In March 2011, the Great East Japan earthquake caused a massive tsunami that
disabled the cooling systems of the Fukushima Daiichi Nuclear Power Plant (福島第一原子力発電所).
Without proper cooling, the plant's nuclear fuel melted
down and several hydrogen explosions resulted.
The plant's owner TEPCO and the Japanese government have since committed billions
of dollars to the site's safe and efficient decommissioning.
To minimize human safety risk, the company has developed and deployed
a series of advanced, radiation-hardened robots to perform a variety of critical tasks.
I have wanted to do this topic since the very start of this channel.
In this video, let's take a look at the Fukushima Robots.
## The Scenario
The plant has 6 units in total. Units 4, 5, and 6 were not operating when the accident happened.
Units 5 and 6 are separated from 1 through 4. They suffered minor damage from the tsunami,
but survived and are essentially fine.
The tsunami did damage Unit 4, and it also suffer a hydrogen explosion due
to exhaust spillover from the neighboring Unit 3.
But because the reactor was shut down at the time,
the fuel rods suffered little damage, making the radiation dosage tolerable.
Units 1-3 suffered the worst damage. They were in operation when the tsunami hit.
These three reactors had meltdowns.
Units 1 and 3 also experienced hydrogen explosions that released radioactive materials.
## Three Mile Island
The first nuclear response robots were developed by the United States
after the accident at Three Mile Island in 1979.
Two robots were employed for site inspection in high-radiation areas inaccessible to people.
The first was the Surveillance and In-Service Inspection robot,
first deployed in August 1982 three years after the accident. It looks like a little
crawler and took photographs, collected samples and made radiation readings.
The robots got increasingly advanced and more capable. The year after
SISI, another robot named Fred was introduced. Fred was special in that
he was equipped with a high-pressure water spray for decontamination work.
To de-fuel the Three Mile Island reactor core - which was flooded with radioactive
water - three Remote Reconnaissance Vehicles were built and deployed.
These robots were very practical. They were tethered, built with familiar technologies,
and kept very small. They flushed, scraped, and cleaned the walls with great success.
## Chernobyl
After Chernobyl, the Soviet Union called upon hundreds of thousands
of human liquidators - the so-called bio-robots - to clean up rubble and debris.
There is a great scene in the HBO mini-series Chernobyl that follows
a group of such liquidators as they shovel up pieces of graphite and toss it off the roof.
This was necessary partly because radiation levels were so high that available robots failed.
And partly because of bureaucratic issues. That dramatic rooftop ceiling scene
apparently occurred because the commission was denied permission
to use helicopters to sweep up the debris from a far distance.
Robots were eventually brought in. Notably, the Specialized Transport Robot or STR-1,
which was a wirelessly controlled bulldozer for removing debris.
It was apparently adapted from an unmanned lunar vehicle.
## PackBot and T-Hawk
The first robots arrived at Fukushima on April 2011 - three weeks after the accident.
Four military multi-purpose robots from a series called PackBot
from the American company iRobot Corporation USA were brought in
to enter the damaged and irradiated Unit 1, 2, and 3 reactor buildings.
PackBot was first used in 2000 in Iraq and Afghanistan for detecting
and defusing improvised explosive devices. Small and lightweight,
it weighs about 30 kilograms and looks like a pair of metal extension arms on a tread.
After a brief training session at the Unit 5 reactor, they went to Unit 2,
opened a double door airlock gate and strolled inside.
There, they measured radiation dosage, atmospheric temperatures, and oxygen concentrations.
A bigger, beefier robot from iRobot - nicknamed Warrior - was later brought
in for additional cleaning work. Throughout the second half of 2011, Warrior vacuumed and cleared
out obstacles in the area - joining others like Bobcat and the Swedish-built Brokk-90.
Another notable military robot used at the time was the T-Hawk,
a small unmanned aerial vehicle from Honeywell. From April 10th to July 24th,
it flew over Units 1 through 4 to review the situation and collect dust samples.
## RESQ-A
PackBot, Warrior, and T-Hawk were American robots. This sparked a mild controversy.
Japan is known for their robotics technology. Where are their robots?
After a criticality accident in September 1999,
three different technology centers in Japan developed a line of nuclear disaster robots.
Notably, there was the REmote Surveillance Squad,
or RESQ robots - built by what is now named the Japan Atomic Energy Agency or JAEA.
