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 STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
January 16, 2003
NRC INFORMATION NOTICE 2003-02:
RECENT EXPERIENCE WITH REACTOR
COOLANT 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 inform
addressees 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 coolant
system (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 was
close 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, the
licensee 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 licensees evaluation indicated that 98 percent or better of the structural wall remained
intact 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 1
On November 30, 2002, a control rod dropped into the core. The licensee suspected a fault in
the 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 communications
listed in NRC Bulletin 2002-01, Reactor Pressure Vessel Head Degradation and Reactor
Coolant Pressure Boundary Integrity.
Discussion
A number of mechanical and welded connections exist above the reactor pressure vessel head
that, 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 pressure
vessel 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 any
questions about the information in this notice, please contact the technical contact listed below
or the appropriate project manager from the NRCs 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 Regulation
Technical contacts: V. Hodge, NRR
E. Sullivan, NRR
301-415-1861
301-415-2796 E-mail: cvh@nrc.gov
E-mail: ejs@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
ML030160004 DOCUMENT NAME: C:\\ORPCheckout\\FileNET\\ML030160004.wpd
- See Previous Concurrence
INDICATE IN BOX: C=COPY W/O ATTACHMENT/ENCLOSURE, E=COPY W/ATT/ENCL, N=NO COPY
OFFICE
RORP:DRIP
EMCB:DE
EMCB:DE
EMCB:DE
NAME
VHodge
TSullivan
SCoffin
BBateman
DATE
01/13/03*
01/13/03*
01/14/03*
01/15/03 OFFICE
DLPM:PD-2 RORP:DRIP
RORP
NAME
AHowe
TReiss
BBeckner
DATE
01/14/03*
01/15/03
01/16/03
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit
Attachment 1 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information
Date of
Notice No.
Subject
Issuance
Issued to
_____________________________________________________________________________________
2003-01
Failure of a Boiling Water
Reactor Target Rock Main
Steam Safety/Relief Valve
01/15/2003
All holders of operating licenses
or 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.
2002-35
Changes to 10 CFR Parts 71
and 72 Quality Assurance
Programs
12/20/2002
All holders of 10 CFR Part 71
quality assurance program
approvals and all 10 CFR Part 72 licensees and certificate holders.
2002-34 Failure of Safety-Related
Circuit Breaker External
Auxiliary Switches at Columbia
Generating Station
11/25/2002
All holders of operating licenses
or construction permits for
nuclear power reactors.
2002-33
Notification of Permanent
Injunction Against Neutron
Products Incorporated of
Dickerson, Maryland
11/21/2002
All teletherapy and radiation
processing licensees.
2002-29 (Errata)
Recent Design Problems in
Safety Functions of Pneumatic
Systems
11/06/2002
All holders of operating licenses
or construction permits for
nuclear power reactors.
2002-32
Electromigration on
Semiconductor Integrated
Circuits
10/31/2002
All holders of operating licenses
for 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 are
issued by subscribing to the NRC listserver as follows:
To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following
command in the message portion:
subscribe gc-nrr firstname lastname