Radiation levels inside units 1 and 3 at Fukushima Daiichi topped out at 49 and 57 millisieverts per hour Sunday, according to readings from robots. That dose would put workers at their yearly limit in about five hours, adding even more complexity to the challenges faced by engineers at the severely damaged nuclear plant.
For the last two days, small robots have inspected areas where radiation is believed to be too high for workers to enter, according to Japan’s Nuclear and Industrial Safety Agency. The new robots join aerial drones and remotely operated heavy machinery that continues to clear rubble left from the March 11 earthquake, tsunami and subsequent station blackout and hydrogen explosions. Made by the same company that sells the Roomba vacuum robot, the iRobot PackBots were programmed to open doors and navigate reactor buildings with radiation sensors.
Separately, NISA indicated unit 4's outer building is flooded with water 5 meters deep, Kyodo news reported Monday. In addition to the tsunami that washed over the plant, water used to flood reactor cores and spent fuel tanks in the place of damaged cooling systems is believed to be flooding three other units. Owner Tokyo Electric Power Co. already is in the process of removing an estimated 20,000 tons of irradiated water from units 1 though 3 so workers can reach the reactors.
(Water sample taken from the spent fuel tank of unit 4. Efforts to cool the tank may have resulted in standing water in the building. Source: TEPCO.)