United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-29: Cracking of Cladding and its Heat-Affected Zone in the Base Metal of a Reactor Vessel Head

                                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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                                                            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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