Bulletin 78-11: Examination of Mark-1 Containment Torus Welds
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
July 21, 1978
IE Bulletin 78-11
EXAMINATION OF MARK-1 CONTAINMENT TORUS WELDS
Description of Circumstances:
On June 28, 1978, Vermont Yankee Power Company (VYPC) reported that five
non-penetrating surface crack indications and one nine-inch long surface
crack were found in the overlay weld-to-torus base metal heat affected zones
during the process of performing modifications (addition of strengthening
gussets) to the torus support columns at the Vermont Yankee Nuclear
Facility. These modifications were being performed as part of VYPC's overall
program to restore the originally intended design safety margins for the
Vermont Yankee Mark I containment system (re: Mark I Containment Long Term
Program).
Based upon initial indications of the depth, VYPC performed an analysis of
the structural capability of the torus shells in the affected areas to
support continued operation of the facility and proceeded to attempt to
grind out the nine-inch crack. On June 30, 1978, the crack was still
apparent after grinding to the calculated depth of 0.25 inches. The plant
was placed in a cold shutdown condition on July 2, 1978. An evaluation is in
progress.
The welding operations at Vermont Yankee were performed at locations on the
torus shell which were lower than the water level in the torus. Although the
underlying causes of the cracking have not yet been determined, the presence
of water on the opposite side of the torus shell during the welding
operations appears to have been a primary contributor. Consequently, a
generic concern has arisen that the potential for cracking could exist when
welding is performed on a torus containing water.
In view of the above, on July 7, 1978, the NRC Office of Inspection and
Enforcement verbally requested licensee's to perform close visual
inspections on similarly made torus weldments at Peach Bottom Unit Nos 2 and
3, Quad Cities Unit Nos. 1 and 2, Hatch Unit No. 1 and Monticello
facilities. These inspections revealed no apparent linear indications
through the painted surface. However, Monticello reported
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IE Bulletin No. 78-11 - 2 - July 21, 1978
that magnetic particle examination, together with visual inspection revealed
two relevant surface linear indications, 1/2-inch long and 1-1/2 inches long
respectively, which were verified by liquid penetrant tests after removal of
the paint. Both indications were reportedly removed by grinding at less than
1/8-inch depth.
Action to be Taken by Licensee
Licensees for Peach Bottom Units 2 and 3, Quad Cities Units 1 and 2, Hatch
Unit 1 and Monticello are further requested to provide the following
information:
Item A
1. Provide descriptions of the welding procedures, procedure
qualifications, welder qualifications and electrode controls employed
in strengthening the support column to torus connections (i.e.,
addition of gussets, saddle supports, webs, etc.).
2. Provide a description of the preventive measures used to assure that
condensation did not occur on surfaces to be welded prior to and during
the welding.
3. Describe the chronology of nondestructive examinations performed
subsequent to such welding operations. Include procedures, methods and
techniques, the time period of NDE application after specific welds
completed have been at ambient temperature, and the results of these
examinations.
Item B
Where the NDE documentation (Item A3) is not sufficiently definitive to show
that welding to the torus was nondestructively examined after the completed
welds were at ambient temperature for a minimum period of 72 hours, the
following measures should be taken.
1. Remove paint from surfaces of the overlay weld and torus base metal
heat affected zones (if not already done so) by rotary wire brushing or
equivalent means.
2. Examine the exposed interbead fusion zone of the overlay weld and the
associated base metal heat affected zones utilizing magnetic particle
techniques in accordance with the applicable section of the ASME Code.
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IE Bulletin No. 78-11 - 3 - July 21, 1978
3. Any indications detected as a result of MT are to be evaluated as to
their acceptability in accordance with the applicable ASME code.
Examinations that detect relevant linear indications may be
supplemented by other nondestructive methods and techniques to
determine the character of the flaws (i.e., estimated size, shape,
depth, orientation, etc.).
4. Results of the field examination of individual weldments is to be
documented.
Within ten days of the date of issue of this Bulletin, report in writing to
the Director of the appropriate NRC Regional Office, the information
requested in Item A, and your proposed plan of action and schedule regarding
Item B if the 72 hour minimum period described above was not met. A copy of
your report as submitted to the Regional Office should be sent to the United
States Nuclear Regulatory Commission, Office of Inspection and Enforcement,
Division of Reactor Operations Inspection, Washington, D. C. 20555.
Approved by GAO, B180225(R0072); clearance expires July 31, 1980. Approval
was given under a blanket clearance specifically for identified generic
problems.
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