Chapter 9
Lessons Learned from the Nuclear Power Plant
Accident and Recovery Efforts
Field of expertise: Maintenance Engineering

Yutaka Watanabe & Takayuki Aoki
Translated by Catherine Sachi Kikuchi

SummaryThe decommissioning of the Fukushima Daiichi Nuclear Power Plant requires an unprecedented
effort to remove fuel debris while maintaining the main facilities, which are damaged and have
reduced safety functions, under conditions that are difficult to access due to high radiation dose
rates and contamination. To minimize the risk of additional external release of radioactive
materials, it is essential to create methods for efficient and effective fuel debris removal and longterm facility maintenance management for the damaged main facilities, as well as a strategic
approach for early transition from the current state to a safely manageable state. New technology
development and human resource development are urgently needed for this purpose.
Keywords: nuclear power plant, accident, decommissioning, risk reduction, safety assurance,
basic research, human resource development

Introduction
On March 11, 2011, the Great East Japan Earthquake occurred. This earthquake is said tobe a once-in-a-thousand-year event. In addition to the magnitude of the shaking, theaccompanying huge tsunami hit the Fukushima Daiichi Nuclear Power Station (hereinafter referredto as 1F) of Tokyo Electric Power Holdings Co. This triggered the first nuclear accident in Japan.
Although there were no fatalities due to radiation, the accident caused serious problems, including
evacuation of the surrounding residents due to the external release of radioactive materials,
collapse of local communities, and environmental pollution.

1: What Happened at 1F at that time?
The reactors at 1F (Units 1-3), then in operation, shut down as control rods wereautomatically inserted into the reactor cores by detecting the massive shaking caused by thequake. Although the external power supply was interrupted by it, the emergency diesel generators
started normally and supplied power to the pumps and other equipment, thus maintaining the1cooling function for the reactors. However, about an hour later, a tsunami of 14 to 15 meters high,which was much higher than expected, came in and flooded on the site and in the buildings. The
emergency power supply facilities in the buildings were flooded and became inoperable. As aresult, it became impossible to supply power to important facilities such as the pumps andemergency generators used to cool the reactors, thus losing the cooling function.
Nuclear fuel generates a large amount of so-called decay heat1, in which radioactive materials
produced by fission emit α-, β-, and γ-rays, to stabilize themselves even after fission stops. Since
each of the reactors in 1F Units 1-3 lost its function to remove decay heat, the coolant in thereactor gradually evaporated and the water level dropped., the coolant in the reactors gradually
evaporated due to the loss of the function to remove decay heat, and the water level dropped.
Therefore, as an emergency response, the injection of fresh water for firefighting was attemptedfrom outside, and after it ran out, seawater injection was also attempted. However, these attempts
were ultimately unsuccessful, resulting in a serious accident in which the fuel melted and fellthrough the wall of the reactor pressure vessel (RPV). In addition, the pressure in the primarycontainment vessel (PCV) surrounding the reactor became excessive and damaged the PCV,causing hydrogen2 generated in the reactor core to leak out and cause an explosion. This
explosion damaged the reactor buildings and led to the external release of radioactive materials,
resulting in widespread contamination of the environment (Figure 9-1).

Figure 9-1. Progression of the accident at the Fukushima Daiichi Nuclear Power Station, units 1-3(From the website of Tokyo Electric Power Company Holdings, Inc.)As described above, at the site of the 1F accident, emergency responses to the accident
that had not been anticipated so far were forced to be taken without a scenario. Meanwhile, thereactors and spent fuel pools were continuously flooded with water using fire
trucks and temporary pumps to bring them under control, and the circulation cooling system thatwas constructed during this time has enabled steady cooling to the present day.
1

Immediately after the shutdown, about 97MW of decay heat was released at Unit 1, and about 167MW at
Units 2 and 3.
2

It is known that when a loss-of-coolant accident occurs and the fuel becomes exposed, the temperatures riseto 800°C~900°C or higher, and the zirconium used in the fuel cladding reacts with water vapor to produce heatand hydrogen.

2

The investigation and examination of the accident at 1F has been conducted and reports
have been issued by four organizations: the Fukushima Nuclear Accident Independent
Investigation Commission by the National Diet of Japan (National Diet Investigation), theInvestigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo
Electric Power Company (government investigation), the Fukushima Nuclear Accident Independent
Investigation Commission (private investigation), and the Tokyo Electric Power Company
Fukushima Nuclear Accident Investigation Committee (TEPCO investigation). Each of thesereports concluded that the direct cause of the Fukushima accident was the loss of all power due tothe tsunami, although some unresolved issues were pointed out therein. Subsequently, in 2013,
the Nuclear Regulation Authority established the Study Group on the Analysis of the Accident at
Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station, which stated, "It istrue that there are still some technical issues at the Fukushima Daiichi Nuclear Power Station thatcannot be confirmed at this time due to the difficulty of conducting on-site surveys and otherlimiting factors, and that must await further investigation and study”. The committee compiled aninterim report in 2014, in which it stated, "In our investigations to date, we have largely completed
our examination of the issues that were pointed out as unresolved in the National Diet of Japan
Fukushima Nuclear Accident Independent Investigation Commission report, but there are still someissues that we cannot start on-site investigations for reasons such as high radiation doses.
Therefore, continued field investigations, evaluations, and studies are required. However, thereport also states that at this point, no facts have been confirmed that would contradict the main
events that have been reported so far and the direct causes of these events.

