The nation's top nuclear regulator is casting doubt on whether reactors in the U.S. are prepared for the type of prolonged power outage that struck a nuclear power plant in Japan.

Nuclear Regulatory Commission chairman Gregory Jaczko said Thursday that a U.S. requirement that a plant be able to last at least four hours without any electrical power "doesn't seem to be reasonable" after the Japan disaster. After that time, the radioactive core's cooling systems could fail unless some power is restored.

Of the 104 nuclear reactors in the United States, 87 can last four hours in a blackout. Another 14 can cope for eight hours, and three can last for 16 hours.

The Fukushima Daiichi plant had no electrical power for days after the earthquake and tsunami.

©2011 The Associated Press. All rights reserved.