outage longer than allowed by a limiting condition of operation. Further, the licensee determined that the dealloying degradation of the piping is a slow process and rapid or catastrophic failure is not a consideration. In addition, the ECWS is a low pressure system with normal operating pressure of approximately 50 psig and therefore severe failure consequences associated with high energy lines are not applicable for the ECWS.
l 3.6 Licensee's Alternative Proaram
)
1.
Weekly monitoring for qualitative assessment of leakage (quantitative if measurable leaks are observed). The licensee stated that there was no measurable leakage.
2.
Continuation of ECWS large bore piping periodic walkdowns. This walkdown is a I
regularly scheduled VT-2 examination. The licensee stated that this inspection technique has proven to be an effective means of identifying dealloyed/ cracked components prior to deterioration of structuralintegrity margins below ASME Section XI requirements.
l 4.0 STAFF EVALUATION AND CONCLUSIONS 4.1 Ooerabiliiv Determination. Root Cause Analysis and Structural intearity Evaluation The licensee determined that the flaw was located in the ECWS which is classified as ASME Code Class 3 system. The flaw was located in Unit 2 3-inch cast pipe flange. The flawed pipe material was identified to be cast aluminum-bronze which is inherently ductile material (ASME SB 271 CA 952 material). Upon discovery of the flaw, the licensee performed an evaluation of the flaw using the guidance provided in Generic Letter 90-05 and found that the flaw satisfies the through-wall criteria prescribed in GL 90-05 and that the flaw meets the criteria for a non-Code repair. The licensee determined that the operability of the system will not be impaired because there was no measurable leakage, and any leakage would be detected before the flaw reaches a limiting size that would affect the operability of the ECWS.
The licensee performed a root cause analysis of the flaw, and determined that the degradation resulted from dealloying. The licensee believes that a preexisting flaw was present to enable initiation of the dealloying process. Ultrasonic examination of the Unit 2 3-inch flange found no linear indications. Ultrasonic examination and all other types of nondestructive testing have not been able to delineate the depth and extent of dealloying. The limited extent of discoloration indicated that the degree of dealloying is relatively minor. The problem of dealloying of castings has also been described in previous communications with the NRC.
i l
l i
k 4
4.2 Auamented insoection The flaw was located in a 3-inch cast flange. The flaw was visually and ultrasonically examined to assess the flaw. Augmented weekly inspections were performed and designed to enhance the detection of even minor changes in the external expression of through-wall dealloying whether it is the quantitative measurement of leakage or the size of the area of discoloration.
4.3 Temocrary Noncode Reoair and Monitorina Provisions The licensee monitored the flawed area weekly for qualitative assessment of leakage. In addition, the licensee performed its ECWS large bore piping periodic walkdowns. If a measurable leak were identified, a quantitative assessment of the leak would have been performed. In its December 22,1997, letter the licensee stated that repair of the flaw will be deferred until adequate time is available for the repair, but no later than the next scheduled Unit 2 refueling outage (during the latter part of 1998).
By letter dated December 14,1998, the licensee stated that a repair that meets ASME Section XI Code requirements was completed on May 12,1998. During a phone call with the licensee on December 30,1998, the staff asked the licensee how they were able to perform the repair prior to the Unit 2 refueling outage, which began on October 3,1998. The licensee explained that at the time the flaw was discovered, it was not possible to perform the repair within the time allowed by the limiting condition for operation because the time needed to perform the repair includes preparation time and installation time. Preparation time includos purchase and/or l
fabrication of replacement parts. When the licensee had completed the preparatory work and found an opportunity to perform the installation within the time allowed by the limiting condition l
for operation, the licensee decided to perform the repair prior to the Unit 2 refueling outage.
The staff finds that the licensee's explanation is consistent with its December 22,1997, letter.
4.4 Staff Conclusions The staff has determined that the licensee's flaw evaluation has been consistent with the guidelines and acceptance criteria of GL 90-05. The staff, therefore, finds the licensee's structuralintegrity and operability assessments to be acceptable. The licensee monitored the flawed area weekly for qualitative assessment of leakage. In addition, the licensee continued its ECWS large bore piping periodic walkdowns until the Code repair was performed in May 1998. The licensee's actions constituted an acceptable temporary alternative to the Code requirements. Furthermore, the staff finds that performance of an immediate Code repair would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety since the repair may not have been completed within the time period permitted by the limiting condition for operation and thus an isolation of the affected ECWS piping would have been required. Such an isolation is not in the best interest of plant safety, given the magnitude of the flaw and the reasonable assurance provided by the licensee's attemative program. Further, the licensee determined that the leakage can be detected before the flaw reaches a limiting size that would affect the operability of the ECWS (there was no leakage).
o-h i
5 r
The staff, therefore, concludes that the licensee's proposed alternatives to the Code requirements are authorized pursuant to 10 CFR 50.55a(a)(3)(ii).
Principal Contributors: G. Georgiev T. Alexion j
r Date: January 21,1999 i
l l
i l
t j-i