Boric Acid Corrosion of Carbon Steel Reactor Pressure Boundary Components in PWR Plants (Generic Letter No. 88-05)
UNITED STATES
NUCLEAR REGULATORY COMMISSION
WASHINGTON, D.C. 20555
March 17, 1988
ALL LICENSEES OF OPERATING PWRS AND HOLDERS OF CONSTRUCTION PERMITS FOR PWRS
GENTLEMEN:
Subject: BORIC ACID CORROSION OF CARBON STEEL REACTOR PRESSURE BOUNDARY
COMPONENTS IN PWR PLANTS (GENERIC LETTER 88-05)
Pursuant to 10 CFR 50.54(f), the Nuclear Regulatory Commission is requesting
information to assess safe operation of pressurized water reactors (PWRs)
when reactor coolant leaks below technical specification limits develop and
the coolant containing dissolved boric acid comes in contact with and
degrades low alloy carbon steel components. The principal concern is whether
the affected plants 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) Part 50 when the concentrated boric acid solution or boric
acid crystals, formed by evaporation of water from the leaking reactor
coolant, corrode the reactor coolant pressure boundary. Our concerns
regarding this issue were prompted by incidents in PWR plants where leaking
reactor coolant caused significant corrosion problems. In many of these
cases, although the licensees had detected the existence of leaks, they had
not evaluated their significance relative to the safety of the plant nor had
they promptly taken appropriate corrective actions. Recently reported
incidents are listed below.
(1) At Turkey Point Unit 4, leakage of reactor coolant from the lower
instrument tube seal on one of the incore instrument tubes resulted in
corrosion of various components on the reactor vessel head including
three reactor vessel bolts. The maximum depth of corrosion was 0.25
inches. (IE Information Notice No. 86-108, Supplement 1)
(2) At Salem Unit 2, leakage occurred from the seal weld on one of the
instrument penetrations in the reactor vessel head, and the leaking
coolant corroded the head surface. The maximum depth of corrosion was
0.36 inches. (IE Information Notice No. 86-108, Supplement 2)
(3) At San Onofre Unit 2, boric acid solution corroded nearly through the
bolts holding the valve packing follow plate in the shutdown cooling
system isolation valve. During an attempt to operate the valve, the
bolts failed and the valve packing follow plate became dislodged
causing leakage of approximately 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 injection
valve dripped onto the high pressure injection nozzle. The maximum
depth of corrosion was 0.5 inches, which represented a 67 percent
penetration of the pressure boundary. (IE Information Notice No.
86-108)
8803220364
.
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(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
from an experimental program. (IE Information Notice No. 86-108, Supplement
2)
Although failure of the reactor coolant pressure boundary did not occur in
every instance, all of these incidents demonstrated the potential adverse
consequences of boric acid corrosion.
The corrosion caused by the leaking coolant containing dissolved boric acid
has been recognized for some time. Since 1979, the NRC has issued five
information 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 for
Nuclear Power Operations issued a report discussing the effect of low level
leakage from the gasket of a reactor coolant pump and concluded that
significant corrosion of the pump studs could occur during all modes of
operation. In December 1984, the Electric Power Research Institute issued a
summary report on the corrosion of low alloy steel fasteners which, among
other things, discussed boric acid-induced corrosion. The information
contained in these documents clearly indicated that boric acid solution
leaking from the reactor coolant system can cause significant corrosion
damage to carbon steel reactor coolant pressure boundaries.
Office of Inspection and Enforcement (IE) Bulletin 82-02 requested licensees
to identify all of the bolted closures in the reactor coolant pressure
boundary that had experienced leakages and to inform the NRC about the
inspections to be made and the corrective actions to be taken to eliminate
that problem. However, the bulletin did not require the licensees to
institute a systematic program for monitoring small primary coolant leakages
and to perform maintenance before the leakages could cause significant
corrosion damage.
In light of the above experience, the NRC believes that boric acid leakage
potentially affecting the integrity of the reactor coolant pressure boundary
should be procedurally controlled to ensure continued compliance with the
licensing basis. We therefore request that you provide assurances that a
program has been implemented consisting of systematic measures to ensure
that boric acid corrosion does not lead to degradation of the assurance that
the reactor coolant pressure boundary will have an extremely low probability
of abnormal leakage, rapidly propagating failure, or gross rupture. The
program should include 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 identifying those locations where
conditions exist that could cause high concentrations of boric acid on
pressure boundary surfaces.
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(2) Procedures for locating small coolant leaks (i.e., leakage rates at
less than technical specification limits). It is important to establish
the potential path of the leaking coolant and the reactor pressure
boundary components it is likely to contact. This information is
important in determining the interaction between the leaking coolant
and reactor coolant pressure boundary materials.
(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.
(4) Corrective actions to prevent recurrences of this type of corrosion.
This should include any modifications to be introduced 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 application of protective
coatings/claddings.
Additional insight into the phenomena related to boric acid corrosion of
carbon steel components is provided in the attachment to this letter.
The request that licensees provide assurances that a program has been
implemented to address the corrosive effects of reactor coolant system
leakage at less than technical specification limits constitutes a new staff
position. Previous staff positions have not considered the corrosion of
external surfaces of the reactor coolant pressure boundary. Based on the
frequency and continuing pattern of significant degradation of the reactor
coolant pressure boundary that was discussed above, the staff now concludes
that in the absence of such a program compliance with General Design
Criteria 14, 30 and 31 cannot be ensured.
