Nearly a decade after the three meltdowns at Fukushima No. 1 nuclear power plant, plans are underway to finally remove nuclear fuel debris from the three reactors.
But in order to remove it, Tokyo Electric Power Company Holdings Inc. (Tepco), the operator of the plant, needs to ensure there is a place to store the debris once it is retrieved. This is thought to be the reason why the government is rushing to give the green light to releasing tritium-laced water piling up at the plant into the Pacific — to give room for the storage of fuel debris.
But removing the fuel debris — a crucial step in the decommissioning process — is an enormous task on its own, with measures that need to be resolved emerging one after another.
At a three-day online meeting of the Atomic Energy Society of Japan from Sept. 16, an official from the International Research Institute for Nuclear Decommissioning (IRID) who is in charge of technical development regarding the decommissioning of the Fukushima plant, explained the plan, or the lack thereof, to remove the debris at reactor No. 2.
“We will consider what kind of measures to take, comparing tactics and developing techniques,” the official said, with a hint of frustration at not being able to come up with a specific way yet.
Tepco is expected to start retrieving fuel debris from reactor No. 2 next year — the biggest and the most difficult step in the decommissioning process. If successful, Tepco will move on to the third and final stage of the decommissioning roadmap.
For the past decade, engineers have struggled with multiple obstacles, including the high radiation levels as well as the radioactive water that continues to accumulate at the plant. Those setbacks have forced Tepco to amend the roadmap multiple times, but the schedule for removing the fuel debris has remained unchanged.
With only a month left until the end of this year, the deadline to remove the debris nears.
“It’s Tepco’s commitment to the people of Fukushima to steadily proceed with the decommissioning based on the roadmap,” said an official on nuclear safety at the Fukushima Prefectural Government.
Nuclear industry insiders at home and abroad are watching closely what would become a test case for decommissioning — something the world has never witnessed before. Fukushima Minpo has examined what is happening with that process, chiefly the retrieving of fuel debris, for each of the reactors.
Reactor No. 1
In March 2017, Tepco sent in a boat-like robot to assess the condition of fuel debris piled up at the bottom of the containment vessel inside reactor No. 1. But nearly a meter-thick layer of accumulated sediment, including metal, blocked the way of the robot.
“It was beyond our expectations,” a Tepco official said helplessly.
Among reactors Nos. 1 to 3 which suffered meltdowns, reactor No. 1 is the only one for which Tepco hasn’t been able to obtain detailed information on the state of debris inside. It appears that the bulk of the fuel melted and fell to the bottom of the containment vessel after the earthquake-triggered tsunami immediately knocked out the water cooling system.
There is a pedestal at the center of the containment vessel that supports the reactor pressure vessel. But the situation inside the pedestal, which is believed to contain a considerable amount of debris, is unclear, making it hard to decide on when the debris can be retrieved.
Research institutions and government officials tasked with the decommissioning say that removing the debris at reactor No. 1 remains “uncharted territory.”
In 2015, Tepco conducted an interior survey of reactor No. 1 using muons, a type of cosmic ray. Using muons that pass through light objects but are blocked by heavy substances such as uranium, they tried to locate where and how much debris exists inside the containment vessel.
According to estimates by institutions involved in the process, out of 279 tons of debris inside, more than 90% is believed to be located at the bottom of the containment vessel.
Tepco and other agencies plan to investigate the interior of the containment vessel and inside the pedestal supporting the pressure vessel again using different approaches by the end of March.
Currently, the containment vessel is constantly filled with water to maintain a cold shutdown, which refers to a condition in which water used to cool nuclear fuel remains below boiling point, preventing the fuel from reheating.
To grasp what it is like inside, multiple types of boat-shaped robots that can operate underwater have been developed. They are equipped with cameras to investigate the interior and an ultrasound sonar to measure the thickness and shape of sediments.
They are also aiming to identify where the debris is located by measuring the flow of neutrons from uranium — the entire process of which would take a couple of months.
Officials at Tepco and other agencies are undergoing training to operate the robots, but there are various uncertainties regarding the interior of the reactor, which has been drastically changed by the meltdown.
“We must expect a situation where we wouldn’t be able to access the core structure, including the inside of the pedestal right underneath the reactor pressure vessel,” said an IRID official.
They believe the key to removing debris from reactor No. 1 is to grasp the situation at the bottom part of the containment vessel where most of the fuel has melted down to.
It is estimated that sediment has accumulated up to a meter above the fuel debris. Some believe the sediment is sandy and soft based on video images, but scientists say they need more detailed data. Depending on the amount of radioactive material, they have to be even more careful in taking out the sediment.
Immediately after the meltdown, it took time to cool the fuel down after a chain reaction of uranium fission caused fuel temperature to soar. It is highly possible that concrete at the bottom of the containment vessel reacted with radioactive material, causing erosion. At present, it is difficult to estimate how much concrete has eroded.
“The process and difficulty of the work will differ largely depending on the degree of concrete erosion by debris,” said a nuclear power expert. “It is indispensable to come up with the technology and work procedure that enables investigation and removal of debris without causing damage to the containment vessel.”
Reactor No. 2
At a research facility in Oxford, England, about 10,000 kilometers from the Fukushima No. 1 nuclear power plant, there is a gigantic metal robotic arm with six joints, stretching up to 22 meters.
Engineers there are currently developing the robotic arm to retrieve radioactive fuel debris melted at the bottom of reactor No. 2.