There were three of them for taking measurements and collecting samples.
However, the robots were given little if any upgrades after
their completion in 2001 - apparently due to a lack of budget.
So when the real thing happened ten years later, the robots were not ready.
With so little maintenance done and the robots now over 10 years old,
many critical components have since gone out of stock.
Worse yet, there were no trained operators. The original manufacturer was unable to help,
because the original engineer left the project.
This farce can be construed as a loss of face. So over the next few months JAEA re-engineered
the RESQ-A robot into the JAEA-3 - grafting on parts from a Brokk they had on site.
After a series of trials, it finally started its missions on September 2011.
But by then, another Japanese robot had stolen its thunder.
## Quince
PackBot was a success, but it still suffered limitations.
Its radio communication system kept the robots from going too far deep into the first floor.
Furthermore, the treads meant that it cannot easily go up and down stairs.
This was a problem. TEPCO not only still needed robots to perform ordinary dose measurement
missions, but there was also a new mission. A robot had to go into the reactor building
basement to install a water gauge and take samples of the contaminated water there.
TEPCO turned to a series of search-and-rescue robots
developed by the NEDO project at the Chiba Institute of Technology.
They called the robot series Quince - which is
Spanish for 15 but I’m not sure if that’s what they were going for.
Quince is another robot with a continuous track. However it was
developed to be waterproof and highly mobile across difficult terrain. The
original practical scenario had been underground malls.
However, TEPCO requested that the team make a few modifications to Quince.
The most significant of which was to harden the robots against radioactivity.
## Radioactivity
A nuclear accident releases a variety of radiation including alpha particles,
beta particles, gamma rays, x-rays and neutron particles.
When radiation particles interact with a semiconductor material,
two effects can largely happen.
The first is called a displacement effect. This is when particles like neutrons and alpha particles
collide with the semiconductor and displace their atoms - shifting their structural arrangements.
This in turn changes the device's properties - resulting in higher current leakage,
decreased lifetime, and so on. Which is bad.
The second is an ionization effect.
Electrons, protons, and X-rays pass through semiconductors, creating electron-hole pairs.
This can cause permanent glitches and abruptly weird behavior - which I also think is bad.
One of these two can happen. Or both. A single event can disable the whole thing.
Or their effects can accumulate over time, eventually bringing the whole thing down.
## Hardening
This damage happens on multiple levels: Device,
Circuit, and System. So radiation hardening needs to be performed on all of these levels as well.
Misleadingly, the "device" term here refers to things like a transistor. To rad-hard a device you
can change the transistor layout, add insulating substrates like with the silicon-on-insulator fab
approach, or make the device bigger. Bigger gate sizes have higher current capacities.
Circuits are made up of multiple devices. Radiation hardening here includes duplicating
critical circuits or adding other circuits that monitor the main circuit in case of damage.
Circuit and device level protections are generally hardware-based only. With system level designs,
we can introduce the benefits of software. In addition to obvious things like adding
radiation armor and redundancies, we can introduce preventive scrubbing algorithms that scrub errors.
## Passing the Test
Quince was made from conventional electronics so the team had no idea
how resistant to radiation it actually was. If the resistance was low then a 2 cm-thick lead
plate would have to be added. This added weight potentially threatened the robot's mission.
Subsequent gamma ray testing on Quince's components including its 180 nanometer CPU,
motherboard, and motor driver boards found that the robot's essential components can survive
a dose of about 200 grays, or 20,000 rads.
Initial measurements during the Fukushima disaster response measured about 100 grays. So the team
decided that Quince could work for over 100 hours and risk just about a 10% chance of failure.
The team did a few more modifications and in June 2011,
the new Quince entered the reactor buildings.
In October 2011, after five successful missions,
a broken communications link whilst on the third floor forced the team to abandon the
robot. Quince 2 and 3 were redesigned to fix this flaw and deployed February 2012.
## Decommissioning
After the immediate emergency, the Japanese government's next goals are to gradually
decommission the plant and rebuild its surrounding area. This is a multi-step process overseen
by both TEPCO and the International Research Institute for Nuclear Decommissioning or IRID.
First, you have to do some cleanup.
The accident's hydrogen explosions scattered rubble across the site. This has to be removed.
