Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
April 30, 1990
Information Notice No. 90-29: CRACKING OF CLADDING AND ITS
HEAT-AFFECTED ZONE IN THE BASE METAL OF A
REACTOR VESSEL HEAD
Addressees:
All holders of operating licenses or construction permits for nuclear power
reactors.
Purpose:
This information notice is intended to alert addressees to a potential
problem related to cracking of cladding and its heat-affected zone in the
base metal of a reactor 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
contained in this information notice do not constitute NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances:
During the 1990 refueling at Quad Cities Unit 2, the licensee observed
cracks in the cladding and its heat-affected zone in the base metal of the
reactor vessel head. The cracks were initially observed visually as rust
stains on the cladding. Subsequent liquid penetrant and ultrasonic testing
(UT) revealed 34 surface and 15 subsurface flaws. The longest surface flaw
was 30 inches and circumferentially oriented in the direction of welding.
The longest subsurface flaw was approximately 20 inches and
circumferentially oriented. All flaws were circumferentially oriented
except for one surface flaw that was transversely oriented in the direction
of welding.
The licensee excavated 9 flaws and removed 5 boat samples. The excavations
and boat samples indicated that the deepest penetration into the base metal
was 0.225 inch beyond the base metal-clad interface, but was contained
within the heat-affected zone of the cladding.
Discussion:
The licensee observed cracks and linear indications both on the surface and
beneath the surface of the cladding on the reactor vessel head of Quad
Cities Unit 2. The cracking is located mainly in a region of the head where
the cladding originally applied by submerged arc welding intersects the
shielded
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IN 90-29
April 30, 1990
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metal arc back-clad. (Back-clad is applied to components where sections of
the component are welded together and each section had previously been
clad.) Metallographic analysis indicates that the surface flaws were
propagating through the cladding as interdendritic stress corrosion cracks.
Some cracks continued into the base metal heat-affected zone. The factors
contributing to the initiation of the surface cracks were high residual
stresses in the cladding, low ferrite content of the cladding, and the
oxidizing environment (large amounts of oxygen and peroxide mixing with
condensed steam) on the inside surface of the reactor vessel head. The
cracks were located in the back-clad region adjacent to the head-to-flange
weld. The cladding in this region had been subjected to a considerable
amount of grinding during fabrication. The licensee believes that the
grinding during fabrication was a significant contributor to the high
residual stress in the cladding.
The cause of the cracks that were found beneath the surface is believed to
be fabrication related. The nuclear steam supply system vendor, the
licensee, and the staff are still evaluating the available information to
establish the root cause of the cracking problem and its generic
implications. General Electric issued RICSIL No. 050, dated April 23, 1990
to BWR owners on this subject.
The surface cracks can be detected by penetrant testing of the back-clad
region and the subsurface cracks can be detected with enhanced UT procedures
that are performed by experienced nondestructive examination personnel. The
presence of extensive machining or grinding marks or rust stains on the
cladding surface could indicate a potential area of cracking.
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: Barry J. Elliot, NRR
(301) 492-0931
William H. Koo, NRR
(301) 492-0928
Attachment: List of Recently Issued NRC Information Notices
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