Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
September 30, 1993
NRC INFORMATION NOTICE 93-79: CORE SHROUD CRACKING AT BELTLINE REGION WELDS
IN BOILING-WATER REACTORS
Addressees
All holders of operating licenses or construction permits for boiling-water
reactors (BWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees that cracks have been observed in the weld regions
of the core support shroud in boiling water reactors. 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 are not NRC requirements;
therefore, no specific action or written response is required.
Description of Circumstances
During the current refueling outage at Brunswick Unit 1 (a BWR-4 reactor),
in-vessel visual inspection revealed cracks at weld regions of the core
support shroud. The shroud is a stainless steel cylinder that serves to
direct the flow of water inside the reactor vessel. The shroud is completely
contained inside the 15.2 centimeter [6 inch] thick reactor vessel. The
structural integrity of the reactor vessel is not impacted by the cracks in
the shroud.
Carolina Power and Light Company (CP&L), the licensee for Brunswick, found
both circumferential and axial cracks in the shroud. The circumferential
cracks were located in the inside shroud surface in the heat-affected zone
(HAZ) of weld H-3 and extended 360 degrees around the circumference of the
shroud (see Figures 1 and 2). Weld H-3 is a horizontal weld which fuses the
top guide support ring to the lower shroud. The first axial crack discovered
was located on the outside shroud surface of weld H-4 in the lower shroud.
CP&L performed additional visual testing (VT) and ultrasonic testing (UT) of
the shroud and removed boat samples to evaluate the length and size of the
cracks.
Discussion
In 1990, crack indications were reported at core shroud welds located in the
beltline region of an overseas reactor (BWR4). This reactor had completed
approximately 190 months of power operation before the cracks were discovered.
As a result of this discovery, General Electric (GE) issued Rapid Information
Communication Services Information Letter (RICSIL) 054, "Core Support Shroud
9309290050.
IN 93-79
September 30, 1993
Page 2 of 3
Crack Indications," on October 3, 1990, to all owners of GE BWRs. The RICSIL
summarized the cracking found in the overseas reactor and recommended that at
the next refueling outage plants with high-carbon-type 304 stainless steel
shrouds perform a visual examination of the accessible areas of the seam welds
and associated HAZ on the inside and outside surfaces of the shroud.
Since early July, CP&L has performed VT inspections of the Unit 1 inside and
outside shroud surface in the vicinity of welds. These inspections were
performed in accordance with GE RICSIL 054 and discovered cracks in the weld
regions. CP&L determined that in order to perform an adequate VT it was
necessary to remove the outer blade guides, pre-clean inspection areas, and
obtain an improved resolution of "1-millimeter wire" (in lieu of the
Code-prescribed resolution). Camera and lighting positions were also found to
be crucial in performing adequate VTs. Also, CP&L has worked with GE to
develop more sophisticated UT equipment to identify how deeply into the shroud
metal the crack extends.
Additional VT inspections revealed more axial cracks at the inside surface of
weld H-4 as well as cracks at welds H-1, H-2, and H-5 of the shroud. One of
the additional cracks, a circumferential crack at weld H-5, appears to be
approximately 76.2 centimeters [30 inches] in length. The majority of the
cracks are located in the HAZ of the welds, although one crack was discovered
in the central region of shroud plate P-6. The crack in P-6, however, may be
associated with a possible weld repair of a surface defect in the plate after
its fabrication at the mill.
The results from the boat samples indicated intergranular stress-corrosion
cracking (IGSCC) as the mechanism. Preliminary results suggest that the crack
in the HAZ of weld H-3 may be 3.8 centimeters [1.5 inches] or more in depth.
The location of this crack is shown in Figure 2.
As a result of the shroud cracks being discovered on Unit 1, CP&L re-examined
the results of the inspection performed during the 1991 refueling outage of
Unit 2. The re-examination revealed three minor crack indications in the HAZ
of weld H-2. The licensee concluded that the cracks do not pose a concern to
normal operation of the reactor.
CP&L plans to repair the Brunswick Unit 1 core shroud before the plant is
brought back into service. CP&L intends to restore the strength of the shroud
by adding stiffening braces around the top portion of the shroud. However,
the licensee will continue to examine and evaluate the cracks in the core
shroud.
General Electric issued Revision 1 to RICSIL 054 on July 21, 1993, to update
the information on the core support shroud cracks and to provide revised
interim recommendations to perform visual examination of accessible areas of
the shroud at all GE BWRs during the next scheduled outage. The NRC has been
informed by GE that they are evaluating the Brunswick results and will provide
.
IN 93-79
September 30, 1993
Page 3 of 3
updated information to owners of GE BWRs. The NRC staff is evaluating the
implications of the shroud cracks for reactor core configuration and emergency
core cooling system performance under accident conditions at operating plants
and will consider the need for additional generic communications.
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 Office of
Nuclear Reactor Regulation (NRR) project manager.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: R. A. Hermann, NRR J. Medoff, NRR
(301) 504-2768 (301) 504-2715
P. Byron, Region II T. Greene, NRR
(919) 457-9531 (301) 504-1175
Attachments:
1. Figure 1: Weld and Plate Locations in the Beltline
Region of the Brunswick Unit 1 Core Shroud
2. Figure 2: Details of Weld Locations H-2 and H-3 in the
Brunswick Unit 1 Core Shroud
3. List of Recently Issued NRC Information Notices
.
