Fukushima radiation levels at highest level since 2011 meltdown
Cranes over the Fukushima Daiichi plant in February 2016. The decommissioning process is expected to take about four decades
Extraordinary readings pile pressure on operator Tepco in its efforts to decommission nuclear power station
Radiation levels inside a damaged reactor at the Fukushima Daiichi nuclear power station are at their highest since the plant suffered a triple meltdown almost six years ago.
The facility’s operator, Tokyo Electric Power (Tepco), said atmospheric readings as high as 530 sieverts an hour had been recorded inside the containment vessel of reactor No 2, one of three reactors that experienced a meltdown when the plant was crippled by a huge tsunami that struck the north-east coast of Japan in March 2011.
The extraordinary radiation readings highlight the scale of the task confronting thousands of workers, as pressure builds on Tepco to begin decommissioning the plant – a process that is expected to take about four decades.
The recent reading, described by some experts as “unimaginable”, is far higher than the previous record of 73 sieverts an hour in that part of the reactor.
A single dose of one sievert is enough to cause radiation sickness and nausea; 5 sieverts would kill half those exposed to it within a month, and a single dose of 10 sieverts would prove fatal within weeks.
Tepco also said image analysis had revealed a hole in metal grating beneath the same reactor’s pressure vessel. The one-metre-wide hole was probably created by nuclear fuel that melted and then penetrated the vessel after the tsunami knocked out Fukushima Daiichi’s back-up cooling system.
“It may have been caused by nuclear fuel that would have melted and made a hole in the vessel, but it is only a hypothesis at this stage,” Tepco’s spokesman Tatsuhiro Yamagishi told AFP.
“We believe the captured images offer very useful information, but we still need to investigate given that it is very difficult to assume the actual condition inside.”
The presence of dangerously high radiation will complicate efforts to safely dismantle the plant.
A remote-controlled robot that Tepco intends to send into the No 2 reactor’s containment vessel is designed to withstand exposure to a total of 1,000 sieverts, meaning it would survive for less than two hours before malfunctioning.
The firm said radiation was not leaking outside the reactor, adding that the robot would still prove useful since it would move from one spot to the other and encounter radiation of varying levels.
Tepco and its network of partner companies at Fukushima Daiichi have yet to identify the location and condition of melted fuel in the three most seriously damaged reactors. Removing it safely represents a challenge unprecedented in the history of nuclear power.
Quantities of melted fuel are believed to have accumulated at the bottom of the damaged reactors’ containment vessels, but dangerously high radiation has prevented engineers from accurately gauging the state of the fuel deposits.
Earlier this week, the utility released images of dark lumps found beneath reactor No 2 that it believes could be melted uranium fuel rods – the first such discovery since the disaster.
In December, the government said the estimated cost of decommissioning the plant and decontaminating the surrounding area, as well as paying compensation and storing radioactive waste, had risen to 21.5tn yen (£150bn), nearly double an estimate released in 2013.
Radiation level in Fukushima reactor could kill within a minute
Images show black lumps on grating for maintenance work below the No. 2 reactor’s pressure vessel at the Fukushima No. 1 nuclear power plant. TEPCO says melted fuel likely caused at least two holes in the metal grating, including an opening measuring 1 meter by 1 meter. (Provided by Tokyo Electric Power Co.)
Radiation levels that can kill a person in a minute and holes created by melted nuclear fuel could further delay decommissioning operations at the No. 2 reactor of the Fukushima No. 1 nuclear plant.
Tokyo Electric Power Co., operator of the crippled plant, said Feb. 2 that the maximum estimated radiation level near what is believed to be melted fuel in the reactor was 530 sieverts per hour, the highest so far since the triple meltdown in 2011.
In its investigation into the interior of the No. 2 reactor, TEPCO also confirmed at least two holes on grating for maintenance work below the bottom of the reactor’s pressure vessel.
The images show the area at the bottom of the No. 2 reactor’s pressure vessel to the metal grating below.
“The holes were likely made when the melted nuclear fuel fell from the pressure vessel and melted the grating,” a TEPCO official said.
The findings were made by studying images taken from a video camera attached to a pipe that was inserted into the reactor on Jan. 30.
Radiation levels were estimated at 20 sieverts per hour, 50 sieverts per hour and 530 sieverts per hour at three spots inside the reactor’s containment vessel.
The company estimated the doses from the extent of disturbances in the images caused by radiation.
Although a TEPCO official said “there is a margin of error because radiation levels were not measured directly,” the company believes the scattered melted nuclear fuel inside the containment vessel was emitting high levels of radiation.
After a number of failed attempts, the remote-controlled camera took the first pictures of possible melted fuel at the plant.
However, closer inspection of the images have revealed additional problems for TEPCO, which had believed most of the melted fuel had remained inside the reactor’s pressure vessel.
TEPCO plans to send an investigative robot, called Sasori (scorpion), into the containment vessel this month to more accurately measure radiation doses at various spots and take additional footage of the scattered nuclear fuel.
The utility plans to use the data to determine a fuel-removal method.
But the robot was expected to use the circular grating, measuring 5 meters in diameter, to move around. One of the holes is 1 meter by 1 meter, a potential pitfall for the robot, which is 59 centimeters long and 9 cm high.
TEPCO said it will consider a different route for the robot in its survey.
Fumiya Tanabe, an expert on nuclear safety who analyzed the 1979 Three Mile Island nuclear accident in the United States, said the findings show that both the preparation for and the actual decommissioning process at the plant will likely prove much more difficult than expected.
“We have few clues on the exact locations, the sizes and the shapes of the nuclear fuel debris,” he said. “The planned investigation by the robot needs a rethink. Work to decommission the plant will require even more time.”
TEPCO said it will need 30 to 40 years to complete the decommissioning process. The utility plans to start work to remove the melted nuclear fuel at the No. 2 and two other stricken reactors in 2021 after deciding on a removal method in fiscal 2018.
TEPCO has yet to determine the location and the condition of the melted fuel in the other two reactors.