Tepco’s scorpion-shaped robot. | IRID
Six years after the triple meltdown at the Fukushima No. 1 nuclear power plant, recent investigations underneath the damaged reactor 2 using cameras and robots came close to identifying melted fuel rods for the first time.
Experts say getting a peek inside the containment vessel of reactor 2 was an accomplishment. But it also highlighted how tough it will be to further pinpoint the exact location of the melted fuel, let alone remove it some time in the future.
The biggest hurdle is the extremely lethal levels of radiation inside the containment vessel that not only prevent humans from getting near but have also crippled robots and other mechanical devices.
Safely removing the melted fuel would be a best-case scenario but the risks and costs should be weighed against the option of leaving the melted fuel in the crippled reactors, some experts said.
“The work to probe inside the containment vessels and remove the fuel debris will be extremely tough because of the high radiation levels,” said Hiroshi Miyano, who heads a panel of the Atomic Energy Society of Japan, which is discussing ways to decommission the Fukushima plant and making recommendations to the government.
The government and Tokyo Electric Power Company Holdings Inc. are trying to find a way to remedy the situation but existing methods and technologies may not be sufficient, Miyano said.
In search of melted fuel
The world’s attention turned to the melted fuel rods in late January when Tepco inserted a 10-meter-plus tube equipped with a camera into the containment vessel of reactor 2 to capture images under the pressure vessel that housed the fuel rods.
The images showed black lumps scattered beneath the pressure vessel.
When the March 11, 2011, Great East Japan Earthquake and monstrous tsunami hit, the plant suffered a blackout and lost its key cooling system, triggering meltdowns in reactors 1, 2 and 3. The melted nuclear fuel rods penetrated the pressure vessels and fell into the containment vessels.
Tepco had put cameras inside the containment vessels several times in the past six years but January’s probe was the first to apparently find melted fuel debris.
“We understand that this is a big milestone. We could finally get to see what it was like underneath the pressure vessel,” said Yuichi Okamura, general manager of Tepco’s nuclear power and plant siting division.
“This is critical information in order to remove the fuel debris.”
But Tepco hasn’t confirmed that the black lumps are melted fuel, saying they could be paint or cable wrappings, and further investigation is needed.
Capturing the images may be progress but the robot and camera forays have not provided enough information about how to deal with the melted fuel.
Last month, Tepco sent a remote-controlled, scorpion-shaped robot in to further probe inside the reactor 2 containment vessel. But the robot failed before it reached under the pressure vessel after a tire became stuck.
The robot’s dosimeter measured radiation levels of 210 sieverts per hour — enough to kill humans instantly.
While 210 sieverts per hour indicate the melted fuel was nearby, the radiation crippled the robot’s electronics, including its semiconductors and cameras, indicating that the further use of robots to pinpoint the melted fuel will be difficult, robotics experts said.
There are computer chips “designed to withstand a certain level of radiation, but the level inside the containment vessel is totally different,” said Satoshi Tadokoro, a professor at Tohoku University who is an expert on disasters and rescue robots.
The radiation can damage a robot’s chips that serve as their brains, causing the devices to lose control, said Tadokoro, whose robots have also been used at the Fukushima plant.
“On top of the high level of radiation, the entrance (to the containment vessel) for the robot is very small,” restricting what types of robots can be used to hunt for the melted fuel, he said.
Tepco said the opening it created on the side of the reactor 2 containment vessel is about 11 cm in diameter.
Fuel removal strategy
Tepco is set to conduct internal probes of the reactor 1 containment vessel this month and is preparing similar missions for reactor 3.
The government and utility then plan to adopt a basic fuel removal strategy this summer and fine-tune the plan next year, with the actual fuel removal taking place in or after 2021.
There are essentially three options for the strategy, according to the Tokyo-based International Research Institute for Nuclear Decommissioning (IRID), which is developing technologies for the Fukushima plant decommission.
One option is to flood the containment vessels with water and use a crane above the reactors to hoist up the melted fuel. The second option is to carry out the same process but without water. The third is to install removal equipment through the side of the containment vessel.
There are merits and drawbacks to each option, said Shoji Yamamoto, who heads the team developing technologies to create the fuel removal devices at IRID.
The flooding option can block radiation using water, but if the fuel melts into the water, it could pose a risk of recriticality. The debris may need to be cut into pieces for removal, but this process would enable water to get between multiple pieces, creating the condition for recriticality. For nuclear chain reactions to happen there needs to be a certain distance between nuclear fuel and water.
If there is no water, the recriticality risk is minimal but the massive radiation levels cannot be blocked, Yamamoto said.
Tepco’s Okamura said being able to block radiation with water is a huge plus, but noted the reactor 2 containment vessel had cracks and holes that could let injected coolant water escape.
With the Three Mile Island nuclear accident in the U.S., the flooding option was used to retrieve the melted fuel in the 1980s. But the key difference was that all of the melted fuel stayed inside the pressure vessel, so it was easier to flood the reactor.
Because the melted fuel in reactors 1, 2 and 3 at the Fukushima plant all penetrated the pressure vessels and fell into the containment vessels, extracting it from the top or the side was a tough call, Yamamoto said, noting it was important to know the exact location of the melted fuel.
The distance between the top of the pressure vessel and the bottom of the containment vessel is about 45 meters and some parts inside the pressure vessels will need to be removed if Tepco tries to remove the debris inside the containment vessels from the top.
“If we know that the melted fuel is concentrated in the containment vessels, it will be more efficient to remove it from the side” because the entry point is closer, Yamamoto said.
Whatever option is decided, Yamamoto stressed that maintaining the fuel removal device will be difficult because the radiation will probably cripple it.
“The fuel removal device will be controlled remotely … it will be broken somewhere down the line and the parts will have to be replaced, considering its (ability to withstand) radiation,” he said.
“Given that, maintenance will have to be done remotely, too, and that will be a big challenge.”
To remove or not
Another option altogether is for Tepco to leave the melted fuel where it is.
During a media tour of the Fukushima No. 1 plant last month, Okamura of Tepco said the utility intended to collect the melted fuel because leaving it was “not an appropriate way” to manage nuclear fuel.
Miyano of the Atomic Energy Society of Japan said the debris must be removed because radioactive materials, including nuclear fuel, must be strictly controlled under international rules requiring strict monitoring.
Domestic nuclear power plant operators have to report the amount of nuclear fuel they have to the Nuclear Regulation Authority, which then reports to the International Atomic Energy Agency.
“There is the question of whether the government and Tepco decide not to remove the fuel debris. That would be an international issue,” said Miyano, adding that a consensus from the international community would be needed.
At the same time, Miyano said debate and analysis will be required to decide which choice would be best by looking at various factors, including how much it will cost to pick up all the melted fuel and where to store it.