ML20203E730
| ML20203E730 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 02/17/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20203E711 | List: |
| References | |
| NUDOCS 9802270117 | |
| Download: ML20203E730 (7) | |
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O' l'. UNITED STATES
!* j NUCLEAR REGULATORY COMMISSION g g WASHINGTON, D.C. 2006H001 f
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE MECHANICAL NO22L E SEAL ASSEMBLY SOUTHERN CALIFORNIA EDISON COMPANY
. SAN ONOFRE NUCLEAR GENERATING STATION. UNITS 2 AND 3 DOCKET NOS. 50-361 AND 50 362
1.0 BACKGROUND
By letter dated July 11,1997, Southern Califomia Edison (SCE or the licensee) requested relief from the 10 CFR 50.55a repair requirements as implemented through the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code to permit installation of mechanical nozzle seal assemblies (MNSAs) as repairs of currently installed Inconel alloy 600 (A600) instrument and sample nozzles on reactor coolant system components in San Onofre Nuclear Generating Station (SONGS), Units 2 and 3. These nozzles are welded to the shell walls with J-groove ("J") welds, wuich have been found to be susceptible to wking as a result of stress corrosion. SCE intends to replace all A600 nozzles with inconel e.m o90 (A690) nozzles, except on the pressurizer and the steam generator cold leg channel heads.
The replacement of these nozzles will start with the 1998 mid-cycle outages. The MNSAs .
would be installed during the mid cycle outages on the pressurizer A600 nozzles, on any A600 nozzles found leaking on the steam generator cold leg channel heads, and on any leaking A600 nozzles located on the reactor coolant system (RCS) hot legs below the mid-loop water level.
In its letter dated November 20,1997, the staff requested additional information from SCE
- regarding the MNSAs. SCE responded to the request for information by letter dated December 12,1997.
The staff met with SCE on December 23,1997 to discuu the use of the MNSAs. In its letter dated January 5,1998, SCE provided a written response to staff questions identified during the meeting. As a part of this submittal, SCE agreed with a recommendation by the NRC to change their submitta! from a request for relief to a request for an attemative, pursuant to 10 CFR 50.55a(a)(3)(i). A follow-up letter dated January 29,1998, from the licen ee provided additional clarification of the MNSA design, and included a change to the Technical Specifications (TS)
Bases to reflect how any leakage from a MNSA would be handled.
The "J' welds on instrument nozzles have the dual purpose of rettraining the nozzle in place and preventing leakage through the annulus between the nozzle and the wall. MNSAs are intended to be installed on leaking, or poter.tially leaking, existing instrument nozzle penetrations on RCS components, in lieu of using external welds or half nozzle replacements b repair these leaking nozzles, as is typically done. The MNSA is intended to provide sealing and structuralintegrity by acting as a complete replacement of the "J' weld.
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I 2-2.0 DISCUSSION
- 2,1 Description A MNSA is a mechanical devim consisting of a split gasket / flange assembly placed around the instrument nozzles. The gasket is made of Grafoil packing, a graphite compound which is compressed within the assembly to prevent RCS leakage past the nozzle. This assembly is bolted into holes drilled and threaded on the outer surface of the RCS component wall. Another assembly is bolted to the flanges, which serves as the structural attachment of the nozzle to the wall. This assembly serves to carry the loads in lieu of the "J" welds on the A600 nozzles.
MNSAs are designed, fabricated and constructed, using approved materials by Asea Brown Boveri/Combus6on Engineering (ABBICE) as a Class 1 component in accordance with the ASME Boiler and Pressure Vessel Code, Section lil,1989 Edition.
2.2 Licer, sing Basis Sectior; 50.55z of Title 10 of the Code of Federal Regulations as implemented through the ASME Code,Section XI, Paragraphs IWB-3132 or IWB-3142 require mechanical removal of the flaw, of repairs to the components to the extent necessary to meet the acceptance standards of IWB-3000. The licensee has requested, pursuant to 10 CFR 50.55a(a)(3)(l), that the MNSAs :
be allowed as an attemative to the specified requirement. The request is based on the -
licensee's conclusion that the proposed alternative provides an acceptable le"el of quality and -
safety. The licensee provided the following reasons to support this conclusion:
- 1. The MNSA is designed, fabricated, and constructed, using approved material in accordance with the rules of ASME BP&V Code, Section Ill.
