Nuclear power plant accident 9 years Decommissioning work How far has it gone? Are you not progressing? Feb 20 19:57
The accident at TEPCO's Fukushima Daiichi Nuclear Power Station that shook Japan and the world. The government and TEPCO say that it will take up to 40 years for all decommissioning work to be completed. Now that nine years, about one quarter of that, are about to end, how far is the work going? Or is it not progressing? The series "Nuke accident nine years Fukushima Daiichi NPS seven questions". It is the second of all seven times.
(Science and Culture Department, Abe Tomoki, Nuclear Accident Coverage Team)
"Meltdown" at Units 1 to 3 "Hydrogen explosion" at Units 1, 3, and 4
Many of you do not feel like this.
"I'm very interested in the Fukushima Daiichi accident, but it's too complicated and unclear what happened to which unit."
In order to respond to these situations, this special issue will carefully organize each unit in an easy-to-understand manner.
The Fukushima Daiichi Nuclear Power Plant has six reactors from Unit 1 to Unit 6. Units 1 to 3 were in operation at the time of the accident. Units 4 to 6 were undergoing periodic inspections.
The cooling systems of three units, Units 1 to 3, were stopped due to the effects of the tsunami and other factors. The meltdown caused nuclear fuel to melt down.
In addition, the generated hydrogen accumulated at the top of the building, causing a “hydrogen explosion” at Units 1, 3, and 4.
At that time, what was happening at each unit?
At that time, the first unit was
The first situation was worse for Unit 1.
A control rod was automatically inserted to detect the shaking of the earthquake and stop the fission reaction, and the reactor was shut down immediately.
However, the equipment for receiving electricity from outside was damaged by the earthquake. The tsunami caused emergency generators and power panels to flood, losing all power.
The cooling system stopped working, and the nuclear fuel became hot and melted down, causing a meltdown.
The next day, on the 12th, a large amount of hydrogen generated from the cover of the melted nuclear fuel and the like accumulated at the top of the building, causing a hydrogen explosion. The top floor of the reactor building blew off leaving only the framework.
At that time Unit 2
At Unit 2, the cooling system was operating at first, but the cooling system was shut down on March 14 three days after the accident, causing a meltdown.
Although Unit 2 did not explode hydrogen, a large amount of radioactive material was lost due to failures such as a "vent" operation to lower the internal pressure to prevent major damage to the "containment vessel" containing the reactor. It is believed to have been released outside.
At that time Unit 3
At Unit 3, the cooling system continued to operate with the survived battery, but stopped on the 13th, two days after the accident.
Nuclear fuel melted down, causing a hydrogen explosion. The top floor of the building blew off like Unit 1.
At that time Unit 4
At that time, Unit 4 was undergoing periodic inspections, and all nuclear fuel had been removed from the reactor and moved to a flooded fuel pool. As a result, meltdown at reactors like Units 1 to 3 was avoided.
However, hydrogen flowed in from the Unit 3 through pipes, and on March 15, four days after the accident, a hydrogen explosion occurred in the reactor building, causing heavy damage to thick concrete walls and ceilings.
Since the cooling of the fuel pool stopped, there was a growing concern that a large amount of fuel would melt and high concentrations of radioactive material would be released.
For this reason, attempts were made to release water to the pool by the SDF and fire departments.
Fortunately, the pool did not run out of water and the nuclear fuel did not dissolve, but the government was temporarily considering evacuation of the metropolitan area.
Decommissioning work advanced
It is nine years since the worst nuclear accident in the world.
Decommissioning is said to take up to 40 years, but about one-fourth is about to take place.
We will summarize how far the work has progressed so far, focusing on dealing with nuclear fuel, which is the most concerned about safety.
Decommissioning work for Unit 1 How far?
Unit 1 whose upper part of the reactor building was blown off by a hydrogen explosion. The fuel pool on the top floor holds 392 nuclear fuels.
In preparation for this removal, debris from the explosion has been continuously removed.
In addition, with the policy of covering the entire building with a cover, installation work will be performed in the future.
On the other hand, robot-based investigations are being conducted on what happens to what is called "debris", which is the nuclear fuel that melts down and melts down below the containment.
At Unit 1, a survey in March 2017, six years after the accident, found sand-like sediments that did not exist before the accident. However, the debris itself has not been confirmed yet, and investigation will continue.
Decommissioning work for Unit 2 How far?
Unit 2 escaped the hydrogen explosion. As with Unit 1, the fuel pool still contains 615 nuclear fuels.
Although the hydrogen explosion did not take place, the radiation level on the top floor where the fuel pool is located is too high for humans to access.
Then, we plan to make a hole in the wall of the building, put a crane, etc., and take out fuel from the pool.
Unit 2 is the most advanced debris investigation.
Debris was observed at the bottom of the containment vessel, which appeared to be debris. In February of last year, the robot was successfully moved and grasped by a remote-controlled robot.
Therefore, next year (2021), the government and TEPCO plan to start extracting debris with Unit 2 first.
Decommissioning work for Unit 3 How far?
At Unit 3, the removal of nuclear fuel from the fuel pool began last April. 566 nuclear fuels are being sequentially transferred to a pool in another building.
In the investigation of melted-off debris, in July 2017 sediments that appeared to be debris were identified.
However, the water level inside the containment vessel is higher than that of Units 1 and 2, and it is necessary to consider ways to lower the water level for debris removal.
Decommissioning work for Unit 4 How far?
As mentioned above, Unit 4 had no nuclear fuel in the reactor, but the fuel pool contained 1,535 nuclear fuels, mainly spent fuel.
