(AP) - Federal regulators want nuclear power plants in California and Washington state to conduct additional, in-depth research into earthquake risks.

The Nuclear Regulatory Commission said in a statement Wednesday that the research at the Diablo Canyon reactors near San Luis Obispo and the Columbia generating station in Richland, Washington, must be completed by June 2017.

Among U.S. plants, the agency says Diablo Canyon and Columbia face "the highest ... hazard" when potential strong shaking is evaluated against the plants' designs.

Bill Dean of the NRC's Office of Nuclear Reactor Regulation says evidence so far leaves the agency confident that the plants are safe to continue operating while operators do more analysis.

Anti-nuclear activists say the directive confirms concerns that Diablo Canyon's design might not account for the threat of nearby faults.

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