Tokyo Electric on Sunday confirmed lethally high radiation levels inside the primary containment vessel of reactor 1 at the heavily damaged Fukushima No. 1 nuclear plant but found they were not nearly as high as those recently logged in reactor 2.
Using a camera-equipped robot on Saturday, Tepco logged 7.8 sieverts per hour on some grating inside the vessel and 1.5 sieverts per hour in water pooled at its bottom.
Those figures are far lower than the 210 sieverts per hour measured at one spot in the PVC of the No. 2 reactor last month, but they are still extremely high.
The four-day probe launched by Tepco on Saturday is aimed at locating the melted fuel rods inside the No. 1 reactor building.
The primary mission of the robot is to investigate the bottom of the containment vessel to see if it can capture images of the melted fuel. Debris is believed to have penetrated the vessel and fallen into the surrounding containment vessel as a result of the heavy damage inflicted by the March 2011 tsunami that devastated eastern Tohoku.
The pressure vessel is the main component of the reactor and contains the fuel rod assemblies. Finding the exact location and condition of the melted fuel is considered critically important to dismantling the reactors.
However, the high radiation inside poses a daunting challenge for those involved in the decommissioning work.
In photos handed out to the media, a valve is shown covered in a yellowish substance that the utility said could be rust.
Another photo shows the grating that the robot, which is attached to a cable, was traveling on.
The utility had sent the robot into the PVC of reactor 1 two years ago but it could only capture images of the grating at the time.
Tepco said the robot can withstand up to 1,000 sieverts before malfunctioning. It traveled about 5 meters on Saturday and will eventually make its way to the other side of the concrete structure through a space that runs beneath the pressure vessel, which houses the core.
If the robot reaches its goal, computer simulations by Tepco show that there is a chance that melted fuel rods could be found there, Tepco said.
In January and February, Tepco investigated the containment vessel of the No. 2 reactor.
It is now preparing to conduct a similar robot probe of the reactor 3.