NUCLEAR REACTOR SITES

Manuel G. Bonilla

Two nuclear reactor sites in California were the first in the United States for which active faults were an important safety consideration. Investigation of these sites in the 1960s by Menlo Park USGS scientists, among others, showed that the threat of surface faulting was very serious and led to the abandonment of both sites. Furthermore research stimulated by these investigations led to concepts regarding site evaluation that have been applied to the siting of reactors, dams, pipelines, waste-disposal facilities, and other engineering works.

The Bodega Head reactor site, 82 km north-northwest of San Francisco, California, is close to the western edge of the San Andreas Fault Zone and about 2 km west of the fault break associated with the 1906 earthquake. Because of the nearby San Andreas fault, excavations for the reactor were minutely examined by geologists, principally Julius Schlocker and Manuel Bonilla of the USGS. During the excavation process a fault was found in the sediments overlying the bedrock. Because the site is not close to the main break of the San Andreas fault, determination of the degree of activity and importance of this fault included a careful review by Schlocker and Bonilla of the surface ruptures that occurred at some distance from the main 1906 break. Although the utility company proposed a design that would accommodate faulting, the Atomic Energy Commission and its consultants concluded that, although the design might protect the reactor from faulting, the piping and other connections leading out of the reactor probably could not withstand both the faulting and the shaking from the expected magnitude 8+ earthquake on the San Andreas fault. Furthermore they stated that "experimental verification and experience background on the proposed novel construction method are lacking," and concluded that "Bodega Head is not a suitable location for the proposed nuclear power plant at the present state of our knowledge" (U.S. Atomic Energy Commission, 1964, p. 13, 14). On October 30, 1964, three days after this statement was released, the utility announced that it had abandoned plans for a nuclear power plant at the site (Novick, 1969).

The Malibu nuclear reactor site, on the coast 45 km west of Los Angeles, California, lies within the Malibu coast zone of deformation and just south of the Malibu coast thrust fault, a major east-west fault. Investigations related to the project, principally by USGS geologists Robert Yerkes and Carl Wentworth, showed that a fault of unknown age crossed the site and that several faults having displacement in the late Pleistocene existed in the Malibu coast zone outside the plant site (Yerkes and Wentworth, 1965). In addition to these local conditions, the regional setting was an important factor in evaluation of the site. The site lies in an east-west belt of moderate seismicity that contains the Malibu coast zone. This zone forms the northern border of a structural block whose eastern border is inferred to be 30 km to the east, along the right-lateral, northwest-trending Newport-Inglewood zone of faults and folds, which has been active in the Holocene. The inference was made that the structural block is currently moving relatively northward, with right-slip on its eastern border and thrusting on its northern border. This combination of local and regional evidence led to the conclusion that the east-west structural zone containing the nuclear reactor site is tectonically active (Yerkes and Wentworth, 1965; Marblehead Land Company, 1966). The value of such regional analysis is recognized in U.S. nuclear plant siting criteria, which, in defining a capable fault, include those faults that exhibit "a structural relationship to a capable fault ... such that movement on one could be reasonably expected to be accompanied by movement on the other" (U.S. Nuclear Regulatory Commission, 1977, p. 413). The Atomic Energy Commission ordered that the proposed Malibu reactor would have to be designed to accommodate fault displacement. Both the amount of differential ground displacement and the adequacy of any proposed design criteria to accommodate the displacement were required to be determined (U.S. Atomic Energy Commission, 1967). Six years later, the applicant withdrew the construction permit application, and the project was abandoned.

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