NRC Generic Letter 1988-05
UNITED STATESNUCLEAR REGULATORY COMMISSIONWASHINGTON, D. C. 20555MAR. 17, 1988ALL LICENSEES OF OPERATING PWRS AND HOLDERS OF CONSTRUCTION PERMITS FOR PWRSGENTLEMEN:Subject: BORIC ACID CORROSION OF CARBON STEEL REACTOR PRESSURE BOUNDARYCOMPONENTS IN PWR PLANTS (GENERIC LETTER 88-05)Pursuant to 10 CFR 50.54(f), the Nuclear Regulatory Commission is requestinginformation to assess safe operation of pressurized water reactors (PWRs) whenreactor coolant leaks below technical specification limits develop and thecoolant containing dissolved boric acid comes in contact with and degrades lowalloy carbon steel components. The principal concern is whether the affectedplants continue to meet the requirements of General Design Criteria 14, 30,and 31 of Appendix A to Title 10 of the Code of Federal Regulations (CFR) Part50 when the concentrated boric acid solution or boric acid crystals, formed byevaporation of water from the leaking reactor coolant, corrode the reactorcoolant pressure boundary. Our concerns regarding this issue were prompted byincidents in PWR plants where leaking reactor coolant caused significantcorrosion problems. In many of these cases, although the licensees had detectedthe existence of leaks, they had not evaluated their significance relative tothe safety of the plant nor had they promptly taken appropriate correctiveactions. Recently reported incidents are listed below.(1) At Turkey Point Unit 4, leakage of reactor coolant from the lowerinstrument tube seal on one of the incore instrument tubes resulted incorrosion of various components on the reactor vessel head including threereactor vessel bolts. The maximum depth of corrosion was 0.25 inches.(TE Information Notice No.86-108, Supplement 1)(2) At Salem Unit 2, leakage occurred from the seal weld on one of theinstrument penetrations in the reactor vessel head, and the leaking coolantcorroded the head surface. The maximum depth of corrosion was 0.36 inches.(IE Information Notice No.86-108, Supplement 2)4 (3) At San Onofre Unit 2, boric acid solution corroded nearly through the boltsholding the valve packing follow plate in the shutdown cooling systemisolation valve. During an attempt to operate the valve, the bolts failedand the valve packing follow plate became dislodged causing leakage ofapproximately 18,000 gallons of reactor coolant into the containment.(IE Information Notice No.86-108, Supplement 2)(4) At Arkansas Nuclear One Unit 1, leakage from a high pressure injectionvalve dripped onto the high pressure injection nozzle. The maximum depthof corrosion was 0.5 inches, which represented a 67 percent penetration ofthe pressure boundary. (IE Information Notice No.86-108)r 2 36) A'-2 irose at7?
-2-(5) At Fort Calhoun, seven reactor coolant pump studs were reduced by boric.acid corrosion from a nominal 3.5 inches to between 1.0 and 1.5 inches.(IE Information Notice 80-27)Additionally, corrosion rates of up to 400 mils/month have been reported froman experimental program. (IE Information Notice No.86-108, Supplement 2)Although failure of the reactor coolant pressure boundary did not occur inevery instance, all of these incidents demonstrated the potential adverseconsequences of boric acid corrosion.The corrosion caused by the leaking coolant containing dissolved boric acidhas been recognized for some time. Since 1979, the NRC has issued fiveinformation notices (80-27; 82-06;86-108; and 86-108, Supplements 1 and 2)and Bulletin 82-02 addressing this problem. In June 1981, the Institute forNuclear Power Operations issued a report discussing the effect of low levelleakage from the gasket of a reactor coolant pump and concluded that significantcorrosion of the pump studs could occur during all modes of operation. InDecember 1984, the Electric Power Research Institute issued a summary report onthe corrosion of low alloy steel fasteners which, among other things, discussedboric acid-induced corrosion. The information contained in these documentsclearly indicated that boric acid solution leaking from the reactor coolantsystem can cause significant corrosion damage to carbon steel reactor coolantpressure boundaries.Office of Inspection and Enforcement (IE)Bulletin 82-02 requested licenseesto identify all of the bolted closures in the reactor coolant pressure boundarythat had experienced leakages and to Inform the NRC about the inspections tobe made and the corrective actions to be taken to eliminate that problem.However, the bulletin did not require the licensees to institute a systematicprogram for monitoring small primary coolant leakages and to perform maintenancebefore the leakages could cause significant corrosion damage.In light of the above experience, the NRC believes that boric acid leakagepotentially affecting the integrity of the reactor coolant pressure boundaryshould be procedurally controlled to ensure continued compliance with thelicensing basis. We therefore request that you provide assurances that aprogram has been implemented consisting of systematic measures to ensure thatboric acid corrosion does not lead to degradation of the assurance that thereactor coolant pressure boundary will have an extremely low probability ofabnormal leakage, rapidly propagating failure, or gross rupture. The programshould include the following:(1) A determination of the principal locations where leaks that are smallerthan the allowable technical specification limit can cause degradationof the primary pressure boundary by boric acid corrosion. Particularconsideration should be given to identifying those locations whereconditions exist that could cause high concentrations of boric acid onpressure boundary surfaces.
