Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 84-18
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 7, 1984
Information Notice No. 84-18: STRESS CORROSION CRACKING IN PRESSURIZED
WATER REACTOR SYSTEMS
Addressees:
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
Purpose:
This information notice is being issued to remind all holders of pressurized
water reactor (PWR) licenses and construction permits that PWR systems are
susceptible to stress corrosion cracking in the presence of various
corrodants. Information is also presented on actions which, if properly and
conscientiously implemented, can significantly reduce the likelihood of such
cracking.
Discussion:
Stress corrosion cracking in boiling water reactor (BWR) primary pressure
boundary piping is currently receiving considerable industry and NRC
attention. This circumstance may lead to an unwarranted conclusion that
similar problems do not occur in PWRs. The reactor coolant system (RCS) of a
PWR has a hydrogen overpressure maintained as an oxygen getter during power
operation. As a result, the primary pressure boundary piping of PWRs have
generally not been found to be affected by stress corrosion cracking.
However, there are two conditions where significant potential exists for
inadvertent introduction of contaminants into PWR fluid systems. The first
opportunity is unacceptable levels of contaminants in the boric acid
purchased. The second is the free surface of the spent fuel pool which can
be a natural collector of airborne contaminants. During refueling operations
there is direct communication between the reactor coolant system and the
spent fuel pool, as well as increased free surface to collect any airborne
contaminants caused by concurrent maintenance activities. At Three Mile
Island Unit 1, during the extended shutdown caused by the Unit 2 accident,
sodium thiosulfate in some way was introduced into the reactor coolant
system and caused extensive stress corrosion attack on the Inconel 600*
steam generator tubes. The thiosulfate solution was normally kept in a
storage tank to be available as an
*Inconel 600 is an alloy trade name of International Nickel Company.
8402090028
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IN 84-18
March 7, 1984
Page 2 of 3
additive to the containment spray system fluid. This design concept was
employed only in reactors designed by Babcock and Wilcox, and is no longer
used.
Other systems which utilize borated water, and, therefore, are also made
with austenitic materials, may not receive the same attention which is given
to the RCS fluid. These systems are extensively cross-connected, and some
equipment serves more than one system function. Thus, contaminants
introduced at any point may appear elsewhere. Because of inadvertent safety
injection actuation, potentially contaminated water can enter the reactor
coolant system.
Stress corrosion cracking generally requires the presence of three factors:
a high level of local stress, material that is sensitive to attack, and the
presence of an active anion corrodant. Examples of such corrodants are
oxygen, chlorides, fluorides, sulfides, and other sulfur ions. In BWRs,
oxygen appears to be the corrodant ion. The first two factors appear as an
inherent result of the normal welding process which was used for assembling
piping systems in reactors currently operating. The third factor can be
controlled independent of the fabrication process used.
In September 1980, the NRC published NUREG-0691, "Investigation and
Evaluation of Cracking Incidents in Piping in Pressurized Water Reactors."*
That NUREG discusses pipe cracking from a variety of causes in austenitic
and nonaustenitic materials. Information is contained on the various
instances of cracks through May 1980. Additional information is contained in
NUREG-0679* published in August 1980.
Since the publication of NUREG-0691 and 0679, additional instances of stress
corrosion attack have been reported.
On December 16, 1981, while transferring spent fuel in the storage pool for
Prairie Island, the top nozzle for one fuel assembly separated from the body
of the assembly. The cause was not immediately apparent. On May 12, 1982,
the licensee submitted a report which indicated that the cause had been
identified as stress corrosion cracking. No specific corrodant was
identified but corrosion products on the crack surfaces contained Si, Al,
Cu, and Cl. None of the other fuel assemblies in storage were similarly
affected. It is not known for certain whether the corrosion cracking
occurred during operation or in the storage pool, but the presumption is
that the cracking occurred in the storage pool.
On January 29, 1983, Northern States Power Co. (the licensee) notified the
NRC that Prairie Island Unit 1 had been shut down because of a leak detected
in a pipe connecting the boric acid storage tanks to the safety injection
system. This pipe is part of the system used to mitigate the consequences of
a main
*Available in microfiche form from National Technical Information Service,
Springfield, VA 22161.
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IN 84-18
March 7, 1984
Page 3 of 3
steamline break, and is required by the plant technical specifications to be
operable whenever the unit is at power. Extensive stress corrosion cracking
was identified during piping inspections. Unit 1 remained shut down until
mid-April 1983, when it was returned to power operation following repairs.
Metallurgical examination of sections of piping removed during the repair
effort disclosed extensive stress corrosion attack. A deposit of iron oxide
on the inner wall of the pipe contained 79 to 110 ppm of chlorides, 114 to
204 ppm of sulfates, and 10 to 84 ppm of fluorides. The piping system was
normally stagnant and heat-traced to 180F to keep the concentrated
boric acid in solution. The source of the contaminants is believed to be
impurities in the purchased boric acid which were concentrated under
stagnant, heated conditions.
PWR accident mitigation systems are normally in a standby condition and
hence provide a fertile environment for stress corrosion cracking. In
addition to technical specification surveillance requirements to exercise
pumps and valves on a regular schedule, some licensees have initiated
measures to recirculate and test system fluids for potential contaminants to
facilitate prompt removal of any identified contaminants. In this
connection, Northern States Power Co. at Prairie Island is utilizing ion
exchange chromatography to detect the presence of potentially harmful
contaminants and reports that this is a practical, effective technique.
No specific action or response is required by this information notice. If
you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office, or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: J. B. Henderson, IE
492-9654
Attachment:
List of Recently Issued IE Information Notices
