ML20155A260
| ML20155A260 | |
| Person / Time | |
|---|---|
| Site: | Rancho Seco |
| Issue date: | 06/01/1988 |
| From: | Andognini G SACRAMENTO MUNICIPAL UTILITY DISTRICT |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| GCA-88-306, GL-88-05, GL-88-5, NUDOCS 8806090312 | |
| Download: ML20155A260 (6) | |
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^4 C)suun SACRAMENTO MUNICIPAL UTILITY DISTRICT O 6201 S Street, P.O. Box 15830. Sacramento CA 950521830,(916) 452 3211 AN ELECTRIC SYSTEM SERVING THE HEART OF CALIFORNIA GCA 88-306 Director of Nuclear Reactor Regulation U.S.
Nuclear Regulatory Commission Mail Station Pl-137 Washington, DC 20555 Docket 50-312 Rancho Seco Nuclear Generating Station License No. DPR-54 RESPONSE TO GENERIC LETTER 88 "BORIC ACID CORROSION OF CARBON STEEL REACTOR PRESSURE BOUNDARY COMPONENTS IN PWR PLANTS"
Dear Sir:
The Sacramento Municipal Utility District hereby provides the information requested in Generic Letter 88-05, "Boric Acid Corro-l sion of Carbon Steel Reactor Pressure Boundary Components in PWR Plants," received by the District on Apri' 1,
1988.
Attachment I describes the actions taken to materially and procedurally reduce the effects of boriu acid corrosion.
Attachment II describes the program the District will implement to further reduce the poten-tial for corrosion.
The District believes the ongoing effort to replace low alloy stee) fasteners with stainless steel components, in addition to the preventive maintenance inspections now performed, provides a high level of confidence that Rancho Seco complies with General Design Criteria 14, 30, and 31.
The program described in Attach-ment II will provide an added margin of assurance that the RCS pressure boundary integrity will be maintained.
Members of your staff requiring additional information or clari-i fication may contact Steve Crunk at (209) 333-2935, extension 4913.
Sincerely,
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Q#A G.
arl do ini Ch ef E ecuti e Officer, Nuclear l
Attachments
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D'Angelo, NRC, Rancho Seco J.
B. Martin, NRC, Walnut Creek 00
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RANCHO SECO NUCLEAR GENERATING STATION D 14J40 Twin Cities Road. Herald, CA 95638 9799: (209) 333-2935
ATTACHMENT I Actions 19 Date The District has already made several material changes at Rancho Seco to reduce the potential for boric acid corrosion of carbon steel components.
To date, Rancho Seco has taken steps to re-duce, where practical, the use of low alloy steel stud material in borated water systems.
Plant modifications were implemented which replaced carbon steel bolting material with stainless steel in various valves in the Reactor Coolant System (RCS).
This was done as part of an on-going program which allows for the replace-ment of carbon steel fasteners.
Other RCS valves with carbon steel fasteners were replaced in their entirety.
A long-range program exists at Rancho Seco to replace RCS valves prone to leakage with "packless" valves in an effort to reduce leakage from the RCS.
Carbon steel reactor coolant pump (RCP) studs with wastage from boric acid corrosion were evaluated and replaced as necessary.
Further modifications, such as replacing RCP gaskets, are already planned to reduce the potential for borated water leakage and subsequent corrosion.
One RCP gasket refurbishment was performed in 19f35; refurbishments to two additional pumps are planned for the next scheduled refueling outage.
Rancho Seco has procedures in place which direct the inspection of plant systems for evidence of leakage of borated water and the resultant boric acid corrosion.
Surveillance Procedures SP.58 and SP.59 test the RCS to ensure primary boundary integrity following: (1) the opening of a mechanical joint in the RCS or, (2) modifications and welded repairs to the RCS.
Another sur-veillance procedure specifically inspects the RCPs for gasket leakage and stud deterioration.
This procedure also directs the performance of thickness measurement of the RCP studs during refueling intervals.
Additionally, the Rancho Seco preventive maintenance program in-l cludes "Repetitive Tasks" which require inspection of areas of the Auxiliary Building, Reactor Building, and Tank Farm for boric acid crystal accumulation and system leaks.
These inspections are performed every 90 days when areas are accessible.
An inspection for degraded fasteners was performed in response to IE Bulletin 82-02, Degradation of Threaded Fasteners in the Reac-l tor Coolant Pressure Boundary of PWR Plants.
Although other mechanisms for fastener failure were also addressed in the Bulle-tin, the resulting inspection performed by the District identi-l fled those locations where there was evidence of boric acid l
corrosion of fasteners.
Also, an inspection of all valves poten-l tially having boric acid damaged fasteners in the Reactor Build-1 l
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ing, Auxiliary Building, and Tank Farm was performed in response to a 1987 INPO evaluation finding.
Those valves identified as having boric acid damaged fasteners were repaired prior to the 1988 restart of Rancho Seco.
