United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-17: Reactor Vessel Top Guide and Core Plate Cracking

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C.  20555

March 10, 1995


NRC INFORMATION NOTICE 95-17:  REACTOR VESSEL top GUIDE AND CORE PLATE  
                               CRACKING


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 significant cracking has been observed in the
weld regions of the reactor vessel top guide and core plate in an overseas
BWR.  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 1994 inservice inspection of the Wuergassen BWR in Germany,
significant cracking was visually observed in the reactor vessel top guide and
core plate.  The cracks were circumferentially oriented along the weld regions
and were located in the rim areas of the top guide and the core plate (see
figure in Attachment 1).  The top guide and the core plate were made of
niobium stabilized austenitic stainless steel (SS) (equivalent to American
Iron and Steel Institute Type 347 SS) and were post-weld heat treated during
fabrication.  The Type 347 SS material had a relatively high carbon content
and a minimum niobium-to-carbon ratio.  Samples of this material had passed a
standard sensitization test.  Significant cracking was also found in the core
shroud, which was made of the same material.  The root cause of the observed
cracking is still under evaluation.    

Discussion

Early in 1991, minor cracking not associated with a weld was observed in a
cross beam of the top guide in a domestic BWR (Oyster Creek).  Subsequent
monitoring and assessment of the cracking showed that the structural integrity
of the top guide was maintained.  The cracking observed in the overseas BWR is
considered significant because it was the first time cracking was found in the
ring weld regions of the reactor vessel top guide and core plate.



9503030304.                                                            IN 95-17
                                                            March 10, 1995
                                                            Page 2 of 3


On November 22, 1994, General Electric (GE) issued Rapid Information
Communication Services Information Letter No. 071, "Top Guide and Core Plate
Cracking," which discussed the cracking event reported in the overseas BWR. 
The overseas BWR had accumulated approximately 13 years of hot operating time
and the average conductivity of the reactor water during the worst five fuel
cycles had been 0.22 microsiemens per centimeter [0.56 micromho per inch]. 
Although the overseas BWR was not a GE BWR and the GE-design BWRs have several
different configurations of the top guide and core plate assemblies, there are
similarities in the designs.  For example, GE noted that welds existed in GE
BWRs (domestic BWRs) in areas that were cracked in the overseas BWR.  In the
overseas non-GE BWR, Type 347 austenitic SS was used for fabricating the top
guide and the core plate, instead of Type 304 austenitic SS, which was used in
domestic BWRs.  GE has tested Type 347 SS for its resistance to intergranular
stress corrosion cracking (IGSCC).  On the basis of tests, GE has concluded
that sensitized Type 347 SS has a susceptibility to IGSCC equivalent to that
of sensitized high carbon Type 304.  When Type 347 SS is not sensitized, it
has a susceptibility to IGSCC equivalent to that of Type 304L SS that is not
sensitized.  GE concludes that domestic BWRs with a similar amount of hot
operating time may expect cracking to occur in the top guide and the core
plate.

The BWR Vessel and Internals Project (BWRVIP), by letter of January 3, 1995,
reported the GE evaluation of the safety significance of this cracking event
as it pertains to domestic BWRs.  The nonproprietary portion of the BWRVIP
letter is given in Attachment 2.  A BWRVIP report for all internals,
discussing IGSCC susceptibility ranking, safety consequences, inspection
scopes and methodologies, flaw evaluation, repair strategies and mitigation of
degradation is expected in the latter half of 1995.

The NRC staff will monitor the inspections of top guides and core plates in
the industry.  The staff is evaluating the safety implications of cracking in
these areas to determine whether additional generic communication is needed.

Related Generic Communications

On September 30, 1993, the NRC issued Information Notice (IN) 93-79, "Core
Shroud Cracking at Beltline Region Welds in Boiling-Water Reactors," in
response to the discovery of cracking of the core shroud welds at Brunswick
Unit 1 plant.  Following the additional discovery of core shroud cracks at
Dresden Unit 3 and Quad Cities Unit 1 in 1994, the following additional
generic communications were issued:  1) IN 94-42, "Cracking in the Lower
Region of the Core Shroud in Boiling-Water Reactors," on June 7, 1994;       
2) Supplement 1 to IN 94-42 on July 19, 1994; and 3) Generic Letter 94-03,
"Intergranular Stress Corrosion Cracking of Core Shroud in Boiling Water
Reactors," on July 25, 1994.


.                                                            IN 95-17
                                                            March 10, 1995
                                                            Page 3 of 3


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 Project Support
                                    Office of Nuclear Reactor Regulation

Technical contacts:  W. H. Koo, NRR       E. M. Hackett, NRR 
                     (301) 415-2706       (301) 415-2751
                     
                     T. A. Greene, NRR
                     (301) 415-1175

Attachments: 
1.  Figure, "Location of Cracking in 
      Top Guide and Core Plate"
2.  Letter from BWR Vessel and Internals 
      Project to U.S. Nuclear Regulatory Commission, 
      "Request for Information Regarding the Impact 
      of BWR Core Plate and Top Guide Ring Cracking," 
      January 3, 1995

(See File IN95017.WP1 for Figure 1)
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