Radiation inside the containment vessel of the No. 2 reactor at the Fukushima No. 1 nuclear plant measures as high as a deadly 530 sieverts per hour, the highest since the 2011 disaster, plant operator Tokyo Electric Power Co. (TEPCO) announced on Feb. 2.
TEPCO calculated the radiation dose from video noise on footage it took inside the containment vessel in late January, when a camera was inserted to examine conditions inside and scout a route for a scorpion-like observation robot scheduled to go into the vessel later this month.
Deployment of the robot is also being reconsidered after two gaping holes were found along the robot's planned path over a 5-meter-wide circular walkway inside the containment vessel, close to where the 530-sievert radiation dose was detected.
The holes in the metal grate walkway -- one of unknown size and the other measuring about 1 meter square -- make both routes considered for the robot impassable.
"We will consider re-evaluating what observations we can take with the robot," Yuichi Okamura, an acting general manager with TEPCO's on-site nuclear power division, told reporters at a Feb. 2 news conference.
Piles of a black and dark brown substance several centimeters thick and thought to be melted nuclear fuel were also observed on the walkway, creating a further possible obstruction to the robot. Meanwhile, examination of the 1-meter-square hole suggests the walkway was struck with tremendous force, hinting that there may be a large amount of melted fuel below.
"It is possible that the nuclear fuel rods melted onto the control rods and then dripped down," Tokyo Institute of Technology professor of nuclear engineering Yoshinao Kobayashi told the Mainichi Shimbun. "It's highly likely that part of the bottom of the pressure vessel broke and the melted fuel flowed down (onto the walkway), and then the grating warped and gave way due to the fuel's heat."