The District believes the ongoing effort to replace carbon steel and low alloy steel fasteners and components with stainless steel components, in addition to the inspections now performed as part of the Preventive Maintenance program, provide a high level of confidence that Rancho Seco complies with General Design Criteria 14, 30, and 31.
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ATTACHMENT II Planned Antions The District will enhance the existing plant procedures by imple-menting a program to locate actual and identify potential bora-ted water leakage paths.
This program will identify potential areas of boric acid corrosion, evaluate the impact on the reactor coolant pressure boundary of any identified borated water. leak-age, and implement corrective actions to prevent the reoccurrence of this type of corrosion.
This program will address the following:
(1).
"A determination of the principal locations where leaks that are smaller than the allowable technical specification limit can cause degradation of the primary pressure boundary by boric acid corrosion.
Particular consideration should be given to identify-ing those locations where conditions exist that could cause high concentrations of boric acid on pressure boundary surfaces."
The District feels confident that the principal locations.where leakage of boric acid solutions or degradation due to concentra-tions of boric acid solutions would occur have been identified.
.Also identified are the existence of configurations that could
' allow leakage of borated water into or within an enclosure con-taining carbon steel pressure boundary components.
Principal areas where leakage and degradation could occur include:
Control Rod Drive Mechanism nozzle flanges Once-Through Steam Generator primary manways Pressurizer manway Pressurizer relief valve flanges Pressurizer heater bundles RCP closure rings Hot and Cold Leg RTDs Valves with carbon steel bolting materials These areas (some of which require insulation removal for inspec-tions) will be inspected during detailed system walkdowns discas-sed in the following sections.
(2)
"Procedures for locating small so11 ant leaks (i.e., leakage rates at less than technical specifie vton limits).
It is impor-tant to establish the potenti:1 path oi the leaking coolant and the reactor pressure boundary components it is likely to contact.
This information is important in deterwining the interaction between the leaking coolant and reactor coolant pressure boundary materials."
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Rancho Seco will procedurally establish the methods for monitor-ing the potential locations for boric acid degradation. The pro-cedure will direct detailed "top-to-bottom" system walkdown inspections to be performed whenever the plant enters Cold Shutdown for greater than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and the Reactor Building becomes accessible.
This walkdown procedure will ensure complete system and area walkdowns dedicated to locating small coolant leaks which could lead to corrosion of the reactor coolant pres-sure boundary due to the concentration of boric acid, including the inspection of all locations enumerated in Item 1.
This pro-cedure will be developed by the next scheduled refueling outage (planned for 3rd quarter, 1989).
If any part of the reactor coolant pressure boundary is opened for inspection or maintenance, the appropriate existing surveil-lance procedure (SP.58 or SP.59) will be performed to ensure the primary boundary integrity.
(3)
"Methods for conducting examinations and performing engineering evaluations to establish the impact on the reactor coolant pressure boundary when leakage is located.
This should include procedures to promptly gather the necessary information for an engineering evaluation before the removal of evidence of leakage, such as boric acid crystal buildup."
The system walkdown procedure discussed above will also address evaluation of the impact of an identified leak on the primary coolant pressure boundary.
The evaluation will primarily deal with the impact of possible pressure boundary degradation caused by boric acid corrosion.
The walkdown procedure will document information gathered during the performance of the walkdown prior to the removal of evidence of leakage, such as general area map-ping (to detect possible spray of boric acid from the leak), map-ping of liquid flow (to help establish precise leak location),
and measurement of crystal deposition.
This procedure section will incorporate information forthcoming from the B&W owners Group Materials Committee study of boric acid corrosion.
(4)
"Corrective actions to prevent recurrences of this type of corrosion.
This should include any modifications to be intro-duced in the present design or operating procedures of the plant that (a) reduce the probability of primary coolant leaks at the locations where they may cause corrosion damage and (b) entail the use of suitable corrosion resistant materials or the applica-tion of protective coatings / claddings."
Rancho Seco will continue to eliminate leakage of borated water onto carbon steel components.
Consideration will continue to be given to the replacement of low alloy steel fastening materials in the RCS pressure boundary where practicable.
Additionally, v
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the District will evaluate design modifications for corrosion protection (such as the creation of drainage pathways) and the use of deflectors and protective coating on carbon steel parts where replacement with stainless steel is impractical.
one method to reduce the effects of boric acid corrosion is to increase the awareness of personnel to the problem.
The District will conduct training for appropriate maintenance,smaintenance planning, engineering, and operations personnel on the effects of boric acid leakage, methods for conducting examinations, and per-forming engineering evaluations to evaluate the impact on the reactor coolant pressure boundary when leakage is detected.
Training will be conducted for maintenance personnel in the proper maintenance practices for bolted flanges and components to prevent leakage.
This training will be based on the EPRI Good Bolting Practices Manual and accepted industry standards.
The District will review current RCS pressure boundary closure assembly practices and procedures against the recommendations from the B&W Owners Group and incorporate any changes deemed necessary.
It is the District's position that the program described in this attachment will provide an added margin of assurance that RCS pressure boundary integrity will be maintained.
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