It’s been a decade since the accident at the Fukushima Daiichi Nuclear Power Station and we are moving forward with decommissioning while reducing risk as much as possible.
Through contaminated water countermeasures, the amount of contaminated water being generated has been significantly reduced. Fuel removal work from the spent fuel pools was completed at Unit 4 in 2014 and at Unit 3 in February 2021. Furthermore, the work environment has been improved to the point where workers may wear simple masks and regular uniforms on approximately 96% of the site.
We are sincerely grateful for the vast amount of technical and human support we have received from both within and outside of Japan.
The decommissioning work that lies ahead of us includes many unprecedented and difficult challenges, such as the retrieval of fuel debris. By leveraging to the best of our ability the knowledge and experience we have gained to date, and wisdom from both within and outside the company, we shall attempt to predict various unforeseen circumstances and devote ourselves to decommissioning.
The following is a history of the various initiatives we have engaged in since the accident.
What happened?
On March 11, 2011, an earthquake occurred off the coast of the Sanriku region of Japan. Units 1 through 3, which were online, were safely scrammed. Units 4 through 6 had been taken offline for periodic inspections.
The earthquake caused a loss of all external power at the Fukushima Daiichi Nuclear Power Station. However, since emergency diesel generators started automatically, power for the plant was secured and the cooling of reactors continued.
About 50 minutes after the earthquake, the site was directly hit by a massive tsunami. Almost the entire area where reactors were located was flooded by the tsunami. Power supply equipment was rendered inoperable due to internal inundation, and vital safety functions, such as reactor cooling water injection and reactor status monitoring, were lost.
It became impossible to inject additional cooling water into the reactors and as a result, the water in the reactor pressure vessels of Units 1 to 3 evaporated. The chemical reaction between the fuel rods exposed from the water surface and water vapor produced hydrogen that accumulated inside the reactor building and led to hydrogen explosions at Unit 1 and Unit 3.
At Unit 4, we assume that hydrogen flowed into, and accumulated in, the reactor building through the joint to the exhaust stack when the Unit 3 primary containment vessel was vented to mitigate increasing internal pressure, thereby leading to an explosion.
After the accident, every possible means was employed as we tried to inject cooling water into the reactors. Actions to stabilize the reactors were also commenced, such as venting the primary containment vessel to reduce internal pressure.
On March 20, 2011, external power was restored and reactor cooling could recommence.
In April 2011, the “Roadmap towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station” was announced thereby putting forth three major objectives:
I. Cooling reactors and fuel pools
II. Suppressing the discharge of radioactive substances
III. Monitoring/decontamination
Thanks to these initiatives, cold shutdown (below 100℃) was achieved at all reactors in December 2011. We have also been able to keep exposure doses at site borders sufficiently low.
In December 2011, the Japanese government created the “Mid-and-Long-Term Roadmap towards the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station Units 1-4”, and based on this, we have been moving forward with decommissioning work.
Then, in April 2012 and January 2014, decisions were made to decommission Units 1-4, and Units 5 and 6, respectively.
The Mid-and-Long-Term Roadmap is continually revised to reflect field conditions that come to light in the course of decommissioning. (Most recent revision was in December 2019.)
TEPCO created the “Mid-and-Long-Term Decommissioning Action Plan 2020” to stipulate the major processes for the entire decommissioning process to achieve the targets of the Mid-and-Long-Term Roadmap and the Nuclear Regulation Authority’s Risk Map.
As part of decommissioning, we are removing fuel, retrieving fuel debris, implementing contaminated water countermeasures, and treating waste, etc., while also improving the work environment, engaging in research and development, and improving safety.
Let’s have a look at each of these tasks.
Overview
Spent fuel used in the power regeneration process is stored in spent fuel pools. In order to reduce the risks associated with this spent fuel, we are removing, and making preparations to remove, the fuel from the reactor buildings at which accidents occurred.
Let’s look at the status of progress at Units 1-4.
Completion of the removal of spent fuel from Unit 4
At Unit 4, removal of all 1,533 fuel assemblies commenced in November 2013 and was completed in December 2014.
Construction of a large cover around the Unit 1 reactor building underway
A building cover will be built to encompass the entire structure and from underneath this cover remotely controlled equipment, such as a ceiling crane and heavy machinery, will be used to remove rubble.
After removing rubble, the operating floor will be decontaminated and shielded, after which fuel removal equipment (fuel handling machine and a crane) will be installed.
Construction of a fuel removal platform at Unit 2 underway
An explosion did not occur at Unit 2 because hydrogen was allowed to escape through holes made in the reactor building walls. The plan to remove fuel after dismantling the top of the building was abandoned in consideration of the safety of the local communities.
