United States Nuclear Regulatory Commission - Protecting People and the Environment

Boric Acid Corrosion of Carbon Steel Reactor Pressure Boundary Components in PWR Plants (Generic Letter No. 88-05)

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                               March 17, 1988




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. 



(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 

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. 

                                   - 3 -

(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 

(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 

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. 

                                    - 4 -

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. 


                                   Frank Miraglia 
                                   Associate Director for Projects 
                                   Office of Nuclear Reactor Regulation 

As stated



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)

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 

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 

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 


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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