Information Notice 1990-10, Primary Water Stress Corrosion Cracking (PWSCC) of Inconel 600
| ML031470647 | |
| Person / Time | |
|---|---|
| Issue date: | 02/23/1990 |
| From: | Rossi C E Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| IN-90-010 | |
| Download: ML031470647 (3) | |
Information
Notice No. 90-10: f Z WIndex Site Map FAQ I Help I Glossary I Contact Us j Seaml A_ U.S. Nuclear Regulatory
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Notices > 1990 > IN 9 UNITED STATES NUCLEAR REGULATORY
COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555 February 23, 1990 Information
Notice No. 90-10: PRIMARY WATER STRESS CORROSION
CRACKING (PWSCC) OF INCONEL 600
Addressees
All holders of operating
licenses or construction
permits for pressurized
water reactors (PWRs).
Purpose
- This information
notice is intended to alert addressees
to potential problems related to primary water stress corrosion
cracking PWSCC of Inconel 600 that has occurred in pressurizer
heater thermal sleeves and instrument
nozzles at several domestic and foreign PWR plants. 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
contained
in this information
notice do not constitute
NRC requirements;
therefore, no specific action or written response is required.Description
of Circumstances:
During the 1989 refueling
outage at Calvert Cliffs Unit 2 (CC-2), visual examination
detected leakage in 20 pressurizer
heater penetrations
and 1 upper-level/pressure
tap instrument
nozzle. Leakage was indicated
by the presence of boric acid crystals at the penetrations
and on the nozzle. Non-destructive
examinations (liquid penetrant
and eddy current examinations)
were performed
on 28 thermal sleeves and 3 instrument
nozzles. Crack indi-cations were reported in 24 thermal sleeves, including
the 20 originally
identified
to be leaking as well as the leaking nozzle. No crack indications
were found in the two lower instrument
nozzles. The examinations
showed that all cracks in the sleeves and the nozzle were axially oriented with a maximum length not greater than 10.5 inches. The mode of failure for the thermal sleeves was identified
as PWSCC.The heater sleeves and the instrumentation
nozzles were made of Inconel 600 tubing and bar materials, respectively, supplied by INCO. All thermal sleeves were made in a high strength heat (NX8878) with a reported yield strength of 63.5 ksi. No chemical contaminants
were found on the sleeve fracture surfaces.
A review of the fabrication
records showed that all 120 thermal sleeves in CC-2 were reamed 3.5 inches from the top before welding and all but two were also 9002160093 IN 90-10 February 23, 1990 http://www.nrc.gov/reading-rrn/doc-collections/gen-cornm/info-noticesl1990/in90010.html
03/13/2003 Infonnation
Notice No. 90-10: reamed after welding to facilitate
the insertion
of the heater rods. All cracks in the sleeves were reported to be located inside the reamed region either above or below the J-groove weld.All instrument
nozzles were made from heat no. NX8297 and its yield strength was reported to be 36 ksi. The licensee indicated
that four upper instrument
nozzles, including
the leaking one, had been extensively
reworked when the faulty J-groove welds were repaired.
Based on the results of the investigations, the licensee, Baltimore
Gas and Electric (BG&E), postulated
that the leaking in the pressurizer
thermal sleeves and the instrumentation
nozzle was due to PWSCC. The licensee is in the process of removing a metallurgical
sample from the leaking instrument
nozzle for failure analysis to identify the mode of failure.On February 27, 1986, a small leak (about 0.15 gpm) was observed on a 3/4-inch-diameter
upper pressurizer
level instrument
nozzle at San Onofre Nuclear Generating
Station (SONGS) Unit 3 while the plant was in hot standby. An axial flaw about 5/8 inch in length was identified
on the inside diameter surface of the nozzle. The flaw appeared to originate
from the end of the nozzle inside the pressurizer
and extended beyond the attachment
weld (1/2 inch in depth) approximately
1/8 inch into the annulus area of the nozzle assembly.
The flawed nozzle was cut out, including
the entire attachment
weld. The results of the metallurgical
examination
performed
on the flawed nozzle assembly indicated
that the cracking was PWSCC.In spring 1989, leakage from instrument
nozzles was observed in two foreign PWRs (one from each 1300-MW plant) when the hydrostatic
pressure testing of the primary system was performed
during the first refueling
outage. The instrument
nozzles were made of Inconel 600 material.
The installation
of the nozzles included mechanically
rolling a portion of the nozzle into the pressurizer
shell. Nondestructive
examinations (NDEs) were performed
on the leaking nozzles and found the cracks to be principally
axial in orientation;
however, some circumferential
cracking was observed.
Destructive
examination
of these two leaking nozzles to identify the root causes has not been completed.
