United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-17: Cracking of Vertical Welds in the Core Shroud and Degraded Repair

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                         WASHINGTON, D.C.  20555-0001

                                 April 4, 1997

                               SHROUD AND DEGRADED REPAIR


All holders of operating licenses or construction permits for boiling-water
reactors (BWRs).


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the discovery of cracks in vertical welds in the
core shroud and degradation of core shroud repairs at Nine Mile Point Nuclear
Station, Unit 1 (NMP1).  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

Cracking of Vertical Welds

During the current refueling outage (March 1997), Niagara Mohawk Power
Corporation, the licensee for NMP1, was performing an augmented inservice
inspection of welds in its repaired core shroud in accordance with Boiling
Water Reactor Vessel and Internals Project (BWRVIP), "Guidelines for
Reinspection of BWR Core Shrouds" (BWRVIP-07).  On March 14, 1997, the
licensee informed the NRC that it found significant cracking in vertical welds
V9 and V10.  (Welds V9 and V10 extend for 228.6 centimeters [90 inches] near
the mid-plane of the shroud).  The indications were located in the heat-
affected zones both on the outside diameter and to a lesser extent on the
inside diameter.  Approximately two-thirds of the length of these welds were
cracked to a depth of 50 to 80 percent of the wall.  As specified by BWRVIP-07
criteria, the licensee expanded the inspection sample to include other
vertical shroud welds.  

Niagara Mohawk had last examined vertical welds during the 1995 refueling
outage as part of its pre-modification inspection plan.  At that time, the
licensee performed enhanced visual examinations from the inside-diameter
surface of four vertical welds (V9, V10, V11, and V12) for a length of 15.2
centimeters [6 inches] near their intersection with the H5 circumferential
weld.  No crack indications were found in these areas during the 1995
inspection or during the current inspections.

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                                                            April 4, 1997
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Also, in the spring of 1996, Georgia Power Company (GPC) reported indications
in vertical core shroud welds V5 and V6 at the Edwin I. Hatch Nuclear Plant,
Unit 1.  GPC conducted the inspection in accordance with the guidance in
BWRVIP-07.  GPC reported two indications of 5.1 centimeters [2 inches] and
30.5 centimeters [12 inches] in the V5 weld on the outside 
diameter.  They also reported one indication 81.3 centimeters [32 inches] long
and four small (3.05 centimeters [1.2 inches]) indications in V6 on the
outside diameter.  Inspections of the inner diameter of both the V5 and V6
welds showed no cracking.  A limited visual inspection of 54.7 centimeters [18
inches] of the vertical welds V5 and V6 near the intersection with the
horizontal H4 welds in the fall 1994 outage had not revealed any cracks.  At
least a part of the region of one of the indications that was found in 1996 in
the V6 vertical weld had been visually inspected in the fall 1994 outage and
was found to be uncracked.

Degraded Repair at NMP1

Niagara Mohawk found the core shroud repair, which had been installed during
the 1995 refueling outage, was degraded.  The repair consisted of four
pretensioned tie rods running vertically from the top to the bottom of the
core shroud.  Inspections showed that the core shroud lower wedge support was
not in contact with the reactor vessel wall and the outside-diameter surface
of the shroud as required.  The function of this wedge support is to restrain
the shroud laterally during a seismic event, and it is held in place by a
spring latch device that failed on the tie rod at the 90-degree azimuth
location.  According to the licensee's preliminary evaluation, the lack of
contact could have affected the performance of the lower wedge support during
a postulated seismic event. 

The failed latch (called a "retainer clip") in the lower part of the 90-degree
tie rod assembly resulted in shifting of the associated lateral support block
downward to the end of the tie rod.  Visual inspection of the other three tie
rod assemblies revealed some discoloration of latches in the 270-degree and
350-degree tie rod assemblies, indicating possible degradation.  The lateral
support block for the 350-degree assembly was found to have shifted about
0.635 centimeter [1/4 inch] downward.  The licensee is evaluating the
potential consequences of a postulated multiple loss of retainer clips.

The licensee also found the nut at the top of the 270-degree tie rod to be
loose, although the retaining device for the nut remained in place.  The other
three tie rod nuts will be checked for tightness.  It was also observed that
the lateral support for the 270-degree tie rod assembly, which was contacting
the inner radial surface of a recirculation nozzle instead of the vessel wall
due to an initial mis-installation, had shifted slightly. 


In view of the extent and location of cracking at vertical core shroud welds
observed at NMP1, the BWRVIP intends to revise the shroud inspection guidance
in BWRVIP-07.  In the interim, the BWRVIP has informed its members that
enhanced visual examination of the core shroud outer diameter in the high-
fluence area is appropriate based on the vertical weld cracking observed at
Hatch 1 and NMP1..                                                            IN 97-17
                                                            April 4, 1997
                                                            Page 3 of 3

Niagara Mohawk is currently performing evaluations to determine if the
degraded core shroud repair would have performed satisfactorily during
postulated seismic events and if the shroud would have maintained its
structural integrity under all design-basis events.  

Related Generic Communications

NRC Information Notice 93-79, "Core Shroud Cracking at Beltline Region Welds
in Boiling Water Reactors," dated September 30, 1993.

NRC Information Notice 94-42, "Cracking in the Lower Region of the Core Shroud
in Boiling Water Reactors," dated June 7, 1994.

NRC Information Notice 94-42, Supplement 1, "Cracking in the Lower Region of
the Core Shroud in Boiling Water Reactors," dated July 19, 1994.

NRC Generic Letter 94-03, "Intergranular Stress Corrosion Cracking of Core
Shrouds in Boiling Water Reactors," dated July 25, 1994.

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.

                                          signed by 

                                       Thomas T. Martin, Director
                                       Division of Reactor Program Management
                                       Office of Nuclear Reactor Regulation

Technical contacts:  M. Banic, NRR
                     (301) 415-2771
                     E-mail:  mjb@nrc.gov

                     C. E. Carpenter, Jr., NRR
                     (301) 415-2169
                     E-mail:  cec@nrc.gov

                     Jai Rajan, NRR
                     (301) 415-2788
                     E-mail:  jrr@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013