Two six-year-old boys pretending to have built a mini nuclear power plant has prompted German police and the fire brigade to clear their street, authorities said.

The school children in the western town of Oelde had built the nuclear reactor mock-up out of a computer casing and taped a "radioactivity warning" they had printed out from the internet on its side.

"When the boys returned to their 'nuclear power plant' from a brief stop at home they were sent away again as the area and a wide radius around it had been cleared and blocked off," police said in a statement.

Residents were ordered not to leave their homes and firefighters tested for a radioactive leak.

The boys' parents thought the fire department was conducting a drill until they read about the operation online and what led to it.

They reported to the police station and explained their six-year-olds had not managed to build an actual nuclear reactor.