For this task, Hitachi developed a heavy duty robot called ASTACO-SoRa. It goes around and
puts rubble into a recycling bin using its two heavy duty arms. Other cleaner robots include:
Raccoon and Arounder, which are smaller cleaner bots that sprays down the ground
around Reactor Unit 2 or anything else with a high pressure water jet or dry ice.
And Meister, a heavy duty robot that can drill for samples or vacuum up particles.
## Spent Fuel Removal
Second, each of the units have a spent fuel assembly for storing
fuel that is no longer useful for energy generation. These have to be removed.
In December 2014, TEPCO announced that they had successfully removed the 1,535 spent fuel
rods from the damaged Unit 4 reactor pool. The rods were placed in a cooling pool in Unit 6.
In 2019, TEPCO started removing the 566 spent fuel rods from Unit 3.
This took about 2 years, and in February 2021 they announced that this was completed.
TEPCO will next start working on removing the spent fuel rods for Units 1 and 2.
This is scheduled to start some time in 2023 or beyond.
## Fuel Debris
That was the spent fuel. Now what about the fuel that had actually been in the reactors?
In Units 1-3, the fuel was known to have melted down. It burned through its original
location in the Reactor Pressure Vessel as well as the concrete pedestal it sits on.
Eventually it reached the Primary Containment Vessel or PCV before
emergency efforts pumped in water to cool it. Then it re-solidified to become nuclear
fuel debris - officially named "corium". This has to be inspected and eventually removed.
Due to each of the three reactors' unique circumstances,
IRID used different robots to investigate the state of the melted fuel.
## Unit 1
Unit 1's nuclear reactor PCV is flooded to a depth of 2 meters. There is likely corium at its bottom.
Back in 2012, TEPCO and the team punched a small hole into the PCV
to run through a wired camera and run various water tests.
Later in 2014, IRID developed a "morphing" robot
which was designed to first squeeze through the 100 millimeter wide hole.
After which then it would twist and transform into a U-shape
to travel around a lead plate inserted in front of the hole for radiation shielding.
The first P-Morph as it was called hit upon an unexpected obstacle
as it made its way around the outside of the pedestal and the team had to leave it there.
But improvements were soon made, and the mission was completed. The next step would be to build a
robot to start collecting the pebble-like nuclear fuel debris scattered inside the pedestal.
## Unit 2
Unit 2 suffered a meltdown but there was no hydrogen explosion. For that reason,
the building structure remains intact and is in better working condition than Units 1 and 3.
To inspect the damage, TEPCO produced a robot called the Scorpion.
Like the aforementioned P-MORPH robot, it can crawl through a small hole
and then reconfigure itself like a Scorpion for better mobility thereafter.
After a great deal of work was done to clear the way,
Scorpion was finally deployed in 2017. However once inside,
the left tread failed for unknown reasons and this Scorpion had to be left inside the reactor.
The team finally photographed the entirety of the inside of the reactor location with what looks
like a camera hanging off a fishing rod. Multiple shots were stitched together to create a panorama.
## Unit 3
The corium inside Unit 3 was the last to be photographed. That is because the PCV is almost
entirely flooded to a depth of about 6 meters, which necessitates new approaches and equipment.
The entrance approach used for Unit 2 was not feasible due to flooding.
A robot in 2015 got as far as the first-floor grating outside the pedestal.
And unfortunately the best possible way in was just 14 centimeters across.
It took about 2 years to build a suitable robot for the task. This tiny little thing is just 13
cm across and 30 cm long and is nicknamed the Mini-Manbo - meaning "little sunfish".
In 2017, the Mini-Manbo successfully recorded video
of the nuclear debris. The event made the news.
## Conclusion
Now that the relevant pieces of nuclear fuel have finally been found,
work can start on removing them.
IRID and TEPCO have decided to begin collecting and removing nuclear fuel debris from Unit 2
first. To do this, they will build a massive stainless steel robotic arm
sitting outside the PCV. It will reach in through a hole in the PCV and pick up the debris.
As of this writing, the project start will be delayed until 2023
due to them needing to improve the arm's capabilities and adding several attachments.
IRID expects that it will take at least ten years to remove
all the nuclear fuel debris from the Fukushima site. And even after that,
more work will need to be done over the years to clean and decontaminate.
Regardless, things have improved to the point
that the government has let local residents return to their homes. In April 2019,
the government lifted the evacuation order for Okuma, the town closest to the plant.
However, only a small percentage of the town's residents have returned.
An indication that there remains a great deal of work to be done - by both people and robots.