Fukushima Nuclear Accident Analysis Report
















June 20, 2012
Tokyo Electric Power Company, Inc.

?Foreword

TEPCO sincerely apologizes for the extreme anxiety and inconvenience it has
caused to the local residents around Fukushima Daiichi Nuclear Power Station,
the residents of Fukushima Prefecture, and the broader public due to the accident
at the power station.
In particular, TEPCO is deeply apologetic that numerous people have been
forced to continue evacuating even now because of the emission of radioactive
materials due to the accident.

On the path toward management and stabilization of the accident, Step 2
conditions of reactor cold shutdown as defined in "Roadmap towards Restoration
from the Accident at Fukushima Daiichi Nuclear Power Station," released last
December, have been met. Efforts are currently being made to achieve the
Mid-and-Long-Term Roadmap formulated jointly with the central government
aimed at plant decommissioning measures.
Since the accident, TEPCO has been deeply indebted to the government,
various related agencies, vendors and many individuals and organizations, both
domestic and international, for their meaningful support and cooperation.

TEPCO recognizes that it is its social obligation as a party to this accident to
identify its causes and reflect the lessons learned in business administration
policies to prevent recurrence of a similar accident. Last June, the company set
up an internal Fukushima Nuclear Accident Investigation Committee and has
since been moving ahead to impartially and thoroughly investigate and examine
the accident.
On December 2, 2011, the results of investigations completed by that time and
countermeasures to address the causes and recurrence prevention mainly
regarding equipment were summarized and released as the Interim Report.
Subsequently, information was gathered by conducting hearings and interviews
with concerned parties, checking records, and confirming the field to the extent
possible focusing on items that are especially important to learn as many lessons
as possible from this major accident that resulted in reactor core damage. This
included whether safety-important facilities functioned even after the earthquake,
how data and understanding of conditions of equipment were collected under the
stressful conditions of total loss of power in the field, whether there were mistakes
made in the accident response operations, and whether the chain of command
including the headquarters functioned properly. Further investigations were
conducted to objectively examine event progression assessment results using
collected data and an analytical methodology.
Furthermore, items not covered in the Interim Report have been investigated
and examined, such as initial support of the power station when the accident
occurred, information disclosure, evaluation of radiation control conditions, and
release of radioactive materials.

The results of these investigations have been organized and compiled into this
Fukushima Nuclear Accident Analysis Report.
Based on the facts identified through investigation, this report provides detailed
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?descriptions regarding past and current efforts toward nuclear safety, the
earthquake intensity, tsunami height and their impact on facilities, accident
responses that were taken, and the facility and administrative countermeasures
developed based on the lessons learned.
As a party to this accident, it is the obligation of TEPCO to accurately convey in
detail the facts concerning what happened inside and outside the power station;
what the people involved thought, what judgments they made, and what actions
they took as the accident progressed; and how initiatives were taken toward
securing nuclear safety. Thus, the company has been striving to clarify such facts
to the full extent.
Also, in compiling this report, the Nuclear Safety and Quality Assurance Meeting
Accident Investigation Verification Committee, which is made up of external
experts, was consulted. In addition to reflecting the committee’s “opinions” when
releasing the Interim Report, it also provided various objective advice from the
perspective as experts and as a third party.
This report was developed with a focus on what needs to be done to ensure
nuclear safety. The lessons learned and self-critique will be reflected in the
administration of operations moving forward. Furthermore, it is hoped that this
report will contribute to the improvement of safety at plants both domestic and
abroad and that it is read widely by the broader public.

Once again, TEPCO is keenly aware of its responsibility for the accident.
TEPCO will thoroughly enforce safety first in its business operation in order never
to bring about similar situations again, and will steadily proceed with mid- and
long-term endeavors toward the decommissioning of reactors at Fukushima
Daiichi Nuclear Power Station.

Chairman of the Tokyo Electric Power Company, Inc.
Fukushima Nuclear Accident Investigation Committee
Masao Yamazaki
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?Objectives, Framework and Status of the Accident Investigation

1. Objective
As the party concerned in the accident, to clarify causes of the accident by
investigating and verifying facts, and to incorporate the lessons learned into future
business administration.


