Examination of Accident at Tokyo ElectricPower Co., Inc.’s Fukushima Daiichi
Nuclear Power Station and Proposal ofCountermeasures(Summary)

October 2011

Japan Nuclear Technology InstituteExamination Committee on Accident at Fukushima Daiichi Nuclear
Power Station

Introduction
This report is a compilation of the aggregate efforts of the nuclear powerindustry comprising electric power operators and manufacturers in Japan as
concerns the accident at Tokyo Electric Power Company Incorporated’s
Fukushima Daiichi Nuclear Power Station (“Fukushima Daiichi”) which
resulted from the Great East Japan Earthquake that occurred on March 11,
2011. This report analyzes the accident based on published facts, knowledge
grounded in operational experience accumulated to date, and knowledge
developed through plant design, and formulates measures and proposals to beaddressed in the future.
We believe that implementing these measures and proposals will ensure that
nuclear power stations are even safer.

1. Overview of Fukushima Daiichi Nuclear Power Station and theAccident
1.1 Overview of Fukushima Daiichi Nuclear Power Station
Fukushima Daiichi is located in northeastern Japan (latitude 37 degrees
north and longitude 141 degrees east), and looks out over the Pacific Ocean at a
distance approximately 225km north of Tokyo.
The first unit commenced operation in 1971, and has a total electrical output
of 4,696MW. The principal specifications for each unit are given in the tablebelow.
Unit

Output(MW)

Start ofoperation

Reactortype

Containment
model

Generalcontractor

1

460

March 26, 1971

BWR-3

Mark I

GE

2

784

July 18, 1974

BWR-4

Mark I

GE & Toshiba

3784

March 27, 1976

BWR-4

Mark I

Toshiba

4

784

BWR-4

Mark I

Hitachi

5

784

BWR-4

Mark I

Toshiba

6

1100

BWR-5

Mark II

GE & Toshiba

October 12,
1978
April 18, 1978
October 24,
1979

1.2 Overview of Accident Following Earthquake off Pacific Coast in the Tohoku
RegionAt Fukushima Daiichi, as Units 1 thru 3 were operating and Units 4 thru 6
were in the refueling outage, an earthquake occurred off the Pacific coast in the
Tohoku [Northeastern] region of Japan at 14:46 on March 11, 2011, and Units 1
thru 3 automatically shut down in accordance with the signal indicating“increasing seismic acceleration.”1Following the reactor shutdown, generators shut down and, moreover,functionality of any external power source system equipment was also lost.
However, AC power for each unit was supplied by emergency diesel generators(“emergency DG”).
Subsequently, due to the tsunami which struck the station, all emergency DG
except for one air-cooled unit at Unit 6 shut down, and all AC power sources atUnits 1 thru 5 were lost.
Furthermore, DC power sources were also lost due to inundation at Units 1, 2
and 4, and such function was also lost at Unit 3 as a result of batteries being
depleted, resulting in a situation where all power being supplied to Units 1 thru
4 was lost.
As a result of the loss of AC power sources, existing cooling function forreactors and spent fuel pools was lost, and cooling was attempted using
temporary power sources and alternative coolant injection, but the situation
escalated into one where fuel in the core was damaged and radioactive material
released into the environment.
1.2.1 Earthquake Impact
In observation record of the earthquake, although the response spectrum
based on standard ground motion Ss was surpassed in some periodic bands,
values were equivalent overall. In addition, prior to the tsunami arrival,
equipment on safety systems was functioning. The earthquake response
analysis which Tokyo Electric Power Company conducted of the reactor
building as well as equipment and pipe systems critical for earthquake safety
on the basis of observation records, showed that all equipment important forsafety maintained its functionality.
1.2.2 Tsunami Impact
The height of the tsunami exceeded the water level designed for tsunami as
shown in the diagram below, and almost the entire area around importantbuildings was inundated. Although there was no significant damage to thestructural framework of key buildings, seawater flowed into the buildings
through openings and other penetrations, and a broad expanse of theunderground levels was also inundated through passageways, stairwells andother such routes.
In addition, many of the emergency DGs and electric power panels whichwere installed on the lower levels of the reactor and turbine buildings weredamaged by tsunami. With the exception of Unit 6, functionality of emergency
power sources was lost.
Moreover, all seawater pumps for cooling were damaged by tsunami, andany means of releasing heat from the core or other areas into the sea were lost.
2

Important safety equipment also was damaged by seawater flooding into thebuildings.

