NUCLEAR REGULATORY COMMISSION

UNITED STATES

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NUCLEAR REGULATORY COMMISSION

WASHINGTON, D. C. 20555

Docket No. 50~255

June 13, 1991

Mr. Dinesh C. Kansal Manager of Quality Assurance

Bechtel Power Corporation 9801 Washingtonion Blvd. Gaithersburg, Maryland 20878

Mr. Kansal:

SUBJECT: PALISADES ANCHOR BOLT DESIGN

This letter is follow-up to our discussion on April 23, 1991, regarding anchor bolt design practices used by Bechtel at Consumers Power Company's Palisades Plant. I advised you, and Mr. E. M. Hughes, Engineering Manager, that the NRC staff would forward a series of questions in order to: 1) further evaluate your unique design approach, and 2) ass~ss the need for NRC staff generic

communication addressing this issue.

Background

A meeting was held at NRC headquarters on April 23, 1991 between representatives of Consumers Power Company, Bechtel, and NRC Region I II and

headquarters staff. At issue was the application of a "unique methodology" in calculating ~nchor bolt stress allowables. Bechtel Corporation, under

contract to Consumers Power Company, had used a methodology for evaluating anchor bolt loadings that had no apparent established basis. This methodology

was used Jn twelve applications during the recent refueling outage.

Enclosure I contains two calculation sheets of anchor bolt design for the Palisades nuclear power plant. Two types of anchor bolts are shown on the

calculation sheets: 1/2" diameter Drillco Maxi bolts with 8'' embedment and

3/4" diameter Hilti Kwik bolts with 5" embedment. The distance between the

Drillco Maxi bolts and Hilti Kwik bolts are about 5.3" and 5.95". Two assumptions have been made in the calculations. One assumption is that the concrete holding capacity of Hilti Kwik bolts would not be reduced even though

the kwik bolts are closely located to the Maxi bolts and stress overlapping

between the two types of bolts is clearly shown on the drawing. The other assumption is that the Drillco Maxi bolts are assumed to behave as ductile anchors, whicb means that the steel anchor will fail and the concrete that

. holds the anchor will not. Ductile anchors require much less a safety factor

than non-ductile anchors.

. j l

EVALUATION

:f~i Rf~ With respect to the first assumption, test data have indicated that the

concrete holding capacity of each of th? two bolts closely spaced has been

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reduced compared to the concrete holding capacity of the bolts when they are widely spaced. Therefore, Bechtel needs to demonstrate that the concrete ,holding capacity of the Hilti Kwik bolts would not be reduced when they are closely located to the Drillco Maxi bolts, as assumed in the calculation. If Bechtel can not demonstrate the correctness of its assumption, Bechtel needs to reevaluate the adequacy of the Kwik bolts~

With respect to the second assumption, a majority of the 4/23/91 meeting was spent discussing Bechtel's rational for their design approach. Bechtel asserts the correctness of this assumption by stating that the total concrete holding capacity of four Drillco Maxi bolts exceeds four times the tensile capacity of a Maxi bolt. In calculating the tensile capacity of a Maxi Bolt, Bechtel used the value of 125 ksi as the tensile str.ength of bolt material. The NRC staff has equated the total concrete holding capacity of four Maxi bolts calculated by Bechtel to four times the tensile capacity of a Maxi bolt (yielding a tensile strength of bolt material equal to 150 ksi). This calculation indicates that Maxi bolts would not behave as ductile bolts if the actual bolt strengths reach above 150 ksi, because concrete would fail before bolt rupture. Enclosure II contains drillco Maxi bolt test data (provided by Drillco). This data shows seven bolts failed in the du~tile manner. Only one bolt failed at the? tensile strength of 146.23 ksi. The other six bolts reached beyond 150 ksi tensile stress. Based on this test dat~, the Maxi bolts at Palisades. are likely t~ behave as non-ductile anchors. Therefore, Bechtel need to demonstrate that ~he Maxi bolts at Palisades will behave as ductile anchors as Bechtel assumed. Otherwise, the Maxi b~lts at Palisades need to be reevaluated for their adequacy CIS non-ductile anchors.

CONClUSION AND RECOMMENDATION

The NRC staff has reviewed the two calculation sheets of anchor bolt design for the Palisades nuclear power plant and found two questionable assumptions used_ in the calculations. One assumption is that the concrete holding capacity of Hilti Kwik bolts had not been reduced in the calculation even though they were located closely to Drillco Maxi bolts. This assumption is contrary to available test data. The other assumption is that the Drillco Maxi bolts were assumed to behave as ductile anchors by Bechtel, while the staff has demonstrated that they are likely to behave as non-ductile anchors.

Therefore, the NRC staff requests that Bechtel:

1) substantiate the two assumptions it has made for anchor bolt design at Palisades; . 2) check whether these two assumptions were used by individual designers and checkers for Palisades as isolated cases; or, are they included in Bechtel generic anchorage design guidelines; and, 3) quantify the number of utilities where this design approach was applied and assess the impact of these assumptions on all anchors similarly designed.

For your information Consumers Power Company has been previously advised (4/23/91 and 5/14/91) to verify that anchor bolt stress allowables were not exceeded by reducing the stress capacities in proportion to the ratio of

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anchor bolt separation distances (per manufacturer's recommendations) in order to arrive at a conservative assessment of bolt capacities. I request that you respond in writing to the open issues and questions addressed in this letter. Contact me at (301) 492-1344 should you have any questions.

Si nce're ly,

Enclosure: As stated

/SI

Brian E. Holian, Project Manager Project Directorate III-1 Division of Reactor Projects III/IV/V Office of Nuclear Reactor Regulation

.DISTRIBUTION? ?OotKET FILE?

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