However, the U.K. was under lockdown from late March through June due to the COVID-19 pandemic. And with infections spreading in Europe, the U.K. was once again put under lockdown until Dec. 2 — a move with possible implications for the decommissioning process in Japan. According to the decommissioning roadmap, Tepco is expected to start retrieving the debris from reactor No. 2 next year.
The U.K. Atomic Energy Authority’s Remote Applications in Challenging Environments (RACE) facility has postponed tests initially scheduled for August on whether the robotic device will be able to reach the fuel debris. But with the pandemic forcing RACE to shrink down its research team, it is not clear when the test will be held.
The initial plan was to transfer the robotic arm in the U.K. to a robot simulation center for decommissioning the Fukushima plant, called the Naraha Center for Remote Control Technology Development, located in the town of Naraha, Fukushima Prefecture, sometime after February 2021 and start training engineers there.
An IRID official in charge said it will do its best so that there will be minimal effect on the entire schedule.
Significantly high radiation levels inside the reactor No. 2 containment vessel — about 210 sieverts per hour — is a big obstacle to retrieving the fuel debris. In February 2017, a scorpion-like robot developed by IRID was sent inside the containment vessel to see what it’s like, but ended up stuck in the rubble.
Even though the robotic device is designed to tolerate a certain level of radiation, engineers still need to control it accurately while racing against time before the device malfunctions under the high radiation.
In order to retrieve nuclear fuel debris from the side of the containment vessel, engineers remotely controlling the robotic arm need to make sure it won’t touch complicated structures, including control devices located right below the pressure vessel. Moving a couple of centimeters in the wrong direction could be fatal for the mission at hand.
Training is essential for this kind of sensitive work. But with the pandemic, there is no telling when engineers from the U.K. can come to Japan.
Inside reactor No. 2, there is an estimated 237 tons of fuel debris, of which 42 tons are in the pressure vessel, the biggest pile among reactors Nos. 1 through 3.
While Tepco plans to use the robotic arm to retrieve fuel debris from the containment vessel, it needs to come up with another way to clear the debris inside the pressure vessel, planning everything from scratch. At present, nothing has been decided.
If engineers take out the debris from the bottom of the pressure vessel, the debris itself as well as other parts inside may fall to the bottom of the containment vessel, and that could damage other devices. Since that could become another headache on its own, some sources believe it is not realistic.
If they want to take out the debris from the side or from above the pressure vessel, they need to create a hole in the vessel to put through a device to see what it looks like inside. In order to do that, though, they need to retrieve spent nuclear fuel placed in a pool located at the top of the reactor building. The plan is to start removing the spent fuel rods in reactor No. 2 in fiscal 2024, albeit without an end date in sight.
“Even if we overcome an obstacle, another one appears,” said a Tepco official in charge.
Reactor No. 3
At reactor No. 3, nuclear fuel is believed to have gradually fallen to the bottom of the containment vessel along with other materials, including steel scaffolding, after the fuel temperature rose to about 2,200 degrees Celsius, according to the Japan Atomic Energy Agency.
“The fuel debris is piled up in a very complex manner,” said Masaki Kurata, JAEA’s division leader on reactors at Fukushima No. 1 nuclear power plant.
Research up to present shows that the inside of the vessel has drastically changed from its original state, with fuel debris tangled up with structural parts piled up to three meters high at the bottom of the containment vessel. Researchers are analyzing how to remove the debris.
In March, Tepco revealed a roadmap for decommissioning the Fukushima plant over the next 12 years, with a plan to remove the debris from reactor No. 3 after debris in reactor No. 2 is removed in 2021. It also laid out a plan to lower the water level for reactor No. 3’s containment vessel.
Inside reactor No. 3, fresh water and sea water injected to cool the nuclear fuel fills up to about 6 meters. The water level is above the ceiling of the ground floor of the reactor building, which is more than the amount of water at reactors Nos. 1 and 2.
In order to apply the same tactics used to remove reactor No. 2’s debris — extracting it using the robotic arm while pouring water to cool the fuel — water needs to be extracted. Tepco plans to draw out water through the suppression chamber connected to the bottom of the containment vessel. But at the same time, Tepco needs to inject water to cool the fuel debris, which means it needs to adjust the amount of water injected and drawn out.
“Tasks we need to resolve keep piling up,” said a Tepco source.
One of the headaches is that the suppression chamber is filled to the brim with water. That is putting more pressure on its walls than expected, causing it to deteriorate more quickly. Anti-earthquake measures also need to be installed.
If the fuel debris is no longer underwater, which shields engineers from radiation, it is expected to be harder for them to retrieve it. Tepco is also concerned about the possible rise in radiation levels and dust laced with radioactive materials floating inside the containment vessel.
JAEA research shows that, judging from the temperature and other conditions, the half-melted fuel debris fell off slowly, leaving the possibility that there still may be something that could trigger a chemical reaction.
There have been cases in the past at nuclear power plants when metal caught fire, which was hard to extinguish. It is necessary for Tepco to consider ways to approach this, taking into consideration the possibility that it could trigger a meltdown again and other possible risks as well.
It is also important to analyze debris samples to assess whether the concentration of components has changed while it slowly cooled down and solidified.
“It became clear that what’s happening to the fuel debris largely differs with each reactor,” said JAEA’s Kurata. “It’s important to proceed based on the assessment of the samples taken instead of relying on past precedents that occurred before the Fukushima meltdowns.”
This section features topics and issues covered by the Fukushima Minpo, the prefecture’s largest newspaper. The original articles were published Nov. 2, 3, 7 and 13.
In a time of both misinformation and too much information, quality journalism is more crucial than ever.
By subscribing, you can help us get the story right.