ML20024G139

Confirms Results of Discussions & Conclusions Reached by NRR Subsequent to 760226 Meeting Re Structural Integrity of Mark I Containment Sys

ML20024G139
Person / Time
Site:	Monticello
Issue date:	02/27/1976
From:	Rusche B Office of Nuclear Reactor Regulation
To:	Mayer L NORTHERN STATES POWER CO.
References
NUDOCS 9102070579
Download: ML20024G139 (3)

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'%2' Nuclear Support Services V5tello 414 Nicollet Mall -'8th Floor BRusche Minneapolis, Minnesota 55401 EGCase TBAbernathy Gentlemen:

JR5uchanan ACRS (16) ret MONTICELLO NUCLEAR GENERATING PLANT As a result of a recent additional assessment of the structural integrity of the Mark I containment system used in your reactor facilities, members of our respective technical staffs met in Bethesda on February 26, 1976.

Inis letter will confirm the results of thsse discussions and the conclusions reached by the NdR staff subsequent to the meeting.

On January 28, 1976 the Mark I owners' group specified that the torus support downward load strength ratios were based on an elastic analysis.

Ratios around 1.0 were not of great concern to you because you believed that a redistribution of loads would ooour and that these factors would be reduced when an elastio-plastic analysis was later performed.

On those facilities with strength ratios Greater than 0 9, the NHC staff requested further information, which we are currently evaluating.

On February 19 and 20,1976, a more detailed discussior; of the above technical points occurred.

After studying details t tne stress dis-tribution from Bechtel's computer output, it is our judgment and your consultants agree that redistribution of loads may not be significant for the welds at the top of the torus columns.

Therefore, the margin of safety at these welds are lower than those for other Mark I cocponents.

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FEB 2 71976 Northern States Power Company,

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Liecause of this reduced margin, the seating in Bethesda was called to discuss your plans to promptly restore adequate margins of safety e

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You described and discussed with our staff various operating modifications which would reduce the loads, such as use of an j,

g increased drywell pressure and dooreased torus water level. After discussing your planned sotions and the length of time required to-provide enhanced safety margins using the operational actions, you and other members of the Mark I owners' group iratioated that you would promptly enhance ~ safety margins by pressuring the containment to

. establish a. differential pretsure of at least-1 psig between the 4

drywell atmosphere and torus.

You proposed to assure that such dif Terential was established by Monday, March 1,1976.

Oar technical staff has asaoased tnis proposal and aeree wita your I

technical representativos tnat the estaolish=unt of a differential pressure of at least 1 psi will provide a rejuotion in potential l

loads during a postulated loss of coolant accident and an sesocisted rastoration of the carcins of safety no that the rarcin of safety is aboat a factor of two.

I am pleased with the pronpt response Dy the Mark I owner.s' group to our concern that astety c:arrins be restored in a prompt fashion.

Tnis, therefore confirms our discussions of yesterday that you will establish by Honday, Maren 1,1976 a difforentini pressure of a minious of one psi between the drywell atmosphere and the torus vapor space and that this differential pressure will be maintained whenever the reactor is in operation, until further authorization by tne NRC.

This will respond to i

our directive of yesterday that safety margins for the structure be enhanced promptly.

So.no of the systems may Oc codified tnereafter by a pumpb:ck systo:a to mininize the nitrogen requirocents. Our taff will revies any saca proposal promptly when your plans beco,e more defir.ite.

i Horeover, we also will continuo to word with year staff on pro st ir.p l e-centation of your plans for structural naifications of tac avr.tnincent ysto n in accardunes witn tne catabiicaed progros of tne r.arx I owners' r,r ou p.

Sincerely, Den C. Rusche, Director Office of. Nuclear Reactp Regulation nro N

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Fsbruary 27, 1976 i

cc Arthur Renquist, Esquire The Environmental Conservation A:

Vice President - Law Library s,,/h Northern States Power Company Minneapolis Public Library 414 Nicollet Hall 300 Nicollet Hall Minneapolis, Minnesota 55401 Minneapolis, Minnesota 55401 Gerald Charnoff Shaw, Pittman, Potts 4 Trowbridge 1800 M Street, N. W.

Washington, D. C.

20036 P?*de Howard J. Vogel, Esquire i

Legal Counsel 2750 Dean Parkway

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Minneapolis, Minnesota 55416 Steve Gadler 2120 Carter Avenue j

St. Paul, Minnesota 55108 l

Mr. Daniel L. Ficker Assistant City Attorney 638 City Hall St. Paul, Minnesota 55102 s

Mr. Kenneth Dzugan

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. Environmental Planning Consultant St. Paul' City Planning 421 Wabasha Street St. Paul, Minnesota 55102

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Sandra S. Gardebring Special Assistant Attorney General Counsel for Minnesota Pollution Control Agency 1935 W. County Road B2 Roseville, Minnesota 55113 Anthony Z. Roisman, Esquire Berlin, Roisman and Kessler 1712 N Street, N. W.

Washington, D. C.

20036

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