' 2. The MNSA is designed to prevent separation of the joint under all service loadings. This has been demonstrated by analysis and testing.
- 3. The MNSA is accessible for maintenance, removal, and replacement after installation.
- 4. 1The MNSA prototype has been subject to additional seismic, thermal transient, and hydrostatic pressure testing to demonstrate that the pint will remain leak tight under expected service conditions.
By letter dated January 29,1998, the licensee revised the TS Bases section related to RCS leakage to define leakage past a MNSA as pressure boundary leakage. TS Section 3.4.13 states that no pressure boundary leakage is allowed during operational modes 1 through 4.
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3-3.0 INSTAt i ATION The licensee has committed to replace all A600 nozzles on the RCS hot and cold legs with A690 nozzles, using the 'J" weld technique on the outside pipe surface by the end of the next refueling cycle for each unit. . Dunng the 1998 mid-cycle outages, the licensee plans to replace a!I RC3 co!d leg A600 nozzles, and all RCS hot leg A600 nozzles located above the mid loop water level.
RCS hot leg A600 nozzles located below the mid-loop water level (20 on Unit 2 and 15 on Unit 3), which cannot be replaced without draining below mid-loop (necessitating a full core off. l load), will t'e inspected for leakage Should leakage be identified in one of these nozzles, SCE will install a MNSA as an interim repair. The nozzle and the MNSA will be replaced by the end of the next refueling outage for sach unit with an A690 nozzle and a "J" weld on the outer surface of the pipe. Utiliz.ing the MNSAs for this application will avoid the radiation exposures essr,,cated with reactor disassembly, core offload, reload, and rcector reassembly, SCE plans to install MNSAs on a permanent ? - - < u the 5 remaining pressurizer A600 nozzles (2 in Unit 2 and 3 in Unit 3), as an attemative U y scing these with A690 "J" welded nozzles.
SCE stated that these nozzles are located in an area with difficult accessibility. SCE also plans to install permanent MNSAs on any of the A600 nozzles on the steam generator cold leg
. channel heads (8 per unit), if leakage in these nozzles is detected. The welded replacement technique needed for both of these areas consists of o time-consuming welded pad build up process, which results in higher radiation exposures to personnel when' compared to the radiation exposure levels associated with the installation of MNSAs.
4,0 EVALUATION ,
ASME Section 111, NB-3671,7," Sleeve Coupled and Other Datented Joints
- requires that the joint design (a) make provisions to prevent separation under all service loading conditions, (b) be accessible for maintenans 2, temoval and replacement after service, and (c) meet either of the two following criteria:
(1) a prototype joint has been subjected to performance tests to determine the safety of the joint under simulated service conditions. When vibration, fatigue, cyclic conditions, low temperature, thermal expansion, or hydraulic shock is -
anticipated, the appNable conditions shall be incorporated into the tests. The mechanical joints shall be sufficiently leak tight to satisfy the requirements of the design specification; or (2) joints shall be designed in accordance with the rules of NB-3200.
The staff's evaluation of the MNSA design against each of the elements of NB-3671.7 is contained below.
Part a of NB-3671.7 requires the joint design make provisions to preverit separation under all service loading conditions. The MNSA is designed to maintain its sealing function and also
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restrain the instrument nozzle from ejecting assuming a complete failure of the "J" weld and full system pressure acting against the instrument nozzle. The staff has reviewed the detailed drawings end calculations contained in the licensee's December 12 letter, and concludes that the MNSA can accomplish these two design functions. Therefore, the staff concludes that the 3
MNSA will not separate oder any service loading conditions.
Pari b of NB-36i9.7 requires tha joint to be accessible for maintenance, removal and replacement after service. The MNSA,is bolted in place, and can be easily removed to meet this requirement.
To comply with part c of NB-3671.7, the licensee chose to implement the first option, prototype testing. The objective of the prototype tests was to qualify one MNSA for a nozzle located at the t>nttom of a pressurizer and one MNSA for a RCS hot leg nozzle, by performing hydrostatic, thermal cyclir'g and seismic tests on prototype MNSAs, and verifying leak tightness under operating conditions.
Each nozzle was clamped by a MNSA on a fixture mounted on an autoclave. The nozzles were )
not welded to the mounting fixtures. The hydrostatic test consisted of pressurizing the autoclave to 3175 psig at ambient temperature conditions. Several tests were performed on the RCS hot leg MNSA prototype for short periods of time, and one long duration test for 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br />. No leakage was detected after this test. A similar test was performed for the pressurizer MNSA prototype with a duration of 26 minutes. No leakage was detected after this test.