2. Framework
(1) Fukushima Nuclear Accident Investigation Committee
?? (Committee members)
Chairman: Executive Vice President Masao Yamazaki
Members: Executive Vice President Masaru Takei
Managing Director Hiroshi Yamaguchi Yoshihiro Naito
General Manager of Corporate Planning Department
General Manager of Engineering Department
General Manager of the Corporate Affairs Department
General Manager of the Nuclear Quality Management Department
Total: 8 members

(2) Accident Investigation Verification Committee
A committee consisting of external experts was established under the Nuclear
Safety and Quality Assurance Meeting as an advisory board to provide comments
from a technical and independent point of view on the investigation results compiled
by the Fukushima Nuclear Accident Investigation Committee.

(Committee members)
Chairman : Genki Yagawa (Professor Emeritus, University of Tokyo)
Members: Yuriko Inubushi (Vice Chair, Consumption Science Federation)
Takeshi Kohno (Professor, Keio University)
Yoshihisa Takakura (Director, Tohoku Radiological Science Center)
Nobuo Shuto (Professor Emeritus, Tohoku University)
Hideki Nakagome (Attorney at Law)
Masao Mukaidono (Professor, Meiji University)

3. Method
(1) Fukushima Nuclear Accident Investigation Committee
The following investigations and verifications were carried out:
Manuals related to this accident, such as the nuclear operator Operation Plan
for Disaster Preparation and various operating procedures in use since before
the accident, were examined and checked.
Earthquake and tsunami data collected at the time of this accident, charts
showing plant behavior, data on alarm records, and other records of plant
parameters collected, as well as daily operating journals recorded at the time of
the accident, white boards, and various other records were examined and
checked.
Analytical assessments using data collected at the time of this accident such as
the tsunami inversion analysis, seismic response analysis, and core damage
analysis.
Field survey studies were conducted on major indoor and outdoor facilities by
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Fact-finding investigations by interviews and various records.
(Fact-finding was performed via interviews with a total of 600 people mostly
consisting of disaster response personnel at the power station and comparison with
various records.)

(2) Accident Investigation Verification Committee
Explanations from the Fukushima Accident Investigation Verification Committee
were verified mainly on the following points:
Are the investigation and verification methods proper?
Are the facts based on objective evidence? Are the investigations in keeping
with the progression of the event, and not from a retrospective point of view?
Are the details of the investigation appropriate?
Are the explanations easy for third parties to understand?
In every meeting of the Verification Committee, in addition to members of the
Fukushima Accident Investigation Committee, the site superintendents of
Fukushima Daiichi Nuclear Power Station, Fukushima Daini Nuclear Power Station,
and Kashiwazaki-Kariwa Nuclear Power Station also attended.

4. Committee meetings
(1) Fukushima Nuclear Accident Investigation Committee
June 11, 2011: 1st Meeting of the Fukushima Nuclear Accident Investigation
Committee
Summary of Fukushima Nuclear Power Stations, the conditions of the
earthquake and tsunami and conditions of damage caused by the earthquake
and tsunami
July 26, 2011: 2nd Meeting of the Fukushima Nuclear Accident Investigation
Committee
State of initial response, accident response after the tsunami arrival, and plant
conditions
September 20, 2011: 3rd Meeting of the Fukushima Nuclear Accident
Investigation Committee
Evaluation of the hydrogen explosions, accident analysis and issue
identification, future actions based on accident response
November 5, 2011: 4th Meeting of the Fukushima Nuclear Accident
Fukushima Nuclear Accident Analysis Report (Interim Report) Plan
February 10, 2012: 5th Meeting of
Investigation Committee
Schedule for Final Report and Structure of Final Report
March 29, 2012: 6th Meeting of the Fukushima Nuclear Accident Investigation
Committee
Preparation for Emergency Response, Power Station Support, and Radiation
Control
April 14, 2012: 7th Meeting of the Fukushima Nuclear Accident Investigation
Committee
Identification of administrative issues, efforts made for safety management and
risk management
May 30, 2012: 8th Meeting of the Fukushima Nuclear Accident Investigation
Committee
Fukushima Nuclear Accident Analysis Report (Final Report) Draft