Seaside area

Key building area
Inundation height O.P.+ 11m～15m

Measures taken forEst. tsunami highwater level
O.P.+5.7 m

Reactor bldg.

height of 5.7m.

Turbine bldg.

Site heightO.P.+4m

Seawaterpumps

Reference

Site height O.P.+10m(Units 1~4) *lane O.P. 0 m

SeawallIntake＊Site height of Units 5~6: O.P.+13m

Tsunami Conditions

Scope of Inundation within Power Station (Purple area indicates seawater
inundation and yellow area inidcates a run-up point)32. Analysis of Accident Event Causes and Issues Derived
2.1 Course of Progression of Accident Events
The course along which events progressed is shown in Diagram 2.1-1, using theexample of Unit 1.
The reactor at Unit 1 was cooled by the isolation condenser (“IC”) in accordancewith the shutdown procedures and guided into a cold shutdown, but the tsunami
caused a loss of emergency AC and DC power source functionality, and it ispossible that most of the valves were in a closed state.
Consequently, it is inferred that the core was not cooled, became damaged andhydrogen was generated which leaked from the containment vessel to the reactor
building, leading to the hydrogen explosion.
The reactors in Units 2 and 3 were being cooled using the reactor core isolation
cooling system (“RCIC”) and other functions in accordance with shutdown
procedures.
After the tsunami struck, Unit 2 maintained core cooling using the RCIC evenafter the loss of emergency AC and DC power sources. However, after the RCIC
shut down, time was required until alternative coolant could be injected and theamount of coolant injected was small, so it is inferred that such conditions led tocore damage. Also, a rapid decrease in pressure in the pressure suppression
chamber and containment vessel was confirmed. Subsequently, it is inferred thathydrogen leaked from the reactor pressure vessel and containment vessel.
At Unit 3, core cooling was secured through operation of the RCIC and highpressure coolant injection system (“HPCI”) even after the loss of emergency AC
power sources, but depletion of the batteries led to a loss of the DC power source,
and the RCIC and HPCI lost functionality. Time was required until alternative
coolant could later be injected and the amount of coolant injected was small, so it
is inferred that the core was damaged and hydrogen generated, and that thehydrogen leaked into the containment vessel and reactor building, which led to thehydrogen explosion.
Unit 4 was in the refueling outage and all of the fuel was stored in the spent fuelpool. From the results of fuel observations with a camera and the results ofanalysis of nuclides in the pool water, it is inferred that there was no significant
damage to the fuel. It is possible that hydrogen which exploded in Unit 4 washydrogen which generated in Unit 3 and seeped into Unit 4 through the standby
gas treatment system exhaust to then build up inside the reactor building.
In order to clarify the factors in the progression of events above, the particulars
4divaricating convergence and expansion of events have been arranged andconsolidated into the event tree shown in Diagram 2.1-2.
The subsequent progression of events differs depending on (1) the circumstances
entailed in loss of power source functions due to the earthquake and tsunami, (2)condition of core cooling function, and (3) success or failure of measures to mitigate
events after core damage, as well as other factors.

