The Tokyo Electric Power Company (Tepco), sent a cleaner robot into the Primary Containment Vessel of the damaged Unit 2 reactor at the Fukushima Daiichi Nuclear Power Station on Thursday, which was meant to clear deposits from a rail that would later be used to insert a Scorpion inspection robot. The robot, the company said, did its job for about two hours before failing due to the effect of high radiation.
Before the cleaner robot failed completely, it was retrieved, so that it would not expire in the path of the next robot, the Scorpion, so named for its flexibility, which allows it to get through tight spaces in order to take photographs and take measurements – temperature and radiation doses – inside the damage reactors.
The cleaner robot removed a part of the potentially troublesome deposits (one fifth of the originally planned area) on the rail that will be used to insert the Scorpion robot. Within two hours, however, the camera on the cleaner robot started to go dark, a sign that radiation was beginning to take its toll on the robot.
The cleaner robot also took pictures of the deposits and will help engineers determine whether they will pose obstacles to the more thorough inspection to follow. “The photos will help engineers select the best route for Scorpion to take to the pedestal area, the bottom of the PCV, where fuel melted during the March 2011 accident, fell to, and re-solidified,” Tepco said. The new photographs, together with ones taken earlier, indicate parts of the metal grating, used as a floor inside the PCV when the reactor was running, now may be missing.
Following a remote-controlled telescopic inspection last week that detected a high radiation level inside the PCV, the robot also detected a lethal level of radiation – as high as 650 Siiverts per hour – as it was performing its cleaning task.
Although radiation levels were high “there is no reason to believe that the level itself has increased or 'spiked' as some reports suggested,” said Tepco. The levels reported inside the PVC were estimates based on noise images on the camera screen, and have a margin of error of plus or minus 30 percent.”
Among the points negating the concept of a spike of radiation inside the PVC, outside of the PVC has shown no radiation level changes. Even so, extra precautionary steps were taken to ensure there were no leaks of radiation during the investigation. The penetration pipe through the PVC is tightly sealed. The work area is also pressurized so that air only moves from the outside into the PVC and not the other way around.