Following completion of the hydrostatic tests, each MNSA prototype was subjected to a thermal cycling test consisting of 3 heatup and cooldown cycles. Each cycle consisted of heating the autoclave from ambient temperature to 650*F at a rate of 150*F/ hour and raising the pressure to 2500 psig. The autoclave contained the normal boric acid solution found in the RCS. The elevated temperature / pressure was held for at least 60 minutes, after which the autoclave was cooled down to ambient conditions. No leakage was observed during these tests. At the conclusion of the tests the MNSAs were disassembled and examined for boric acid deposits on any of the internal surfaces of the flanges or the Grafoil gaskets. None were reported.
The seismic testing consisted of subjecting a MNSA mounted on a shako table to five operating basis earthquake events and one safe shutdown earthquake event. The mounting fixture permitted pressurization to 3175 psig and ambient temperature during the seismic testing. The test results indicate that no mechanical damage occurred and no leakage was detected. The staff finds these resu;ts acceptable.
The staff has reviewed the scope and results of the testing described above, and concludes that the requirements of part c of MB-3671.7, demonstration of the safety of the joint under simulated service conditions, has been met for the short-term. Short-term in this case is defined as the period of operation from the mid-cycle cutage to the next refueling outage for each unit.
5-The prototype testing performed on the MNSAs and supplemental information provided by the licensee, did not adequately resolve the staff's concerns relative to the ability of the MNSAs to effectively resist corrosion for long-term service.
There is a potential for boric acid corrosion (BAC)in the pressurizer, steam generator and RCS piping walls, should any borated water leak into the annulus between the nozzle and the wall from a through wall crack in the nozzle. SCE stated that ihe extent of corrosion is expected to be limited, based on experience and an evaluation of the likely environment within the annulus.
SCE stated that it would not expect corrosion in the exterior surfaces of the walls and the MNSA components, since this would require leakage past the Grafoil seal gasket, in response to staff questions, the licensee did address the effects of leakage past the Grafoil in its January 5 letter. The staff agrees that sufficient basis has been provided to demonstrate that degradation of the low alloy steel components would not occur during the time period between installation of the MNSAs during the mid-cycle outage and the next refueling outage.
- Furthermore, the staff concurs that the A 286 tolting materialis resistant to BAC.
To obtain further data on this corrosion mechanism, the llcensee committed !n its December 12 letter to remove an A690 nozzle during the Unit 2 midcyce outags which has been in service since 1993, and inspect the low alloy carbon steel annulus for corrosion. SCE expects corrosion on the order of 3 5 mils per year. A bounding calculation will also be performed which will determine the acceptable limit of wastage. The results of the inspection and the evaluation will be documented and submitted to the staff upon completion. This information will assist the staff's review of the proposed permanent installation of MNSAs in the locations indicated above.
Another potential degradation mechanism is the chemical interaction of the borated water in the annulus with the Grafoil gasket. If a crack in the "J" weld develops, borated water will fill the annulus between the nozzle and the wall Galvanic corrosion of low alloy steel can occur as a result of an electrochemical roaction between the low a3oy steel and the graphite in the Grafoil gasket, or other corrosion resistant mateiials in the presence of an electrically conductive fluid such as borated water. SCE stated that galvanic corrosion would be limited as long as the Grafoil seal maintains structural integrity and no borated water leakage occurs. In the January 5 letter, SCE provided additional information asserting that galvanic corrosion is expected to be minimal even if a leak occurred because the crevice would be deareated and metal nydroxides would be formed raising the crevice pH. However, no specific tests have been conducted to assess the extent and limitation of potential galvanic corrosion. The staff believes that a sufficient basis has been provided for interim operation, but additional information from future tests and inspections is needed to determine the acceptability of the MNSAs for long-term operation.
