Information Notice No. 88-03: Cracks in Shroud Support Access Hole Cover Welds
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 2, 1988
Information Notice No. 88-03: CRACKS IN SHROUD SUPPORT ACCESS HOLE
COVER WELDS
Addressees:
All holders of operating licenses or construction permits for boiling water
reactors (BWRs).
Purpose:
This notice alerts addressees to the potential for cracks in the welds of the
covers of the shroud support access holes within the reactor vessel. The
cracks could result in weld failure with resulting formation of loose parts
and core by-pass flow. The event described highlights the importance of
inspecting the access cover welds. It is expected that recipients will review
the information for applicability to their facilities and consider actions, as
appropriate, to preclude similar problems from occurring at their facilities.
However, suggestions contained in this information notice do not constitute
NRC requirements; therefore, no specific action or written response is
required.
Description of Circumstances:
Jet pump BWRs are designed with access holes in the shroud support plate which
is located at the bottom of the annulus between the core shroud and the
reactor vessel wall. Each reactor vessel has two such holes which are located
180 degrees apart. These holes are used for access during construction and
are subsequently closed by welding a plate over the hole. The covers and
shroud support ledge are Inconel Alloy 600 material. The connecting weld
material is also Inconel 600 (Alloy 182 or 82).
The high residual stresses resulting from welding, along with a possible
crevice geometry of the weld, when combined with less than ideal water
quality, present a condition conducive to intergranular stress corrosion
cracking (ISGCC). This has been recognized by General Electric and, as a
result, they have developed a remotely operated ultrasonic testing capability
for detecting cracks in the cover plate welds. The first use of this custom
ultrasonic testing fixture was at Peach Bottom Unit 3.
On January 21, 1988, intermittent short cracks were found in the weld heat-
affected zone around the entire circumference of the covers at Peach Bottom
Unit 3. It is estimated that cracking exists over 50% to 60% of the circum-
ference with cusps as deep as 70% through the wall. It is believed that cover
plate welds have not been inspected previously on any other BWR. It is
possible that the cracking is generic and may, therefore, affect all BWRs with
jet pumps.
8802010084
. IN 88-03
February 2, 1988
Page 2 of 2
Discussion:
General Electric has identified three concerns if failure of the access hole
cover plates is postulated due to weld cracking:
1. Loose parts - In the event of complete failure of the access cover weld
during normal reactor operation, the slightly higher bottom head area
pressure would lift the cover out of its recess. It would most likely
fall to one side, but there is a potential for it to be swept into the
recirculation pump suction line causing severe pump damage.
2. Core flow bypass (normal operation) - Loss of one or both cover plates
would allow some recirculation system flow to bypass the core, from the
jet pump discharge through the open access hole to the recirculation pump
suction. This flow transient would be readily detectable and would
require reactor shutdown.
3. Core flow bypass (Loss of Coolant Accident) - If the access hole cover
plate welds were to fail as a direct consequence of a recirculation
suction line break, the bypass path would prevent the emergency core
cooling system from reflooding the core to the 2/3 level. The core spray
system would be capable of maintaining adequate core cooling provided
there has been no degradation in the core spray piping.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical
contact listed below or the Regional Administrator of the appropriate regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Warren Hazelton, NRR
(301) 492-0911
Attachment: List of Recently Issued NRC Information Notices
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