-3-(2) Procedures for locating small coolant leaks (i.e., leakage rates at lessthan technical specification limits). It is important to establish thepotential path of the leaking coolant and the reactor pressure boundarycomponents it is likely to contact. This information is important indetermining the interaction between the leaking coolant and reactor coolantpressure boundary materials.(3) Methods for conducting examinations and performing engineering evaluationsto establish the impact on the reactor coolant pressure boundary whenleakage is located. This should include procedures to promptly gatherthe necessary information for an engineering evaluation before the removalof evidence of leakage, such as boric acid crystal buildup.(4) Corrective actions to prevent recurrences of this type of corrosion. Thisshould include any modifications to be introduced in the present designor operating procedures of the plant that (a) reduce the probability ofprimary coolant leaks at the locations where they may cause corrosiondamage and (b) entail the use of suitable corrosion resistant materials orthe application of protective coatings/claddings.Additional insight into the phenomena related to boric acid corrosion ofcarbon steel components is provided in the attachment to this letter.The request that licensees provide assurances that a program has been implementedto address the corrosive effects of reactor coolant system leakage at less thantechnical specification limits constitutes a new staff position. Previous staffpositions have not considered the corrosion of external surfaces of the reactorcoolant pressure boundary. Based on the frequency and continuing pattern ofsignificant degradation of the reactor coolant pressure boundary that wasdiscussed above, the staff now concludes that in the absence of such a programcompliance with General Design Criteria 14, 30 and 31 cannot be ensured.You are required to submit your response signed under oath or affirmation, asspecified in 10 CFR 50.54(f), within 60 days of receipt of this letter. Yourresponse will be used to determine whether your license should be modified,suspended, or revoked. Your response should provide assurances that such aprogram is in place or provide a schedule for promptly implementing such aprogram if one is not in place.This information is required pursuant to 10 CFR 50.54(f) to assess conformanceof PWRs with their licensing basis and to determine whether additional NRCaction is necessary. The staff does not request submittal of your program. Youshall maintain, in auditable form, records of the program and results obtainedfrom implementation of the program and shall make such records available to NRCinspectors upon request.This request for information is covered by the Office of Management andBudget under Clearance Number-3150-0011, which expires December 31, 1989.
e -4-4-Comments onand Budget,Washington,burden and duplication may be directed to the Office of ManagementReports Management, Room 3208, New Executive Office Building,D.C. 20503.Sincerely,Frank MiragliAssociate Director for ProjectsOffice of NucTear Reactor RegulationAttachment:As stated, AATTACHMENTBORIC ACID CORROSION OF CARBON STEEL REACTOR COMPONENTS IN PWR PLANTSBoric acid is used in PWR plants as a reactivity control agent. Itsconcentration in the reactor coolant ranges between 0 and approximately 1weight percent. At these concentrations boric acid solutions will notcause significant corrosion even if they come in contact with carbon steelcomponents. In many cases, however, coolant that leaks out of the reactorcoolant system loses a substantial volume of its water by evaporation,resulting in the formation of highly concentrated boric acid solutions ordeposits of boric acid crystals. These concentrated solutions of boricacid may be very corrosive for carbon steel. This is illustrated byrecent test data, tabulated below, which were referenced in NRCInformation Notice No.86-108, Supplement 2.Concentrationof boric acid Temperature Corrosion rate(percent) Condition (OF) mils/month25 Aerated 200 40025 Deaerated 200 25015 Aerated 200 350-40015-25 Dripping 210 400If all of the water evaporates and boric acid crystals are formed, thecorrosion is less severe. However, boric acid crystals are not completelybenign toward carbon steel, and at a temperature of 5000F, corrosion ratesof 0.8 to 1.6 mils/month were obtained in the Westinghouse tests referencedin the generic letter. Corrosion by boric acid crystals was observed inTurkey Point Unit 4 where more than 500 pounds of boric acid crystals werefound on the reactor vessel head. After these crystals were removed, -corrosion of various components on the reactor vessel head was observed.The most effective way to prevent boric acid corrosion is to minimizereactor coolant leakages. This can be achieved by frequent monitoring ofthe locations where potential leakages could occur and repairing the leakycomponents as soon as possible. Review of the locations where leakageshave occurred in the past indicates that the most likely locations are (1)valves; (2) flanged connections in steam generator manways, reactor headclosure, etc.; (3) primary coolant pumps where leakages occur at cover-to-casing connections as a result of defective gaskets; and (4) defectivewelds.In many of these locations the components exposed to boric acid solutionare covered by insulation and the leaks may be difficult to detect. Ifleak detection systems have been installed in the components (e.g., reactorcoolant pumps from certain vendors), they should be used to monitor forleakage.
