ML20236B868

Responds to to President G Ford.Cps for Unit 2 Granted in 1968.Investigations Conducted by Util & NRC Concluded That Plant Should Be Designed to Withstand Stated Postulated Earthquakes in Area of Plant

ML20236B868
Person / Time
Site:	Diablo Canyon, 05000000
Issue date:	03/26/1976
From:	Gossick L NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO)
To:	Hensey W AFFILIATION NOT ASSIGNED
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References
FOIA-87-214 NUDOCS 8707290267
Download: ML20236B868 (2)

Text

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i INR o 41976 Professor Will'imm H. Hensey, Jr.

i 4550 Franklin Avenue Los Angeles, California 90027

Dear Professor Hensey:

This replies to your letter of January 31 addressed to President GeraJd Ford. The construction permits for Diablo Canyon Unit 2 were granted in 1968. The seismic design proposed by the utility was reviewed by the Nuclear Regulatory Commission and its geotechnical consultants, U. S.

Coast & Geodetic Survey and the U. S. Geological Survey. The inves-tigations conducted by the utility and our review concluded that the Diablo Canyon units should be designed to withstand: 1) n postulated magnitude 81/2 earthquake centered along the San Andreas fault 48 miles from the plant, 2) a postulated magnitude 7.25 earthquake centered along the Nacimiento fault 20 miles from the plant, and 3) a postulated mag-nitude 6.75 earthquake within six miles of the plant. This seismic design took account of all of the faults known within the region at that time together with their potential for earthquake generation.

Subsequently, in 1971, published investigations of the offshore region showed a possibic fault near the Diablo Canyon site. At the onset of our Operating License review in August 1973, we required the utility to investigate the offshore area to determine the earthquake potential of the offshore structure. The investigations conducted by the utility together with investigations conducted independently by the U. S.

Geological Survey have resulted in an understanding of the offshore structure which we believe permits us to make a full assessment of its seismic potential. Our licensing action of the plant was delayed in order that this investigation could be completed.

The investigation has shown the existence of an offshec fenit, the Hosgri fault, at least 90 miles in length. At its nearest approach, the Hosgri fault passes within about 31/2 miles of the Diablo Canyon site. A conservative analysis of the earthquake potential of this fault assuming that it is coupled to the San Semion fault to the north has been made during our review. This suggests that a maximum expectable earthquake on this offshore structure might be as large as magnitude 7.0 to 7.5. An assessment of the effect of ground motion from a postulated earthquake of this size on the Diablo Canyon units is now being mad i

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. Professor William H. Hensey, Jr. 2 The San Onofre plant also received thorough reviews before a construction permit was granted for the units at that site. The ,

investigations of the San Onofre region also included thorough in- l vestigation of the offshore geology. It was determined that a fault exists about five miles offshore from the San Onofre site. The units at the San Onofre site were designed to withstand the maximum earth-quake which we believe could occur on the offshore zone. j r

I hope that this letter has been responsive to your concern. Please i contact this agency if you have further questions.

Sincerely,

}5!gned) Lee y, canick Lee V. Gossick Executive Director for Operations DISTl l DOCKET FILES (50-275-50-323)

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