Exposure to 73 sieverts for a minute would cause nausea and seven minutes would cause death within a month, Tokyo Electric Power Co. said.
The experts said the high radiation level is due to the shallow level of coolant water — 60 cm — in the containment vessel, which Tepco said in January was believed to be 4 meters deep. Tepco has only peeked inside the reactor 2 containment vessel. It has few clues as to the status of reactors 1 and 3, which also suffered meltdowns, because there is no access to their insides.
The utility said the radiation level in the reactor 2 containment vessel is too high for robots, endoscopes and other devices to function properly.
Spokesman Junichi Matsumoto said it will be necessary to develop devices resistant to high radiation.
High radiation can damage the circuitry of computer chips and degrade camera-captured images.
For example, a series of Quince tracked robots designed to gather data inside reactors can properly function for only two or three hours during exposure to 73 sieverts, said Eiji Koyanagi, chief developer and vice director of the Future Robotics Technology Center of Chiba Institute of Technology.
That is unlikely to be enough for them to move around and collect video data and water samples, reactor experts said.
“Two or three hours would be too short. At least five or six hours would be necessary,” said Tsuyoshi Misawa, a reactor physics and engineering professor at Kyoto University’s Research Reactor Institute.
The high radiation level can be explained by the low water level. Water acts to block radiation.
“The shallowness of the water level is a surprise . . . the radiation level is awfully high,” Misawa said.
While the water temperature is considered in a safe zone at about 50 degrees, it is unknown if the melted fuel is fully submerged, but Tepco said in November that computer simulations suggested the height of the melted fuel in reactor 2’s containment vessel is probably 20 to 40 cm, Tepco spokeswoman Ai Tanaka said.
Tepco has inserted an endoscope and a radiation meter, but not a robot, in the containment vessel. It is way too early to know how long Tepco will need to operate robots in the vessel because it is unknown what the devices will have to do, Tanaka said.
A Quince was exposed to radiation of 20 sieverts per hour for a total of 10 hours, and the device worked fine, Koyanagi said. If the team conducts further experiments, it may find out the robot can resist even more radiation, he added.
According to experts, even though high radiation in the containment vessel means additional trouble, it is not expected to further delay the decommissioning the three crippled reactors, a process Tepco said will take 40 years.
The experts noted, however, that removing the melted nuclear fuel from the bottom of the containment vessels will be extremely difficult.
Tepco inserted a radiation meter into the containment vessel of reactor 2 Tuesday for the first time, measuring atmospheric radiation levels at several points inside the vessel. The readings logged 31.1 and 72.9 sieverts per hour.
Tepco has not been able to gauge the water depths and radiation levels of the containment vessels for reactors 1 and 3, as, unlike unit 2, there is no access.