2: Current Status and the Decommissioning Plan of 1F
10 years after the accident, the environment inside the plant has been greatly improved,
and it is now possible to enter the plant without wearing radiation protective clothing or masks.
However, the inside of the reactor buildings of Units 1-3 are still highly contaminated withradioactive materials, and workers and others cannot easily access them. Therefore, investigations
inside the buildings and PCVs have been conducted by strictly practicing the three principles of
reducing external exposure (keeping distance, installing shielding, and limiting time), controllingareas to be entered, wearing special protective clothing, and making extensive use of remotecontrol equipment and robots. As a result, it was found that there was fuel debris and deposits thatappeared to be structures at the bottom of the PCV under the RPV in Unit 1, deposits thatappeared to be fuel debris and structures and a part of the fuel assembly (upper tie plate) in thesame location as in Unit 2, and deposits that appeared to be fuel debris and structures and part ofthe in-core reactor core (CRD guide tube) in the same location as in Unit 3 (Figure 9-2).
To achieve the goals, set forth in the Medium- to Long-term Roadmap developed by theInter-Ministerial Council for Contaminated Water and Decommissioning Issues and the TechnicalStrategy Plan1) to provide direction for fuel debris removal methods, etc., based on this roadmap,and the Risk Map2) developed by the Nuclear Regulation Authority of Japan, TEPCO announced
the Medium- to Long-Term Decommission Action Plan 20203) in March this year. According to theplan, TEPCO will conduct a test removal of fuel debris from Unit 2 over the next three years andwill reflect the information obtained from the test removal in the design of the full-scale removal
equipment, etc., to gradually expand the scale of removal. The plan for Unit 1 and Unit 3 is toproceed with the project while reflecting the experience of fuel debris removal from Unit 2 (Figure9-3).

3Figure 9-2. The status of the Fukushima Daiichi Nuclear Power Station, units 1-3(From the Technology Strategy Plan1), Nuclear Damage Compensation and DecommissioningFacilitation Organization)

Figure 9-3. Fuel debris removal process
The decommissioning of an accident-damaged reactor such as 1F is an internationallyunprecedented challenge that requires removing fuel debris while maintaining the safety functionsof the main facilities that have been damaged and degraded by accident under conditions that aredifficult to access due to high radiation dose rates and high contamination from radioactive
materials. This is trying to execute an unprecedented project in the world. To proceed safely andsteadily with this unprecedented project, strategic efforts are needed to reduce the risk ofadditional release of radioactive materials from the current relatively high-risk state to a state thatcan be safely managed at an early stage, and to ensure that the decommissioning work can4proceed safely and steadily with sufficient time to spare. In addition, it is necessary to introduce
efficient and effective ideas/methods for planning methods of fuel debris removal and long-term
facility management of damaged main facilities to obtain and assure availability of long-term
personnel, reduce radiation exposure, and secure funding. Success will require achieving a wide
variety of research and technological development expansion and execution, as well humanresource development and training.

3: Lessons Learned from the Accident and Future Actions
The Nuclear Regulation Authority of Japan (NRA) established standards to prevent the lossof functions due to common causes and the development of severe accidents as lessons learned
from the 1F accident. These standards are based on the fact that the Fukushima accident caused
a simultaneous loss of safety functions due to common causes such as earthquakes and tsunamis,
and that the subsequent development of severe accidents could not be stopped (Figure 9-4).

Figure 9-4. The basic concept and main requirements of the new regulatory standards
These standards are said to be the most stringent in the world, as they were formulated
after a comprehensive survey of the world's codes and standards, followed by the NRA's own
independent study. Given the hindsight after the accident, the question becomes why did we notanticipate this and why did we not take measures to deal with it? And once the occurrence of suchan accident is assumed, it is possible to develop countermeasures to deal with it. Therefore, toprevent accidents such as the one at 1F from recurring, it is first and foremost necessary todevelop the ability to assume hazards (or risks) or to imagine them. To realize this, it is importantto overview the entire system including the mechanical system (or hardware system) of a nuclear
power plant and the human system that implements safety activities including responses to5

accidents that occur in the mechanical system, and to learn risk assessment and analysis methods
including probabilistic risk assessment methods. Second, it is necessary to have the ability to makeappropriate decisions on whether to implement countermeasures against assumed hazards (or risksources). For this purpose, it is essential to have a system and capability to make decisions onsuch issues as an organization.

