United States Nuclear Regulatory Commission - Protecting People and the Environment

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 
Page Last Reviewed/Updated Tuesday, November 12, 2013