Information Notice 2003-02, Recent Experience with Reactor Coolant System Leakage & Boric Acid Corrosion
| ML030160004 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah, Comanche Peak  |
| Issue date: | 01/16/2003 |
| From: | Beckner W NRC/NRR/DRIP/RORP |
| To: | |
| Hodge, CV, NRR/DRIP/RORP, (415-1861) | |
| References | |
| TAC MB7177 IN-03-002 | |
| Download: ML030160004 (7) | |
UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, DC 20555-0001January 16, 2003NRC INFORMATION NOTICE 2003-02:RECENT EXPERIENCE WITH REACTORCOOLANT SYSTEM LEAKAGE AND BORIC ACID
CORROSION
Addressees
All holders of operating licenses or construction permits for pressurized water reactors (PWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to informaddressees of recently observed reactor coolant leakage at two pressurized water reactor
facilities, one of which resulted in the subsequent degradation of the reactor pressure vessel
head. It is expected that recipients will review the information for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However, suggestions in this
information notice are not NRC requirements; therefore no specific action or written response is
required.
Description of Circumstances
Sequoyah Unit 2 On December 26, 2002, the unit tripped from full power as a result of low reactor coolantsystem (RCS) flow due to a ground fault in a reactor coolant pump motor winding. In the
ensuing shutdown to correct the pump problem, the licensee initiated a search to locate and
correct a suspected RCS leak that, prior to the trip, had resulted in elevated moisture and
activity levels inside containment. During this inspection, the licensee identified an
accumulation of boric acid on the reactor vessel head insulation that resulted from a leaking
reactor vessel level indication system (RVLIS) compression fitting. The leakage had seeped
through a seam in the insulation onto the reactor pressure vessel (RPV) head and resulted in
minor boric acid corrosion of the head. This RVLIS compression fitting had been disconnected
and reconnected during the May 2002 refueling outage. The licensee also identified a small
leak through a canopy seal weld on an empty control rod drive mechanism (CRDM) penetration
that did not result in any boric acid corrosion of the reactor vessel head.Based on the location of the leaking RVLIS fitting, the temperature of the leakage fluid wasclose to the ambient temperature outside the vessel insulation. The licensee estimated the
mass of boric acid crystals on this insulation surface at about 9 kilograms (20 pounds). A seam in the insulation was in this area. On removing the insulation and cleaning the area, thelicensee observed boric acid corrosion of the head near the flange. The licensee determined
that the amount of material loss from the head was small, in the shape of a groove less than
one centimeter (cm) [0.3 inch] wide, about twelve cm [4.6 inches] long, and at most about one- third cm [0.125 inch] deep.The licensee's evaluation indicated that 98 percent or better of the structural wall remainedintact and that no abrupt corners existed in the degraded area. The licensee justified continued
operation based on the minor extent of the degradation.Comanche Peak Unit 1On November 30, 2002, a control rod dropped into the core. The licensee suspected a fault inthe CRDM coils. Failing to identify the cause of the dropped rod while at reduced power, the
licensee decided to shut down. While continuing to troubleshoot the CRDM problem in Mode 3, the licensee observed a leak around the CRDM housing. The leak was from a CRDM canopy
seal weld. Water from the leaking canopy seal weld apparently entered the CRDM coils, causing coil failure. Boric acid crystals were found around the leak site, on the vessel head
insulation, and on the reactor pressure vessel head. The licensee repaired the canopy weld
with a weld overlay and cleaned the CRDM housing, the head insulation, and the head to
remove the boric acid deposits. The amount of boric acid crystals recovered from the head was
about 1 kilogram (2 pounds). The licensee did not find any reactor coolant pressure boundary
degradation. Other operating experiences of similar character may be found in the generic communicationslisted in NRC Bulletin 2002-01, "Reactor Pressure Vessel Head Degradation and Reactor
Coolant Pressure Boundary Integrity."