-2-It is important to determine not only the source of the leakage but alsothe path taken by the leaking fluid by evaluating the mechanism by whichleaking boric acid is transported. In some cases boric acid may beentrained in the steam emerging from the opening in the pressure boundarythat subsequently condenses inside the insulation thus carrying boric acidto locations that are remote from the source of leakage.Boric acid corrosion can be classified into two distinct types: (1)corrosion that actually increases the rate of leakage and (2) corrosionthat occurs some distance from the source of leakage and hence does notsignificantly affect the rate of leakage. An example of the first typeis the corrosion of fasteners in the reactor coolant pressure boundary,for example, in reactor coolant pumps. This type of corrosion can leadto excessive corrosion of studs. The second type of corrosion can contributesignificantly to the degradation of the reactor coolant pressure boundary.At Arkansas Nuclear One Unit 1, a leak developed in a high pressure injectionisolation valve located 8 feet above the high pressure injection nozzlewhich was made of carbon steel. Accumulation of boric acid resulted in anapproximately 1/2-inch-deep corrosion wastage adjacent to the stainless-to-carbon steel weld. Other locations of the nozzle exhibited corrosionto a lesser degree. Corrosion of the reactor vessel head was observed atSalem Unit 2. Corrosion pits were 1 to 3 inches in diameter and 40 to 300mils deep. The source of this corrosion was a defective seal weld in oneof the instrument penetrations. These examples indicate that the corrosionproduced by boric acid could degrade even relatively bulky components. AtFort Calhoun, the diameter of a reactor coolant pump closure bolt wasreduced from 3.5 inches to 1.1 inches by boric acid corrosion. At SanOnofre Unit 2, boric acid corrosion of the valve bolts was responsible for-the failure of the valve and the discharge of .18,000 gallons of primarycoolant into the containment.Because of the nature of the corrosion produced by boric acid, the mostreliable method of inspection of components is by visual examination.Ultrasonic testing performed in accordance with Section XI of the AmericanSociety of Mechanical Engineers Boiler and Pressure Vessel Code may not besensitive enough to detect the wastage. At Fort Calhoun, two successiveultrasonic tests failed to detect corrosion of the reactor pump closurestuds. When ultrasonic testing is used, the licensee should.provideassurances that the results are reliable.
LIST OF RECENTLY ISSUEDSubject.GENERIC LETTERSDate of -Issuance Issued To'GenericLetter.No.GL B8-04GL B8-03EL B6-02DISTRIBUTION OF GEMSIRRADIATED IN RESEARCHREACTORSRESOLUTION OF GENERIC SAFETYISSUE 93, "STEAM BINDING OFAUXILIARY FEEDWATER PUMPS""INTEGRATED SEFETY ASSESSMENTPROGRAM II (ISAP II)"02/23/8802/17/8601/20/86ALL NON-POWERREACTORLICENSEESALL LICENSEES,APPLICANTS FOROPERATINGLICENSES, ANDHOLDERS OFCONSTRUCTIONPERMITS FORPRESSURIZEDWATER REACTORSALL POWERREACTOR*LICENSEESGL 68-01 "NRC POSITION ON IGSCC IN BWRAUSTENITIC STAINLESS STEELPIPING"01/25/88ALL LICENSEESOF OPERATINGBOILING WATERREACTORS ANDHOLDERS OFCONSTRUCTIONPERMITS FORBWRSGL 67-16GL 67-15GL 67-14GL 87-13NUREG-1262, "ANSWERS TOQUESTIONS AT PUBLIC MEETINGSRE IMPLEMENTATION OF 10 CFR55ON OPERATORSLICENSESPOLICY STATEMENT ON DEFERREDPLANTSREQUEST FOR OPERATOR LICENSESCHEDULESINTEGRITY OF REQUALIFICATIONEXAMINATIONS AT NON-POWERREACTORS11/12/8711/04/8708/04/8707/10/87ALL POWER ANDNONPOWERREACTORLICENSEES ANDAPPLICANTS FORLICENSESALL HOLDERS OFCONSTRUCTIONPERMITS FOR ANUCLEAR POWERPLANTALL POWERREACTORLICENSEESALL NON-POWERREACTORLICENSEESGL 87-12 50.54(f) LETTER RE. LOSS OFRESIDUAL HEAT REMOVAL (RHR)DURING MIDLLOOP OPERATION07/09/87ALL LICENSEESOF OPERATINGPWRS ANDHOLDERS OFCONSTRUCTIONPERMITS FORPWRS
.t-4-Comments on burden and duplication may be directed to the Office of Managementand Budget, Reports Management, Room 3208, New Executive Office Building,Washington, D.C. 20503.Sincerely,Frank MiragliaAssociate Director for ProjectsOffice of Nuclear Reactor RegulationAttachment:As statedDISTRIBUTIONCentral FileECEB R/FECEB S/FC. McCrackenC. ThomasF. GillespieT. MartinF. MiragliaF. HebdonC. Berlinger8803220364OFC :E SDB :P :AD RR ADP:- R---- -. ---- --- -- -iNAME-:C cCracken:gr C homa ieF i :TM rtin :FMDATE :03/ g /88 :03/ /88 :03/1&'/88 :03/ 1 /88 :03/ H /88 :OFFICIAL RECORD COPY \