5

Reactor control
March 11, 2011
14:46

Containment vessel control

Reactor scram signal transmitted due to earthquake
・ Reactor automatically shuts down
・ Turbines & generators shut down
・ Main steam isolation valve closes
・ External power source lost・Emergency diesel generators automatically start up

March 11, 2011
14:52

Isolation condenser automatically starts up

March 11, 2011
15:03

Isolation condenser shut down (manually)

March 11, 2011
15:07

Rate of reactor pressure vessel temperature decline adjusted so as not toSuppression pool cooling commenced

exceed 55℃/h

・Reactor pressure: controlled within a range of approx. 6~7MPa
・Primary loop recirculation system B system pump intake
temperature declines

March 11, 2011

Tsunami strikes

1st wave at 15:27

2nd wave at 15:35
March 11, 2011
15:37

Emergency diesel generator B tripped

Emergency diesel generator A tripped

March 11, 2011
15:37

Station black out (SBO)
・Loss of function due to submersion of DC power supply panels (inferred)・SBO shuts down suppression pool cooling

Loss of reactor cooling function

Opening and closing of main steam relief valve (safety valve function)

Reactor water level declines(TEPCO assessment)・TAF (about 18:00 on 3/11)

・

BAF (about 20:00 on 3/11: after tsunami, IC inoperable，About 24:00 on 3/11: one-half capacity IC operation temporarily after tsunami

Inferred to occur
between TAF~BAF

Core damaged due to exposure・Large volume of hydrogen generated as a result of core damage

March 12, 2011
05:46

March 12, 2011
14:30

Fire protection vehicle starts injecting coolant

PCV venting(D/W pressure decrease confirmed)

March 12, 2011
15:36

Hydrogen explosion

Diagram 2.1-1 Course of Accident Event Progression at Fukushima
Daiichi Unit 1 Nuclear Power Station after Earthquake
6

Scram signal

Reactorshutdown

Earthquake Reactor
increases in scramstrength

AC power source
External
powersourceDC power
sourceEmergency DC power
DG
supplyRecovery of
Ensuring long-term cold shutdown
AC power
sourceReactor
IC andExternal
RPV
RHR
RHR
turbine-driven power source, depressurizatio coolant
restoratio
injection
coolant
emergencynn(including
injection
DG, power(including
alternatives)
systems not source
alternatives)
requiring AC interchange
power source(RCIC HPCI)Core coolingCore state
PCVventing(prior to
core
damage)Cold
shutdown,core damage,PCVdamage, etc.

Cooling
ofdamagedcore
Reactor Reactor
depressur coolant-ization
injection

Containment vessel controls
PCV
coolant
injection

PCVventing

Hydrogencontrols(nitrogensubstitution,
etc.)

Reactorbuilding
controls
SGTS,
ventilation, etc.

Final state
Cold shutdown, core damage, PCV damage, etc.

Cold shutdown
Cold shutdown(A)

Cold shutdown (long-term cooling required)
PCV damage (over pressurization)

Same as (A)Same as (A)RCIC/HPCIPCV sound or PCV leakage (over heating)
Cold shutdown (long-term cooling required)(Success)

Radioactivity release (PCV leakage (over heating)and hydrogen burning in building)Batteries collected toprovide tempoary facility

PCV damage (over pressurization)(Ongoing)

１F-3[Earthquake」

Non-recoverable(Not yet implemented)(Ongoing)[Tsunami]

PCV damage (hydrogen explosion)Accident
sequence
PCV damage (over pressurization) presumed to haveCore damage(Actual flow of coolant into reactor is low)

PCV damage (MCCI, etc.)
Core damage

Accident sequence
presumed to haveoccurred at 1F-2

PCV damage (DCH, etc.)
PCV sound or PCV leakage (over heating)Same as (A)PCV leakage (over heating) and hydrogen burning inbuilding

Cold shutdown (long-term cooling required)(Ongoing)
PCV damage (over pressurization)

Batteries collected toprovide tempoary facility

1F-2

RCIC (started up prior
to loss of DC powersource)Non-recoverable

１F-1,2

PCV damage (hydrogen explosion)(Not yet implemented)(Ongoing)

PCV damage (over pressurization)

Core damage(Actual flow of coolant into reactor is low)

PCV damage (MCCI, etc.)[Tsunami]Core damage
PCV damage (DCH, etc.)