4: For Safe and Steady Decommissioning of 1F: Tohoku University’sInitiatives
The decommissioning of 1F, which is expected to last for more than 30 years, is a major
undertaking that places the nation's prestige on the line, and the government is leading an allJapan effort to carry it out. It is necessary for all related organizations in Japan to make concerted
efforts to solve this unprecedented and globally significant problem and to proceed with thedecommissioning safely and steadily.
Under these circumstances, Tohoku University's efforts for the decommissioning of 1F
began the year after the accident, when the University president himself visited the 1F site anddecided that we, as a university in the disaster area, must contribute to solving this unprecedented
problem (the technical challenge). Our university has set "leading the reconstruction and recovery
after the Great East Japan Earthquake" as its university-wide vision under the traditional principles
of "research first", "open-door policy", and "respect for practical learning". As a part of this vision,
we launched the " Decommissioning of Accident-damaged Nuclear Power Plant andEnvironmental Restoration Project" and started the Decommissioning Basic Research and HumanResources Development. This project has been engaged in basic research contributing to thedecommissioning of accidental nuclear reactors and the training of young engineers andresearchers who will be responsible for future decommissioning of nuclear reactors since the 2014
academic year. Based on these experiences, on December 1, 2016, the Center for Fundamental
Research of Nuclear Decommissioning (CFReND) was established as a university-wide
constituent organization of the Institute for Disaster Reconstruction and Regeneration Research tofurther develop its activities and lead Japan's basic and fundamental research on nucleardecommissioning. In April 2020, the Cooperative Research Division on Innovative Technologies for
Fukushima-Daiichi Decommissioning was newly established to conduct joint research with TEPCO
on 1F decommissioning (Figure 9-5). CFReND will serve as a bridge between the University andexternal organizations, will digest the needs of the external organizations and communicate themto the University, and will serve as an interpreter (hub function) to communicate the seeds andideas of the University to external organizations in an easy-to-understand manner (Figure 9-6). It
also contributes to the training of the next generation of human resources who will be responsiblefor decommissioning by utilizing this kind of practical research.

6National University Cooperation

Tohoku University

Institute for Disaster
Reconstruction andRegeneration ResearchSteering Committee

Tohoku Medical Megabank Organization
Research Organization of ElectricalCommunication

Center for Fundamental Research on Nuclear
Decommissioning
Director

Advisory Committee

Support OfficeVice Director

Division of risk management Division of radioactive waste Cooperative research division
on innovative technologies
management studies forstudies for nuclear
for Fukushima-Daiichi
nuclear decommissioning
decommissioning
decommissioning

Figure 9-5. Organization Chart of the Center for Fundamental Research of Nuclear
Decommissioning (CFReND)

Overseas ResearchOrganizations
Domestic Research
Organization
Laboratory inTohoku University
Researchers inTohoku University

CFReND

Proposals orApplications

NDF
METI

Needs, Funds
Domestic Research
Organization

MEXT
JAEA/CLADS

Industry(Utilities, Plant manufacturers,Construction Companies, etc.)
MEXT
Projects

IRID

Laboratory in
Tohoku University

Figure 9-6. CFReND’s Hub function for research collaborations

Conclusion - From the author
To complete the decommissioning of 1F that experienced a severe accident, it is essentialnot only to combine existing technologies but also to develop innovative technologies or toelucidate the principles of phenomena, etc., which will require a high level of challenge in a wide
range of academic fields. Now that the collaboration with TEPCO has been established, the trueneeds and raw data of the 1F site can be obtained accurately and promptly, and it is expected thatCFReND will function to contribute to the safe and steady progress of the 1F decommissioning by
efficiently and effectively demonstrating the research potential of the University in all fields.

7References
Ministry of Economy, Trade and Industry. (2019). TEPCO Holdings, Inc. Mid-and-Long-Term
Roadmap for Decommissioning Fukushima Daiichi Nuclear Power Station. Decommissioning andContaminated Water Countermeasures Meeting, Government of Japan.
https://www.meti.go.jp/earthquake/nuclear/pdf/20191227.pdf (In Japanese)
Based on this roadmap, the following document has been prepared:
Nuclear Damage Compensation and Decommissioning Facilitation Corporation. (2019). TechnicalStrategic Plan 2019 for Decommissioning of the Fukushima Daiichi Nuclear Power Station ofTokyo Electric Power Company Holdings, Inc.
Nuclear Regulation Authority. (2020). Fukushima Daiichi Nuclear Power Station of Tokyo ElectricPower Company Holdings Inc. Mid-Term Risk Reduction Target Map (March 2020 version).
Cabinet of Japan. https://www.nsr.go.jp/data/000306118.pdf (In Japanese)
Tokyo Electric Power Company Holdings Inc. (2020). Decommissioning Mid-to-Long-Term Action
Plan 2020.
https://www.tepco.co.jp/decommission/information/newsrelease/reference/pdf/2020/1h/rf_2020032
7_1.pdf (In Japanese)8