Fire services and police cordoned off several streets and told residents to stay indoors in the western town of Oelde after the two boys left their mock power station on the street when they went home for dinner Monday evening.

"It wasn't a prank, they were just playing," a local police spokeswoman said Tuesday. "The boys tried to go back later to carry on but the fire brigade wouldn't let them through."

The lock-down of the area began when a passer-by saw the metal object with the yellow and black symbol on it, took fright and alerted authorities, the spokeswoman said.

Police sent out warnings on local radio for residents to remain in their homes while a radiation detector was rushed to the scene to investigate the old computer casing and the warning sign, which the boys had printed out from the internet.

After the object had been identified, the boys' parents explained to police the children had gone out to "play nuclear power station" that evening, the spokeswoman said.

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