Same as (A)Cold shutdown (long-term cooling required)
PCV sound or PCV leakage (over heating)

1F-1
PCV damage (over pressurization)

1F-1IC(operated until tsunami arrived; after tsunami arrival, duration of operation is unclear)

Radioactivity release (PCV leakage (over heating)and hydrogen burning in building)(RPV low pressure state)
Non-recoverable

Core damage(Injection of coolant delayed or low actual flow of coolant into reactor)(RPV low pressure state)Core damage(Not yet implemented)

PCV damage (hydrogen explosion)Accident
sequence
presumed to have
PCV damage (over pressurization)
PCV damage (MCCI, etc.)

Diagram 2.1-1 Event Tree Showing Progression of Accident at Fukushima Nuclear Power Station Units 1~ 3

7

2.2

Analysis of Accident Event Causes and Issues Derived

2.2.1 Analysis of Causes from the Progression of Accident Events
From an analysis of the accident using the course of accident event
progression (event tree) 2.1, three major factors, which formed significant
turning points in the progression of the accident and escalated the accident,
have been deduced ((1) inability to supply AC power to the station, (2) inability
to remove heat from the reactor, and (3) leakage of hydrogen into the building
and the hydrogen explosion), and issues pertaining to these factors have been
derived.
The results of a factor analysis regarding the “inability to supply any AC
power source” are shown in Diagram 2.2.1.
(1) Inability to Supply AC Power to the Station
The inability to supply power attributable to the external power source, theinability to supply power using emergency diesel generators, the inability tointerchange power sources, and the inability to restore power at an early stage
have been cited as factors from an equipment and operational aspect as to theinability to supply AC power to the station.
(2) Inability to Remove Heat from Reactor
The inability to depressurize using the main steam safety relief valve, theinability to remove heat using the core spray and residual heat removal system,
the inability to remove heat using accident management equipment(alternative coolant injection equipment), and the inability to vent thecontainment vessel have been cited as factors from an equipment andoperational aspect that failed to remove heat from the reactor.
(3) Leakage of Hydrogen into Building and Hydrogen Explosion
The factor which led to hydrogen leaking into the building has been given tobe hydrogen leaking from the containment vessel and circulating into andaround the common exhaust equipment, and one of the causes of the hydrogen
explosion has been cited as the inability to detect hydrogen leaking into thereactor building.
2.2.2 Consolidation of Issues Confirmed as Functional Aspects
The basic safety functions of a nuclear power station are “stopping,”“cooling,” and “confining,” and the factors which caused these respectivefunctions to malfunction were analyzed. In addition, the basic requirements
necessary for implementing “stopping,” “cooling,” and “confining” operations(main control room heating, ventilating and air conditioning system,
communication equipment and other equipment environments) were also
analyzed.
Moreover, the common factors which impeded the achievement of eachfunction were the “inability to supply electric power” and “inoperability of8

auxiliary cooling,” and these factors were analyzed and issues derived.
The results of an analysis of accident factors according to function for Unit 1
and the issues consolidated are shown in Table 2.2.2.

9Factor Analysis
Implementation Phase

Issues to be Examined【Units 1~4】
Activation of protective devices due tocontact between pylons and electric wires【Units 1 & 2】
Inability to supply power
through external powersourceReliability of external power sources (substations,electric lines, switchyards) against earthquakes
and tsunami

Damage to line switches and powerreceiving circuit breakers at switchyards【Units 3 & 4】
Assumption that tsunami reach andinundate switchyards

Inundation of switchyards【Units 1~4 (except air-cooled diesel generators at Units 2 & 4)】
Inability to supply
AC power source（station black out）