Additional
NDEs were performed
on all the instrument
nozzles of five 1300-MW PWRs. Crack indications
were found in 12 instrument
nozzles.Discussion:
Extensive
laboratory
testing has shown that intergranular
stress corrosion cracking (IGSCC) requires the presence of the following
three key elements: an aggressive
environment, susceptible
material, and sufficient
tensile stresses for crack initiation
and propagation.
PWSCC refers to IGSCC in the primary water environment
of PWRs. The laboratory
demonstration
of PWSCC in Inconel 600 was first reported by Coriou almost 30 years ago. The studies of PWSCC in Inconel 600 have been documented
in numerous published
reports.However, the mechanism
for PWSCC in Inconel 600 is still not well understood.
In PWRs, PWSCC of Inconel 600 was first reported in steam generator
tubing.IN 90-10 February 23, 1990 The cracking to date in the thermal sleeves and the instrument
nozzles of the domestic PWRs has been reported as being only axially oriented.
The safety implication
of an axial crack is not considered
a significant
threat to the structural
integrity
of the components
and most likely will result in a small leak. However, limited circumferential
cracking was reported in the instrument
nozzles of several foreign PWRs. The difference
in the cracking morphology
has been attributed
to the different
type of mechanical
working (rolling vs. reaming) being performed
on these nozzles and thermal sleeves.The appearance
of the crack in the rolled instrument
nozzle is consistent
with that observed in the roll-expanded
region of steam generator
tubing.Circumferential
cracking poses a more serious safety concern because if it were to go undetected
it could lead to a structural
failure of a component rather than to a limited leak.The reported cracking of the pressurizer
thermal sleeves and the instrument
http://www.nrc.gov/reading-rmldoc-collections/gen-comm/info-notices/1
990/in900
1 0.htmI 03/13/2003 Information
Notice No. 90-10: nozzle at Calvert Cliffs Unit 2 and the instrument
nozzle at SONGS Unit 3 are the most current PWSCC events for Inconel 600 in domestic PWRs besides the cracking problem associated
with the steam generator
tubing and plugs.The pressurizer
thermal sleeves in Calvert Cliffs Unit 1 (CC-1) were also made of the same heat of susceptible
material, but the recent inspection
of the CC-1 pressurizer
did not reveal any leaking or cracking of the thermal sleeves. The licensee indicated
that the major difference
in the fabrication
of thermal sleeves between CC-1 and CC-2 is that the pre-installation
reaming operation
was not performed
on CC-1 sleeves. The Combustion
Engineering
Owners Group (CEOG) performed
an evaluation
of pressurizer
heater sleeve susceptibility
to PWSCC for plants designed by Combustion
Engineering.
The CEOG recommended
a visual inspection
program for the thermal sleeves. The inspection
program for the thermal sleeves varied, depending
on the degree of susceptibility
of the sleeve materials.
The sleeve susceptibility
was rated by the elements described
above. The staff notes that at CC-2 the yield strength of the thermal sleeve material is higher than that of the instrument
nozzle material.
However, PWSCC occurred in both heats of materials.
This circumstance
may indicate that the yield strength of the material is not necessarily
a reliable screening criterion
for PWSCC susceptibility.
The CEOG is performing
additional
work to address PWSCC of Inconel 600. The CEOG programs include the following activities:
evaluations
to gain better understanding
of the cracking mechanism
in pressurizer
thermal heater sleeves and instrument
nozzles; an analytical
determination
of a temperature
profile for the heater sleeves;review of the fabrication
history of all Inconel 600 penetrations
in the primary system components;
conduct of a test that is primarily
a system leakage test on a mock-up of the flawed components;
and improvement
of NDE methods for cracking evaluation.
PWSCC of Inconel 600 is not a new phenomenon.
However, very little special attention
has been given to the inspection
for PWSCC in Inconel 600 applica-tions other than that associated
with the steam generator
tubing. As a result of the recently reported instances
of PWSCC in the pressurizer
heater thermal sleeves and instrument
nozzles in several domestic and foreign PWRs, it may be prudent for licensees
of all PWRs to review their Inconel 600 applications
in the primary coolant pressure boundary, and when necessary, to implement
an augmented
inspection
program.IN 90-10 February 23, 1990 This information
notice requires no specific action or written response.
If you have any questions
about the information
in this notice, please contact one of the technical
contacts listed below or the appropriate
NRR project manager.Charles E. Rossi, Director Division of Operational
Events Assessment
Office of Nuclear Reactor Regulation
Technical
Contacts:
William H. Koo, NRR (301) 492-0928 Robert A. Hermann, NRR (301) 492-0911 Attachment:
List of Recently Issued NRC Information
Notices.ENDEND http://www.nrc.gov/reading-rmldoc-collections/gen-comnilinfo-noticesll
990/in900
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