Due to the high risk, nuclear fuel removal started earlier than other units in November 2013 (2013), two years after the accident, and in the following year, and in December 2014 (December 2014) Finished carrying to another building.
Work is behind schedule as a whole
In this way, at each unit, work is first proceeding to move nuclear fuel such as spent fuel remaining in the fuel pool to a safe place. In order to proceed with the decommissioning work in the future, we must do this before we can proceed.
However, the work is overall behind schedule.
High pollution remains inside the building, making it impossible to work unless remote work is performed by robots.
The start of the removal of Unit 3 was originally planned for the end of 2014, but it was actually a little over four years late last April.
The initial plan for the removal of the first unit was FY2017, but work has not yet started, and it is about 10 years behind the plan, and it is expected to start in FY2027 or FY2016 .
The removal of Unit 2 was started in FY2018 in the initial plan, but the work has not yet started yet, and it is expected to start between FY2024 and FY2014, about seven years behind the plan. I am.
Measures such as decontamination on the site of the nuclear power plant
In order to ensure the future decommissioning work, another important work carried out in the last nine years is measures against radioactive materials on the site of the nuclear power plant.
Since the entire site of the Fukushima Daiichi Nuclear Power Plant was contaminated, workers at the decommissioning facility had to wear protective clothing and double rubber gloves on the entire site, making it difficult to move and hindering detailed work. And so on.
For this reason, TEPCO has been conducting decontamination work such as stripping off contaminated soil and paving the ground.
As a result, at present, 96% of Fukushima Daiichi nuclear power plants do not need to wear protective clothing, and masks are no longer required in some of them.
At present, the situation at the time of the accident has changed drastically, with a canteen and a convenience store being set up on the site of the nuclear power plant, and the working environment is being improved.
What is the problem of "contaminated water" that continues to occur?
On the other hand, the Fukushima Daiichi Nuclear Power Station continues to generate 170 tons of contaminated water every day, which is a major stumbling block for decommissioning.
In order to cool down the melted-down nuclear fuel and debris, water is continuously being poured into the "reactor" and the "containment containing the reactor", and this continues to be generated as highly concentrated contaminated water.
In addition, a large amount of groundwater flows from the mountain side of the building, which mixes with contaminated water and causes an increase in the amount of water.
One of the pillars of countermeasures to control the inflow of groundwater is the installation of a “frozen soil wall”.
The permafrost wall is to make a so-called "ice wall" of about 1.5 km in length underground by flowing liquid at a temperature below 30 degrees to pipes buried around the building, and started construction in 2014 (2014) And it was almost completed in 2017 (2017).
However, it is difficult to completely freeze some parts of the ground where there are other facilities and pipes, and it is impossible to stop 100% of groundwater inflow.
Therefore, measures were taken to reduce the amount of groundwater flowing into the building by raising groundwater at the upstream side of the building and draining it to the sea, as well as reducing the amount of groundwater flowing into the building by pumping groundwater through wells called sub-drain around the building. We have tried to reduce the amount of water generated.
As a result, the amount of contaminated water that had been generated about 500 tons per day after the accident was reduced to about one-third, but still about 170 tons are still being generated.
This contaminated water contains the radioactive substance tritium, and the issues surrounding this treatment are featured in the first installment of the series.
Decommissioning work, progressed in nine years? Experts
Mr. Hiroshi Miyano, chairman of the decommissioning study committee of the Atomic Energy Society of Japan, who is an executive of a former manufacturer who is familiar with decommissioning work, said that the work so far can be extended by the conventional technology, and the work to be done is just unknown. I point out that it will be an area.
Mr. Hiroshi Miyano, Decommissioning Committee of the Atomic Energy Society of Japan Chairperson: "I think that there was a great deal of difficulty in measures to control contaminated water, prevent the scattering of radioactive materials, and take out nuclear fuel from spent fuel pools. However, the work up to this point can also be done by extending the conventional technology.The point is that the most important thing is that the retrieval of the most important debris etc. has to be developed in an area that has not been experienced before. It's an unknown territory. ''
Decommissioning work, going into the "unknown territory"
Decommissioning is going to be a more difficult stage.
The biggest challenge is the removal of melted nuclear fuel, the so-called debris.
Debris emits extremely intense radiation, and the melted structures around the reactor mix together, making it extremely unwieldy.
It is still in the reactors of Units 1 to 3 and the containment containing the reactors, and the total amount is estimated to be 880 tons.
At present, it is not possible to remove the PCV with the PCV filled with water because the PCV has holes due to accident damage and it is difficult to repair the holes.
If it can be filled with water, it can block radiation, so it would be desirable to be able to do that, but that is not possible.
For this reason, TEPCO has decided to proceed with a method called “submerged construction method” that takes out in the air.
This is actually unprecedented in the world, and it is necessary to thoroughly implement measures to prevent the scattering of radioactive materials and safety measures in environments with high radiation doses, and plan concrete work for such construction methods in the first place. Is an issue.
In the third part of the series, the government and TEPCO will consider whether the decommissioning work can be completed in the remaining approximately 30 years, and what will be the challenges to proceed with the work.
Series 9 Years of the Nuclear Accident Seven Years Fukushima Daiichi Nuclear Power Plant
(1) What are the other options for "release to sea or air"? Posted on February 13 (2) Decommissioning work How far has it gone? Are you not progressing? Posted on February 20 (3) Decommissioning work Can it be completed in 30 years remaining? Scheduled to be posted in late February
For the fourth and subsequent times, they will be posted at a later date.