Robots sent into three buildings at Japan’s crippled Fukushima Dai-Ichi nuclear station detected radiation too toxic for humans after the plant operator set out a plan to end the crisis in six to nine months.
Measurements show one hour inside the No. 3 reactor building would expose humans to more than one-fifth of the radiation Japan has said is the most workers can endure in a year, the atomic safety agency said yesterday. The buildings were damaged by explosions in the days after a magnitude-9 quake and tsunami on March 11 knocked out cooling equipment, sparking the worst nuclear disaster since Chernobyl in 1986.
A sustained drop in radiation at the Fukushima Dai-Ichi station may be achieved within three months, Tokyo Electric Power Co. said in a statement laying out its plans. Following that, a cold shutdown, where core reactor temperatures fall below 100 degrees Celsius (212 degrees Fahrenheit), may be achieved within six months, it said.
“Tepco needs to be faster than it has outlined on two particular areas: circulating the radioactive water back into the pressure vessels and covering the reactor buildings to prevent radiation releases,” Tadashi Narabayashi, a professor of nuclear engineering at Hokkaido University said today.
Shares of Tepco, as the company is called, fell 4.3 percent to 447 yen today in Tokyo. The stock is down about 80 percent since the quake and tsunami, which left almost 28,000 people dead or missing.
Two iRobot Corp. robots sent into reactor buildings on April 17 to check whether humans can reenter them found radiation levels as high as 49 millisieverts per hour inside the No. 1 unit, and up to 57 millisieverts in the No. 3 unit, the Nuclear and Industrial Safety Agency said.
The cumulative maximum level for nuclear workers was raised to 250 millisieverts from 100 millisieverts by Japan’s health ministry on March 15. Exposure totaling 100 millisieverts over a year is the lowest level at which any increase in cancer is evident, according to the World Nuclear Association in London.
Another robot was sent into the reactor No. 2 building yesterday and recorded levels of 4.1 millisieverts per hour, spokesman Tetsuya Terasawa told reporters today. That level wouldn’t prevent workers going into the building, another spokesman said.
“We judge the radiation limit by the amount, importance and hours of the work, as well as accumulated exposure of each worker,” spokesman Shogo Fukuda said. “We wouldn’t hesitate to send workers in at a level of 4.1 millisieverts per hour.”
Fuel pellets in the No. 1, 2 and 3 reactors may have melted at the plant, the nuclear safety agency said today, in the first official confirmation of damage to the cores.
“It is believed that the fuel pellets in the reactors have melted,” the agency said in a report. “The extent of the melting cannot be confirmed until the fuel rods have been removed.”
In the next three months, Tepco plans to fill the reactor containment vessels at the No. 1 and No. 3 units with water, the company said in its statement on April 17. The utility will seal the vessel of the No. 2 reactor, which is likely damaged, before flooding it.
“If we flood the damaged vessel, the leak of contaminated water will increase,” Tepco Vice President Sakae Muto told reporters the same day. “We will continue injecting water with care and monitor the volume of water leaked.”
The water pumped so far has overflowed into basements and trenches, with some of it leaking into the ocean.
Tepco started draining highly radioactive water from trenches around the No. 2 reactor at about 10 a.m. today, spokesman Osamu Yokokura said.
The power utility plans to move 10 million liters (2.6 million gallons) of the contaminated water to a waste treatment unit and expects to complete the transfer in 26 days, Junichi Matsumoto, an official at Tokyo Electric, told reporters earlier today.
“Once Tepco completes draining the radioactive water from the building of the No. 2 reactor, engineers may find the damaged area of containment vessel and seal it,” said Kazuya Idemitsu, a professor of nuclear engineering at Kyushu University.
Three to six months after the initial phase of its plan, Tepco will attempt a cold shutdown of reactors No. 1, 2 and 3, the company said. Reactors 4, 5 and 6 were shut at the time of the disaster.
The utility will also cover the No. 1, 3 and 4 reactor buildings as a temporary measure to reduce radiation emissions after the structures were damaged by hydrogen blasts last month, according to the statement.
“Tepco should cover buildings as soon as possible, at least before Japan’s typhoon season, because strong wind and rain may cause spillage of radioactive water from spent fuel pools," Narabayashi at Hokkaido University said.