The staff has reviewed these test results and finds them adequate for qualifying the MNSAs for use until the next refueling outages for each unit. The acceptability of long-term use of the MNSAs is contingent on sesolution of the corrosion effects, which is further addressed in Section 5.0.
i 6-Therefore, the staff finds the design in accordance with ASME Section Ill, NB 3671.7 and the rules of NB 3200,1989 Edition, for the interim period of service until the next refueling outages for each unit. The design and installation of a MNSA that meets the applicab' ASME Code requirements for a joint is an acceptable attemative to the applicable ASME Code requirements for repair of instrument nozzles. As indicated above, the MNSA design meets the applicable ASME Code requirements for a joint, and therefore provides an acceptable level of quality and safety compared to the ASME Section XI Code repair requirements. The safety function of the original "J" weld is to prevent RCS leakage, and provide structural integrity for the instrument nozzles. The MNSA will provid6 an equivalent barrier against possible RCS leakage and also provides an equivalent restraining mechanism to prevent nozzle ejection.
Since MNSAs are bolted into 0.5-inch diameter,1-inch deep holes (1.38 inch deep for the alignment holes for the MNSAs installed on the pressurizer bottom) drilled in the component walls, SCE performed revised ASME Section ill fatigue calculations for SONGS Units 2 and 3, to verify that the Code prescribed cumulative usage factor of 1.0 was not exceeded in the components. For the pressurizer shell and bottom head, SCE performed the fatigue calculations in accordance with ASME Section lli NB-3222.4,1971 Edition. SCE also determined that there was adequate reinforcement in the walls for the holes, based on the ASME Code Section lil NB 3334,1971 Edition, rules for reinforcement of holes. For the RCS hot leg piping, the fatigue calculations were performed in accordance with the rules of ASME Section 1ll NB 3222.2,1971 Edition, and the required area of reinforcement was determined in accordance with the tules li ASME Section 111 NB-3643.3,1971 Edition. The staff has reviewed these calculaMns, and finds them reasonable and in accordance with industry design practice.
5.0 FUTURE INSPECTIONS AND TESTS The staff needs further information to determine the acceptability of the MNSAs to perform their intended safety function beyond the next refueling outages for the SONGS units due to (1) the limited testing of these devices with regard to the effect of long term exposure to the operating environment, and (2) the uncertainty about degradation mechanisms under long-term operating conditions and their effect on some of the components of the MNSAs. The licensee has requested the staff to complete its review of the acceptability of long-term installation of the MNSAs by September 30,1998. To allow sufficient time for the staff to complete its review by this deadline, the licensee should provide (within 45 days of the date of this safety evaluation) a submittal containing their augmented inspection program and additional information addressing potentiallong term degradation and corrosion of the MNSAs. This submittal should include, at a minimum, information as discussed above on the following items:
. Inspection of bolt and MNSA components for corrosion.
. Long term integrity of the Grafoil gasket under RCS pressure and thermal operating conditions (no load relaxation and no galvanic corrosion).
. Additional accelerated corrosion tests on all of the materials used to fabricate the subcomponents in the MNSA design.
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6.0 CONCLUSION
The staff concludes that the MNSA design is in accordance with the joint requirements specified in ASME Section 111, NB 3671.7 and rules of NB-3200,1989 ASME Edition. The staff further concludes that the installation of the MNSA conforms with the appropriate ASME Section lli Code requirements, as detailed in the previous section. The design and installation of MNSAs that meet the applicable ASME Code requirements for joints provides assurance that the safety functions of the original"J' weld, prevention of RCS leakage and nonle structuralintegrity, are 1 naintained when the MNSAs are installed around the nozzles. Therefore, the use of the MNSA provides an acceptable level of quality and safety compared to the applicable ASME Code requirements for repair of instrument nozzles. Accordingly, pursuant to 10 CFR 50.55a(a)(3)(i),
the use of MNSAs as an altemative to a Section XI Code repair of the instrument nozzles is authorized from the Unit 2 and 31998 mid-cycle outages until the next refueling outages for the SONGS units. With respect to the intended uses of the MNSAs by the licensee, the staff has concluded that:
- 1. The request foi temporary installation (until the next refueling outages for the SONGS units) of MNSAs on any leaking A600 nozzles located below the mid-loop water level on RCS hot leg piping is acceptable.
- 2. The request for the installation of MNSAs on any leaking A600 nozzles on the steam generator cold leg channel heads is acceptable on an interim basis only until t. e next 9 fueling outages for the SONGS units.
- 3. The request for the installation of MNSAs on the A600 nozzles on the pressurizer is acceptable on an interim basis only unti' the next refueling outages for the SONGS units. 3 Principal Contributors: M. Hartzman, NRR/EMEB R. Hermann, NRR/ECMB Date: February 17, 1998