Instead, we will build a fuel removal platform (access gantry and anticum) on the south side of the building, connect it to the building, and insert fuel removal equipment onto the operating floor, after which fuel will be removed using remotely operated equipment.
Currently, construction and deliberation of the fuel removal platform (access gantry and anticum) on the south side of the reactor building is currently underway.
Completion of the removal of spent fuel from Unit 3
Rubble was removed from the top of the Unit 3 reactor building, which suffered a hydrogen explosion, and the installation of the fuel removal cover was completed in February 2018.
After preparations were completed, fuel removal from the spent fuel pool began in April 2019.
Removal of all 566 fuel assemblies was completed in February 2021.
At the time of the accident, Units 1-3 were in operation, so there was fuel inside the reactor pressure vessels.
Power, including emergency power sources, was lost thereby making it impossible to cool reactors. The fuel overheated and melted. This melted fuel that cooled and solidified is referred to as, “fuel debris.”
Various investigations have been implemented in order to examine the conditions inside the primary containment vessels in preparation for fuel debris retrieval.
To date we have conducted many internal investigations of the primary containment vessels using robots and measurement methods that employ muons, cosmic radiation with strong penetrating ability.
An internal investigation of the primary containment vessel of Unit 1 found deposits at the bottom and on pipes outside the pedestal. At Unit 2 we confirmed that pebble-size deposits can be moved. Information obtained during the contact investigation will be utilized for examining other internal investigations and methods for retrieving fuel debris (removal location and design of equipment etc.) in the future. And, at Unit 3, we found deposits in multiple locations at the bottom of the pedestal, which is the foundation that supports the reactor.
When water used to cool fuel debris comes in contact with that fuel debris, it becomes contaminated water that contains high concentrations of radioactive substances. Furthermore, new contaminated water is also generated when groundwater and rainwater flowing into buildings, such as the reactor buildings and turbine buildings, etc., mixes with contaminated water.
The three principles of contaminated water countermeasures are:
1. Remove the source of contamination
2. Isolate water from contamination sources
3. Prevent the leakage of contaminated water
In order to reduce risks associated with the radioactive substances contained in contaminated water, first, cesium adsorption equipment is used to remove primarily cesium and strontium, which account for the majority of the radioactive substances in the contaminated water. After that, by treating it with multi-nuclide removal equipment (ALPS), most radioactive nuclides, with the exception of tritium, can be removed.
By employing treatment methods that take advantage of the chemical and physical nature of radioactive substances, such as using chemical agents to precipitate radioactive substances and adsorb them using adsorbents, etc., ALPS can remove enough of 62 types of radioactive substances to reduce concentrations to below legally required limits and purify contaminated water.
We have reduced the amount of groundwater flowing into the buildings, by “facing” surface, which prevents rainwater from seeping into the ground, pumping up groundwater from wells located in the vicinity of the buildings, and forming an ice wall under the soil around the buildings. We have also moved forward with countermeasures to prevent rainwater from flowing into damaged buildings.
In order to prevent groundwater from leaking into ocean, steel walls have been built on the ocean side to stop the flow of water and sea-side wells pump up groundwater, thereby keeping the environment in the port safe.
Radioactive substances in the contaminated water are removed in stages to reduce risks, after which the water is stored in tanks on site.
For example, we prevent groundwater that may possibly be contaminated from flowing into the ocean and impacting the environment by using ocean-side impermeable walls, and at groundwater drains, we pump up groundwater stopped by the ocean-side impermeable walls using wells located at seawalls. Groundwater that is pumped up is purified and discharged into the ocean after confirming that it meets the regulatory standards for discharge, thereby reducing the amount of groundwater coming close to the reactor buildings.
Countermeasures to remove the source of contamination
The treatment of highly concentrated contaminated water stored in tanks was completed in May 2015 (excluding a portion of this water) thereby reducing risks.
Measures to isolate water from contamination sources
The amount of contaminated water being generated has been reduced from approximately 540m³/day (May 2014) prior to the implementation of countermeasures, to approximately 180m³/day in FY2019, and approximately 140m³/day in 2020 (annual averages).
Countermeasures to prevent the leakage of contaminated water
We have gradually reduced the concentration of radioactive substances (cesium) within and outside of the power station port to approximately 1/1 million what it was immediately following the accident. Furthermore, we completed the transfer of treated water from flanged tanks to welded tanks thereby reducing the risk of leaks and enabling us to manage stored treated water in a safer manner.
