Tepco said Saturday it has detected radiation of up to 4,000 millisieverts per hour at the building housing the No. 1 reactor at the Fukushima No. 1 nuclear plant.

The radiation reading, which was taken when Tokyo Electric Power Co. sent a robot into the No. 1 reactor building on Friday, is believed to be the largest detected in the air at the plant so far.

On Friday, Tepco found that steam was spewing from the reactor floor. Nationally televised news Saturday showed blurry video of steady smoke curling up from an opening in the floor.

Tepco said it took the reading near the floor at the southeast corner of the building, under which runs a pipe emitting steam. No damage to the pipe was found, the utility said.

The pressure suppression containment vessel is located under the building and highly radioactive contaminated water generated by the reactor is believed to have accumulated there, Tepco said, adding the steam is probably coming from the water.

The utility said its workers have no plan to work near that area, but it will carefully monitor developments.

Meanwhile, tanks for storing radioactive water were on their way Saturday to the plant.

Tepco has said radioactive water could start overflowing from temporary storage areas on June 20, or possibly sooner if there is heavy rainfall.