ML19259C209
| ML19259C209 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 06/06/1979 |
| From: | COMMONWEALTH EDISON CO. |
| To: | |
| Shared Package | |
| ML19259C206 | List: |
| References | |
| NUDOCS 7906140393 | |
| Download: ML19259C209 (2) | |
Text
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Commonwealth Edison NRC Docket Nos. 50-373/374 ATTACHMENT LASALLE COUNTY STATION UNITS 1 & 2 NON-CONFORMANCE OF VENTILATION STEEL STACK
Background
The ventilation stack located above the auxiliary building roof of LaSalle County Station Units 1 & 2 serves as a single elevated point of gas release for the reactor building, turbine building, solid radwaste building, off-gas standby gas treatment, and gland-seal exhaust system.
The stack is classified under "other seismic Category I structures." (See FSAR $ 3.8.4.1.6).
The stack has a total design height of 237'3-1/2,"
with inside diameters of 18'6" at the top and 24'0" at the base, with a 30-ft. long tapered transition section in the middle.
The design sheel thickness varies from 1-1/4" at the base to 5/16" at the top.
The stack is supported at its base by a ring girder, which is anchored to the top of the auxiliary building roof steel at eight locations, 45 degrees apart.
The stack was erected by John Mohr & Sons and is substantially complete.
Description of Non-Conformance After the base ring girder of the stack was erected and fabricated in the field, a crack open to the surface was observed at the juncture of the girder top flange and the stack shell plate.
A full penetration groove weld had been specified at this junction.
Subsequent non-destructive testing by ultrasonic methods indicated that the crack was shallow in depth and extended approximately 50 inches.
This joint defect failed to conform to the acceptance criteria of AWS Dl.1, Section 8.15, required by S&L's Specification J-2982, and the requirements specified in John Mohr & Sons' welding inspection criteria 77-021-400.
Therefore, a corrective action (grinding through the weld perpendicular to the end of the crack) was later inplemented in the field.
Spot grinding to trace the defect has been performed over approximately 17 feet of the joint, and then at the end of the defect the plate was ground through to stop further crack propagation.
2284 134 79061403??3
Commonwealth Edison NRC Docket Nos. 50-373/374.
Ultrasonic Testing Following disclosure of the cracks mentioned above, CONAM Inspection Agency was requested to perform additional ultrasonic testing to determine the adequacy of the ring girder material and stiffener welds.
This testing was conducted under Sargent & Lundy's direction.
Preliminary results from the ultrasonic testing indicate the possible existence of lamellar tearing in parts of the plates that are adjacent to the full penetration groove weld.
In addition, there are indications of laminations in part of the plate.
Assessment of the Problem The problem revealed by the ultrasonic testing is twofold.
The first problem is associated with plate lamina-tion; the other is related to lamellar tearing.
The laminations are discontinuities commonly found in all rolled structural steel products; they do not have any practical influence on the mechanical strength of the material.
Tests on a steel specimen cut from the girder web, conducted by an independent laboratory, verified that the mechaniccl strength of the plate at the site meets the requirement of ASTM A588.
Lamellar tearing is a separation of plate materials (crack) caused by induced strains in the through-thickness direction due to weld shrinkage.
They are often found in restrained full-penetration groove weld joints.
The long crack emanating fram the junction of the girder top flange and the stack shell is believed to be caused by lamellar tearing.
The stack with the defects discussed above has been determined to be able to resist the design basis wind load of 90 mph which has a recurrence period of 100 years.
Therefore, no immediate remedial action is required.
However, a re-enforcement of the stack base ring girder will be required prior to operation to satisfy the full stack design basis which includes consideration of a tornado.
Resolution of the Non-Conformance 2284 135 Normally, cracks by lamellar tears are very stable.
However, as a precautionary measure, a crack-monitoring program has been implemented by Commonwealth Edison's LaSalle County Station personnel.
Should any further deterioration occur, it will be reported,and the effects on the structural integrity of the stack will be assessed.
A permanent repair or re-enforcement of the existing stack base ring girder is now being studied.