You are required to submit your response signed under oath or affirmation,
as specified in 10 CFR 50.54(f), within 60 days of receipt of this letter.
Your response will be used to determine whether your license should be
modified, suspended, or revoked. Your response should provide assurances
that such a program is in place or provide a schedule for promptly
implementing such a program if one is not in place.
This information is required pursuant to 10 CFR 50.54(f) to assess
conformance of PWRs with their licensing basis and to determine whether
additional NRC action is necessary. The staff does not request submittal of
your program. You shall maintain, in auditable form, records of the program
and results obtained from implementation of the program and shall make such
records available to NRC inspectors upon request.
This request for information is covered by the Office of Management and
Budget under Clearance Number 3150-0011, which expires December 31, 1989.
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Comments on burden and duplication may be directed to the Office of
Management and Budget, Reports Management, Room 3208, New Executive Office
Building, Washington, D.C. 20503.
Sincerely,
Frank Miraglia
Associate Director for Projects
Office of Nuclear Reactor Regulation
Attachment:
As stated
ATTACHMENT
BORIC ACID CORROSION OF CARBON STEEL REACTOR COMPONENTS IN PWR PLANTS
Boric acid is used in PWR plants as a reactivity control agent. Its
concentration in the reactor coolant ranges between 0 and approximately 1
weight percent. At these concentrations boric acid solutions will not cause
significant corrosion even if they come in contact with carbon steel
components. In many cases, however, coolant that leaks out of the reactor
coolant system loses a substantial volume of its water by evaporation,
resulting in the formation of highly concentrated boric acid solutions or
deposits of boric acid crystals. These concentrated solutions of boric acid
may be very corrosive for carbon steel. This is illustrated by recent test
data, tabulated below, which were referenced in NRC Information Notice No.
86-108, Supplement 2.
Concentration of boric acid Temperature
Corrosion rate (percent) Condition (F)
mils/month
25 Aerated 200 400
25 Deaerated 200 250
15 Aerated 200 350-400
15-25 Dripping 210 400
If all of the water evaporates and boric acid crystals are formed, the
corrosion is less severe. However, boric acid crystals are not completely
benign toward carbon steel, and at a temperature of 500F, corrosion
rates of 0.8 to 1.6 mils/month were obtained in the Westinghouse tests
referenced in the generic letter. Corrosion by boric acid crystals was
observed in Turkey Point Unit 4 where more than 500 pounds of boric acid
crystals were found 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 minimize
reactor coolant leakages. This can be achieved by frequent monitoring of the
locations where potential leakages could occur and repairing the leaky
components as soon as possible. Review of the locations where leakages have
occurred in the past indicates that the most likely locations are (1)
valves; (2) flanged connections in steam generator manways, reactor head
closure, etc.; (3) primary coolant pumps where leakages occur at cover
to-casing connections as a result of defective gaskets; and (4) defective
welds.
In many of these locations the components exposed to boric acid solution are
covered by insulation and the leaks may be difficult to detect. If leak
detection systems have been installed in the components (e.g., reactor
coolant pumps from certain vendors), they should be used to monitor for
leakage.
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It is important to determine not only the source of the leakage but also the
path taken by the leaking fluid by evaluating the mechanism by which leaking
boric acid is transported. In some cases boric acid may be entrained in the
steam emerging from the opening in the pressure boundary that subsequently
condenses inside the installation thus carrying boric acid to 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) corrosion that
occurs some distance from the source of leakage and hence does not
significantly affect the rate of leakage. An example of the first type is
the corrosion of fasteners in the reactor coolant pressure boundary, for
example, in reactor coolant pumps. This type of corrosion can lead to
excessive corrosion of studs. The second type of corrosion can contribute
significantly to the degradation of the reactor coolant pressure boundary.
At Arkansas Nuclear One Unit 1, a leak developed in a high pressure
injection isolation valve located 8 feet above the high pressure injection
nozzle which was made of carbon steel. Accumulation of boric acid resulted
in an approximately 1/2-inch-deep corrosion wastage adjacent to the
stainless-to-carbon steel weld. Other locations of the nozzle exhibited
corrosion to a lesser degree. Corrosion of the reactor vessel head was
observed at Salem Unit 2. Corrosion pits were 1 to 3 inches in diameter and
40 to 300 mils deep. The source of this corrosion was a defective seal weld
in one of the instrument penetrations. These examples indicate that the
corrosion produced by boric acid could degrade even relatively bulky
components. At Fort Calhoun, the diameter of a reactor coolant pump closure
bolt was reduced from 3.5 inches to 1.1 inches by boric acid corrosion. At
San Onofre 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 primary
coolant into the containment.
Because of the nature of the corrosion produced by boric acid, the most
reliable method of inspection of components is by visual examination.
Ultrasonic testing performed in accordance with Section XI of the American
Society of Mechanical Engineers Boiler and Pressure Vessel Code may not be
sensitive enough to detect the wastage. At Fort Calhoun, two successive
ultrasonic tests failed to detect corrosion of the reactor pump closure
studs. When ultrasonic testing is used, the licensee should provide
assurances that the results are reliable.
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