Radioactive waste generated in conjunction with decommissioning is stored in waste storage facilities safely at the power station. In addition to rubble, this waste also includes gloves, masks, and protective clothing used for decommissioning tasks. Soil and felled trees removed in the course of decontamination, are also sorted and stored as waste.
In March 2016 we created a solid waste storage and management plan, which stipulates plans to eliminate outdoor temporary storage through the construction of storage facilities and volume reduction facilities based upon our 10-year forecast for the amount of solid waste to be generated, and plans to store solid waste more properly through continuous monitoring. This storage and management plan is updated once a year in conjunction with forecast revisions in light of decommissioning progress.
The effects of decontamination
Immediately after the accident workers were forced to wear protective clothing and full facemasks in all areas.
As a result of decontamination through “facing,” which entails spraying the ground with mortar, radiation doses were reduced on site and the dispersion of radioactive substances is being prevented. This prevents clothing from becoming contaminated thereby enabling workers to wear regular uniforms.
In March 2016, the power station site was divided into three zones: Green, Yellow, and Red. At current time, the size of the Green Zone, in which workers need only wear simple masks and regular uniforms, has been expanded thanks to dust dispersion prevention measures and radiation level reductions. In 2019, this was enlarged to 96% of the site.
Developing facilities
Several new facilities have been built based on the opinions of site workers in order to make the environment easier to work in.
In May 2015, we completed construction of a large rest house that can accommodate approximately 1,200 workers. In addition to being able to rest and have meetings in a comfortable space, workers can also enjoy hot meals in the cafeteria, shop at a convenience store, and take advantage of shower rooms.
The new administration office building was also opened in October 2016. This has enabled workers to carry out their duties in a more relaxed environment. And, with the partner companies’ building located right next to the new administration office building, the physical distance between TEPCO employees and contractors has been shortened thereby enabling even closer communication and coordination.
In preparation for accidents, a 24-hour emergency medical center has been created, and a helipad has been built in order to quickly transport injured parties to medical facilities offsite if necessary.
We dismantled approximately half of the top of the Unit 1/2 exhaust stack in cooperation with a local company in order to reduce the risk of toppling even though the exhaust stack had sufficient seismic resistance.
This task, which was completed in May 2020, was implemented using remotely operated equipment in order to reduce worker exposure.
We have implemented various countermeasures during the 10 years since the accident as we aim to complete decommissioning. However, there are many issues that we must solve as we engage in the long-term decommissioning process.
At Unit 1, we plan to complete construction of the large cover by around 2023 in order to reduce dust dispersion during rubble removal. After that, we will commence fuel removal between FY2027 and FY2028. At Unit 2, we shall deliberate and implement plans for effective decontamination and shielding to reduce radiation doses in the operating floor in preparation for the commencement of fuel removal between FY2024 and FY2026. Ultimately, we aim to complete fuel removal from Units 1-6 during 2031.
Fuel debris retrieval is an unprecedented task never attempted anywhere in the world. We will continue to engage in R&D and engineering tasks to apply the fruits of that R&D in the field, along with manufacturing and installing fuel debris retrieval equipment in preparation for trial retrieval from Unit 2.
We plan to reduce the amount of accumulated water in the reactor buildings to approximately half what it was at the end of 2020 between FY2022 and FY2024, and reduce the amount of contaminated water being generated to less than 100m³/day during 2025. The treatment of accumulated water in the buildings was completed in 2020. In order to move our plan forward, we are keeping groundwater levels around buildings low and implementing measures to prevent rainwater from seeping into the ground, such as paving site areas inside the land-side impermeable while (ocean side, mountain side) and repairing damaged parts of building roofs.
Based on government policy, we shall employ careful processes to suitably dispose of treated water (that contains tritium) resulting from the purification of contaminated water generated at the power station, which is being stored in tanks on site. When we discharge water treated with ALPS etc. into the environment in the course of disposal, we shall reduce the concentrations of radioactive substances, with the exception of tritium, as much as possible to comply with government regulatory standards by subjecting it to re-purification (secondary treatment).
In regards to waste treatment, we are quickly providing measurement data useful for ascertaining the nature of waste and determining safe methods of storage/management so as to enable technical forecasts for treatment/disposal policies, and the safety of those policies. We plan to eliminate all outdoor temporary storage of rubble, etc., by 2028.
Decommissioning will take 30 to 40 years. Along with continuing to improve the work environment, we shall proactively engage in the development of new technologies, such as robots and remotely operated equipment, etc., as we safely and steadily complete decommissioning of the Fukushima Daiichi Nuclear Power Station with your understanding and cooperation.