YOKOSUKA NAVAL BASE, Japan — The scenario was the same in the city of Yokosuka’s nuclear disaster drill as it was in years past: A monitoring post at the base detects an abnormal amount of radiation.

The U.S. Navy’s response also was the same: to establish that the radiation wasn’t coming from any nuclear-powered warships.

In Wednesday’s drill the fictitious radiation was coming from a nearby construction site that was conducting X-ray tests.

But, for the first time in six years, the Navy sent seven observers to watch as about 550 people from Yokosuka city office, police, Japan Self-Defense Forces, doctors, local residents and government practiced evacuations, communications and victim care.

The city requested increased Navy participation after last year’s announcement that the nuclear-powered aircraft carrier USS George Washington would replace the USS Kitty Hawk at Yokosuka Naval Base in 2008, said citizen’s safety division official Kazuaki Onuki.

Although the U.S. Navy maintains that there has never been a nuclear accident in 50 years of using nuclear energy to run its warships, the drill prepares for “just in case,” Onuki said.

“It is a way to acquire sense of security,” Onuki said.

Cmdr. David Waterman, spokesman for Commander, Naval Forces Japan, said “the professionalism of those manning the operations center was evident and the well-scripted drill appeared to be a success.”

He referred questions about future disaster planning to the city.