A spokesman for the government's nuclear regulatory body acknowledged that the level made it "tough" for workers to engage in restoring the reactors' key cooling functions for prolonged periods, and that it was seeking ways to mitigate radiation exposure.
Exposure to 250 millisieverts is the permissible level for workers dealing with the ongoing crisis, the worst Japan has seen. Plant operator Tokyo Electric Power Co. said that usually the radiation level inside the reactor building is 0.01 millisievert per hour under normal conditions.
As part of efforts to stably cool the Nos. 1 and 3 reactors, Hidehiko Nishiyama at the Nuclear and Industrial Safety Agency also said the installation of an air-cooling device to take the heat from the water circulating around the reactor is under consideration.
Key cooling functions at the Nos. 1 to 4 units have not yet been restored since the March 11 mega earthquake and ensuing tsunami hit the six-reactor complex, located on the Pacific coast in Fukushima Prefecture.
According to the data made available Monday, the radiation level at the No. 1 reactor building was between about 10 and 49 millisieverts per hour, while at the No. 3 reactor building it was between 28 and 57 millisieverts per hour.
On Sunday, two remote-controlled robots provided by U.S. company iRobot measured radiation levels, temperatures and other data inside the reactor buildings to check whether workers could safely engage in restoration work there.
As for the radiation level measured by workers prior to the tests by the robots, up to 270 millisieverts per hour were detected near the door to the No. 1 reactor building.
Nishiyama said that the areas the robots and the workers checked different areas.
The robots are expected to check the No. 2 reactor building later in the day, where highly radioactive water was found to be flooding the adjacent No. 2 reactor turbine building.
Massive amounts of water have been poured into the reactors and their spent nuclear fuel pools as a stopgap measure to cool them down. But pools of contaminated water have been detected in various parts of the site, an apparent side effect of the emergency measure.
The utility firm known as TEPCO has said that the amount of polluted water at the Nos. 1 to 3 reactor turbine buildings and nearby areas totals an estimated 67,500 tons.