ML19210E669

Forwards Supplemental Info to Deficiency Rept Re Defects in Ventilation Stack.Weekly Insp Program Initiated to Ensure Safety of Structure.Transfer of Fuel from Current Location Not Prudent

ML19210E669
Person / Time
Site:	LaSalle
Issue date:	11/28/1979
From:	Delgeorge L COMMONWEALTH EDISON CO.
To:	James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III)
References
NUDOCS 7912050501
Download: ML19210E669 (3)

Text

Commonwealth Edison q

One First. National Plaza. Chicago. til 9ois Address Reply to: Post Office box 767 Chicago !!!iaois 60690 November 28, 1979 Mr. James G. Keppler, Director Directorate of Inspection and Enforcement - Region til U.S. Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, Illinois 60137 Subj ect: LaSalle County Station Units 1 and 2 Vent Stack - 10 CFR 50.55(e)

Supplemental Report '

NRC Docket Nos. 50-373/374 Re'erence (a): G. Flore111 letter to C. Reed dated October 24, 1979

Dear Mr. Keppler:

Enclosed is the response to the request for additional in-formation (Reference (a)) regarding the subject LaSalle County vent stack.

If you have any additional questions on this matter, please direct them to this office.

Very truly yours,

>- A / ./ ,

, of. NA -wty.&

L. O. De1 George Nuclear Licensing Administrator LaSalle County Station LOD:mae enclo re cc: Director of Inspection & Enforcement 1502 001 So/7 36 ~~

///

701205 03g

Commonwealth Edison NRC Docket Nos. 50-373/374 ENCLOSURE

, Response to NRC Concerns Regarding the Defects in the LaSalle Ventilation Stack

1. As discussed in Sargent & Lundy's Report No. SAD-337, the effects of cracks and linear indications detected in the LaSalle Unit 1&2 stack ring girder have been carefully evaluated. The major confirmed crack in the southeast quadrant has been ground out. The other anomalies are linear indications detected by ultrasonic testing.

These cracks or linear indications are the results of plate lamina-tion and weld metal shrinkage. Such cracks or Indications can only be initiated by the localized strains produced by weld metal shrinkage, which are many times higher than the yield strain of steel or strains due to design loads. The AISC Commentary on highly-restrained welded connections states that there are no known cases of lamellar tears (i.e., the indications referred to above) being initiated or propagated by design loads.

In the calculation of stresses due to the design wind load, S&L has made further conservative assumptions by considering a large portion of the girder-to-stack shell joints ineffective or partially effective, to take into account the presence of discontinuities. Even with such a conservative assumption, analysis shows that the stresses under the 90 mph wind load are substantially below the code permissible stress.

Considering the nature of the cracks and linear indications and the very low stresses under the design wind load, it is concluded that further initiation or propagation of cracks is not probable.

2. To further insure the safety of the structure, a weekly inspection program was initiated by Commonw:alth Edison personnel at the LaSalle Station in May 1979 Inspection results have not shown any new cracks or propagation of old cracks. During the proposed repair, precautions will be taken to preclude or mitigate any deterioriation of the known cracking and other anomalies. As shown in S&L's Drawing No. S-1132, the stack and ring girder will be reinforced by adding stiffeners prior to any repair work. As such, the stack will always have capability to withstand 90 mph winds at any stage of repair. In addition, inspection of cracking will be made on a daily basis on days during which weiding or grinding of the stack is being conducted to insure the structural integrity of the stack during the course of the repair.

3. As discussed in item 1, the stack in its present condition is capable of withstanding the effects of the 00 mph design wind. The only loading that can cause stack failure is the tornado wind. The stack repair is scheduled to begin in January, 1980 and is expected to be completed in April, 1980. The probability of a tornado striking and collapsing the stack in the area of the fuel storage vaults during this period is estimated to be 2x10-5 This probability takes in.to account the occurrence of a tornado of 100 mph winds or greater in the 1502 002

Commonwealth Edison region of the station (which is 10-3/yr/ square mile), the frequency of tornado occurrence during the period from December, 1979 through April, 1980 (0 34), the expected tornado damage area (0.2 square miles), and the probability of the stack falling into the fuel storage area (0 3). Because of this very low probability, we believe that the risk of damage to the new fuel in the event of stack failure is very remote. Given this insignificant risk prebability, and considering the alternative temporary storage facilities available for the new fuel assemblies, transfer of this fuel from its current location is not prudent. Moreover, the additional handling involved with moving the fuel to and returning it from an alternate facility also presents risks of damage which are not justified given the remote probability of a tornado with stack damage in the period preceeding the conpletion of the repair.

4 e

. . _ ~