DiscussionA number of mechanical and welded connections exist above the reactor pressure vessel headthat, historically, have leaked at a number of plants. This leakage of borated water may lead to
degradation of the low alloy steel reactor vessel head by boric acid corrosion. At Sequoyah
Unit 2, the leakage resulted in relatively minor degradation of the reactor vessel head. At
Comanche Peak Unit 1, the leakage resulted in no apparent degradation of the RCS pressure
boundary. In the Sequoyah Unit 2 and Comanche Peak Unit 1 events, the unidentified reactor
coolant leakage had not shown a discernible increase from the very low levels that typically
occur at a PWR facility. Common assumptions that RCS leakage onto a hot surface, such as the reactor pressurevessel head, will not cause corrosion may not be justified and are the subject of ongoing
research. Usually, small quantities of water coming into contact with a surface as hot as the
reactor vessel head would be expected to flash and leave a noncorrosive dry boric acid residue
on the surface. However, at Sequoyah Unit 2 the resulting condition produced an environment
in which boric acid corrosion could occur. This experience challenges current assumptions with
respect to the potential effects of RCS leakage. The NRC is continuing to consider the safety
and regulatory aspects of this experience. This information notice requires no specific action or written response. If you have anyquestions about the information in this notice, please contact the technical contact listed below
or the appropriate project manager from the NRC
's Office of Nuclear Reactor Regulation(NRR). /RA/William D. Beckner, Program Director
Operating Reactor Improvements Program
Division of Regulatory Improvement Programs
Office of Nuclear Reactor RegulationTechnical contacts: V. Hodge, NRRE. Sullivan, NRR301-415-1861301-415-2796 E-mail: cvh@nrc.govE-mail: ejs@nrc.govAttachment: List of Recently Issued NRC Information Notices
ML030160004DOCUMENT NAME: C:\ORPCheckout\FileNET\ML030160004.wpd
- See Previous ConcurrenceINDICATE IN BOX: "C"=COPY W/O ATTACHMENT/ENCLOSURE, "E"=COPY W/ATT/ENCL, "N"=NO COPY
OFFICERORP:DRIPEMCB:DEEMCB:DEEMCB:DENAMEVHodgeTSullivanSCoffinBBatemanDATE01/13/03*01/13/03*01/14/03*01/15/03OFFICEDLPM:PD-2RORP:DRIPRORPNAMEAHoweTReissBBecknerDATE01/14/03*01/15/0301/16/03
______________________________________________________________________________________OL = Operating License
CP = Construction PermitAttachment 1 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICES
_____________________________________________________________________________________InformationDate of
Notice No. SubjectIssuanceIssued to
_____________________________________________________________________________________2003-01Failure of a Boiling WaterReactor Target Rock Main
Steam Safety/Relief Valve01/15/2003All holders of operating licensesor construction permits for
nuclear power reactors, except
those that have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor.2002-35Changes to 10 CFR Parts 71and 72 Quality Assurance
Programs12/20/2002All holders of 10 CFR Part 71quality assurance program
approvals and all 10 CFR Part 72 licensees and certificate holders.2002-34Failure of Safety-RelatedCircuit Breaker External
Auxiliary Switches at Columbia
Generating Station11/25/2002All holders of operating licensesor construction permits for
nuclear power reactors.2002-33Notification of PermanentInjunction Against Neutron
Products Incorporated of
Dickerson, Maryland11/21/2002All teletherapy and radiationprocessing licensees. 2002-29(Errata)Recent Design Problems in Safety Functions of Pneumatic
Systems11/06/2002All holders of operating licensesor construction permits for
nuclear power reactors.2002-32Electromigration onSemiconductor Integrated
Circuits10/31/2002All holders of operating licensesfor nuclear power reactors except
those who have ceased
operations and have certified that
fuel has been permanently
removed from the reactor vessel.Note:NRC generic communications may be received in electronic format shortly after they areissued by subscribing to the NRC listserver as follows:To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the followingcommand in the message portion:subscribe gc-nrr firstname lastname