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(2) Accident Investigation Verification Committee
Committee meetings
June 15, 2011: 1st Meeting of the Accident Investigation Verification Committee
Summary of Fukushima Nuclear Power Stations, conditions of earthquake and
tsunami, and the condition of damage caused by earthquake and tsunami
August 3, 2011: 2nd Meeting of the Accident Investigation Verification
Committee
State of initial response, accident response status after the tsunami arrival, and
plant conditions
September 22, 2011: 3rd Meeting of the Accident Investigation Verification
Committee
Evaluation of the hydrogen explosions, accident analysis and issue
identification, future actions based on accident response
November 10, 2011: 4th Meeting of the Accident Investigation Verification
Committee
Fukushima Nuclear Accident Analysis Report (Interim Report) Plan
April 16, 2012: 5th Meeting of the Accident Investigation Verification Committee
Structure of Final Report, preparation for emergency response, power station
support, radiation control, identifying operational issues and efforts for safety
management and risk management
June 4, 2012: 6th Meeting of the Accident Investigation Verification Committee s Report (Final Report) Draft
In addition, more than 70 individual meetings for detailed explanations and question
and answer sessions were held.
Furthermore, opinions were exchanged with the management of the Nuclear Power
and Plant Siting Division.

Conducting on-site investigations
July 8, 2011, February 1, 2012: Fukushima Daiichi Nuclear Power Station
April 24, 2012, May 10, 2012: Kashiwazaki-Kariwa Nucl
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Table of Contents


1 Report Objective 1

2 Overview of the Fukushima Nuclear Accident 1
2 1 Overview of the Fukushima Daiichi Nuclear Power Station 1
2 2a Daini Nuclear Power Station 2
2 3 Overview of the Accident 2
2 4 Content of Accident Investigation and Composition of This Report 4

3 Overview of State of Tohoku-Chihou-Taiheiyou-Oki Earthquake and Tsunami
Preparations 7
3 1 Scale of the Earthquake and Tsunami 7
3 2 Intensity of Earthquake at the Power Stations 8
(1) Observation Results at Fukushima Daiichi NPS
(2) Observation Results at Fukushima Daini NPS
3 3 Height of the Tsunami at the Power Stations 9
(1) Characteristics of the Tsunami Wave form
(2) Fukushima Daiichi NPS Tsunami Investigation Results
(3) Fukushima Daini NPS Tsunami Investigation Results
(4) Reason for the Difference in Height of Tsunami between Fukushima Daiichi
NPS and Fukushima Daini NPS
3 4 Earthquake Preparations (Seismic Safety Assessment) 15
(1) Chronology of Seismic Safety Assessment
(2) Seismic Safety Assessment (Interim Report)
3 5 Tsunami Preparations 20
(1) Evaluation of Tsunami Height
(2) Arguments Regarding the Tsunami by Pertinent Agencies and Response by
TEPCO
(3) Japan's Earthquake and Tsunami Evaluation after the Sumatra Island
Earthquake
(4) Ground Level of Buildings
(5) Conclusion

4 Preparations for Safety Measures (Excluding Earthquakes and Tsunamis) 43
4 1 Regulations 43
4 2 Operation Plan for Disaster Preparation 44
4 3 Facility Design 44
4 4 Incorporating New Findings [Attachment 4-3] 44
4 5 Preparations for Severe Accidents [Attachment 4-6] 48
(1) Development of Accident Management Measures
(2) Probabilistic Safety Assessment (PSA) efforts for AM Measures
(3) Accident Management Measures and the Fukushima Accident
4 6 Efforts for Safety Culture and Risk Management 57
(1) Efforts to Improve Safety and Quality
(2) Cross-Divisional Efforts for Risk Management


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~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~5 Planned and Actual Preparations for Disaster Response 64
5 1 Nuclear Disaster Preparations (Plan) 64
(1) Development of Disaster Preparations Plan
(2) Basic Structure and Roles of the Off-Site Center
(3) Overview of Off-Site Facility
5 2 TEPCO’s Response Framework in Detail (Plan) 69
(1) Emergency Preparations (General Disasters)
(2) Emergency Response Preparations (Nuclear Disaster)
5 3 Response Status During the Accident 73
(1) Declaration of State of Emergency and State of Nuclear Emergency
(2) Providing Information to the Central Government
(3) Providing Information to Surrounding Communities
(4) Information Disclosure
(5) Personnel Dispatch and Activities
(6) Activities at the Off-Site Center
(7) Withdrawal Issue

