Photo/IllutrationWhat appears to be sand has piled up on a pipe at the bottom of the containment vessel of the No. 1 reactor at the Fukushima No. 1 nuclear power plant. This image was taken by a survey robot on March 20. (Provided by the International Research Institute for Nuclear Decommissioning)

Tokyo Electric Power Co. (TEPCO) extended the mission of a survey robot after it failed to locate melted nuclear fuel at the No. 1 reactor of the Fukushima No. 1 nuclear power plant.

The robot on March 20 was sent into the reactor’s containment vessel, where the melted fuel is believed to have landed at the bottom, but pipes blocked its view, TEPCO said March 21.

The utility lengthened the period of research from four days to five days until March 22 to check other areas with high probabilities of melted fuel.

According to estimates, some of the melted nuclear fuel may have fallen into a cylinder-shaped structure that supports the pressure vessel, flowed out from the structure through a door for workers, and then spread at the bottom of the containment vessel.

The bottom of the containment vessel is covered with accumulated cooling water.

The survey on March 20 took place in an area about 1 meter from the door. The robot’s measurement device, equipped with a camera and a dosimeter, was hung from a stage for workers and lowered into the water below.

Although the pipes blocked the view of the conditions at the bottom of the containment vessel, the camera took pictures of what appears to be sand piling up on the pipes.

The radiation level around the area in the water was 6.3 sieverts per hour.

“Judging from the radiation level, there is a high possibility that what is piling up on the pipes is not nuclear fuel,” a TEPCO official said.