6 Impact of the Earthquake on Power Stations 117
6 1 Plant Status Immediately Before the Earthquake 117
(1) Status of Fukushima Daiichi NPS
(2) Status of Fukushima Daini NPS
6 2 Plant Status Immediately After the Earthquake 117
(1) Status of Fukushima Daiichi Unit 1
(2) Status of Fukushima Daiichi Unit 2
(3) Status of Fukushima Daiichi Unit 3
(4) Status of Fukushima Daiichi Unit 4
(5) Status of Fukushima Daiichi Unit 5
(6) Status of Fukushima Daiichi Unit 6
(7) Status of Fukushima Daini NPS
6 3 Status of Off-Site Power 129
(1) Fukushima Daiichi NPS
(2) Fukushima Daini NPS
(3) Causes of Damage to Off-Site Power Facilities
(4) Summary of Off-Site Power
6 4 Assessment of the Impact of the Earthquake on Facilities 137
(1) Assessment Using Plant Parameters
(2) Results of Seismic Response Analysis Using Observation Records
(3) Results of Visual Checks of Station Facilities
(4) Summary of Impact Assessment on Facilities

7 Direct Damage to the Facilities from the Tsunami 149
7 1 Damage to the Facilities at Fukushima Daiichi NPS 149
(1) Flood Pathways into Major Buildings
(2) Facility Damage due to the Tsunami
7 2at Fukushima Daini NPS 155 ue to the Tsunami
7 3 Summary of Damage to the Facilities due to the Tsunami 159
(1) Fukushima Daiichi NPS
(2) Fukushima Daini NPS
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8 Response Status after the Earthquake and Tsunami 161
8 1 Movement of Personnel On-Site 163
(1) Status of Employees and Contractor Workers Working On-Site Before
Earthquake
(2) Movement of Personnel Immediately After Earthquake Occurrence
(Evacuation/Direction Out of Radiation Control Area)
(3) Movement of Personnel within the MCR
(4) Movement of Employees, Contractor Workers Beyond March 12
8 2 Fukushima Daiichi Unit 1 Response and Station Behavior 167
(1) Response Status Overview
(2) Details of Response Status
(3) Behavior at the Station
(4) Summary
8 3 Fukushima Daiichi Unit 2 Response and Station Behavior 209
(2) Response Status Details
(4) Summary
8 4 Fukushima Daiichi Unit 3 Response and Station Behavior 234
(1) Response Status Overview
(3) Behavior at the Station
(4) Summary
8 5 Fukushima Daiichi Unit 4 Response and Station Behavior 262
8 6 Fukushima Daiichi Unit 5 264
(1) Response Status
(2) Summary
8 7 Fukushima Daiichi Unit 6 Respons 271
(2) Summary
8 8 Fukushima Daini Unit 1 Response and Station Behavior 275
(1) Response Status
(2) Station Parameter Behavior
(3) Summary
8 9 Fukushima Daini Unit 2 Status and Station Behavior 282
(1) Response Status
(3) Summary
8 10 Fukushima Daini Unit 3 Response and Station Behavior 287
(1) Response Status
(2) Station Parameter Behavior
(3) Summary
8 11 Fukushima Daini Unit 4 Respons 291
(1) Response Status
(3) Summary
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9 Handling Spent Fuel Pools (SFP) Cooling 296
(1) Sequence of Events Leading to the Securing of Coolant Injection for the SFPs
at the Fukushima Daiichi NPS
(2) Fukushima Daiichi NPS SFP Cooling
(3) Fukushima Daini NPS SFP Cooling

10 Supporting the Power Station 302
10 1 Supporting Fukushima Daiichi with Personnel 302
(1) Number of Personnel Dispatched to the Fukushima Daiichi NPS
(2) Assistance Activity Details
(3) Assistance Activity Results
10 2 Materials and Equipment Support for Fukushima Daiichi 311
(1) Securing Batteries [Attachment 10-2]
(2) Securing Power Supply Cars [Attachment 10-3]
(3) Securing Fire Engines [Attachment 10-4]
10 3 Spent Fuel Pool Cooling Water Injection/Cooling Assistance 328
10 4 Power Station Assistance Evaluation 329
(1) Problems
(2) Points that can be Evaluated Positively

11 Evaluation of Plant Explosion 334
11 1 Explosion Cause Estimation 334
(1) Explosions Caused by the Gasification of Combustible Liquids
(2) Steam Explosion
(3) Hydrogen Explosion
11 2 Analysis on Explosion Events Using Seismometers 336
11 3 Causes of Hydrogen Explosion 340
(1) Details of Hydrogen Leaking into the Reactor Building
(2) Causes of Hydrogen Explosion at Unit 4
(3) Design and Operation of the SGTS and its Role in this Accident
(4) Efforts to Prevent Hydrogen Explosions
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