Inability to supply powerusing emergency diesel
generators

Inundation of emergency diesel
generators by tsunami

Circuits for external power sources
concentrated at one location at substation

Assumption that tsunami reach and inundatebuilding where diesel generators are installed

Inundation of area where emergency diesel
power generation equipment is installed
Loss of auxiliary equipment cooling
function (seawater pumps)

Interior of buildings inundated throughbuilding openings and penetrations【Air-cooled diesel generators at Units 2 & 4】
Assumption that tsunami reach and inundatebuilding where emergency high-voltage electric
panels are installed

Inundation of electric panels by
tsunamiInundation of area where emergency highvoltage electric panels are installed
Interior of buildings inundated throughbuilding openings and penetrations

Early recovery of powersource not possible

Inundation of adjacent
plants' power supply panels

Not able to maintain AC power
supply from adjacent plants

Inability to interchange powersources (severe accident measure)

Difficulty in deploying
power generating car(emergency disposition)

Difficulty in arriving due to roaddamage and traffic congestion

Power supply panels
not reusable

Inundation of power
supply panels

Access obstructed by flotsam and destruction oncompound roads

Slow progress in laying cable (weight, distance,
work environment)

Securing heavy machineryto ensure accessibility of roads

Multiple routes within compound

Securing equipment and materials tomaintain the work environment insidebuildings and at night, and development ofprocedures and training

Outdoor work while tsunami warnings were being issued as well as nightwork and work which progressed slowing due to the environment

Loss of means ofcommunication

Loss of power source forcommunication
Loss of power source foralternative communicationequipment

Securing power source for communications
and communication equipment(communication means and quantity of

Quantity of alternative
communication devices
Inundation ofcommunication

Inundation of communicationequipment

Maintenace of workforce of
personnel required forradiation control

Ensuring personnel required forradiation control during an

Damage to temporary cable due toexplosion at adjacent plant(After core damage)

Difficulty in working
due to radioactivity
and high doses

Slow progress in securing the number of personnel
required for personal dosemeters, protective
equipment and management system equipment

Difficulty in securing required personnel
following worker exposure
Difficulty in accessing interior ofbuildings owing to high dose rates
Slow progress in work removing debris
owing to radiation contamination

Diagram 2.2.1 Analysis of Factors Leading to Inability to Supply AC Power

10Inundation of radiation control
system equipment and materials
Interchangeablity of equipment andmaterials among electric powerSafety Functions
Emergency shutdown
function for reactor
Stopping

Alternative reactivity
control
Subcriticality maintenanceStandby liquid control system
function

Power supply function

Supportsystems

Related Equipment, etc.
Safety protection systems,
control rods and control rod
drive systems
Recirculation pump trip (AM)
Alternative rod insertion

Alternative power supply
function

Table 2.2.2. Analysis of the 1F U1 accident factors and the issues consolidated
Loss of Function or State of Deteriorated Function

Pressure relief function forreactor coolant pressure
boundary

High-pressure core cooling
function(Operated normally during earthquake)

－

－

－
－

(Not necessary due to scram success)(Not necessary due to scram success)

－
－

－
－

－

(Not necessary due to scram success)－－

External power source×

Loss of external power source due to earthquake

Emergency diesel generator
6.9kV high-voltage power
supply×

Loss of function due to inundation

×

Loss of function due to inundation

480V low-voltage power
supply×

Loss of function due to loss of 6.9kV high-voltage
power supply

125V DC power supply
Power supply interchange

××Power generating car(emergency deployment)×

Loss of function due to inundation

Containment cooling service
water system

×

Auxiliary cooling seawater
system×

Reactor building coolingwater system

×

Main steam safety relief
valve(safety valve function)
High pressure coolant－

Isolation condenser(main unit)△(Operated
temporarily)

Isolationcondenser(support
system)Make upwater purified
systemFire
protection
systemStandby liquid control system(AM)
Alternative coolant injection
function(high pressure)

Reactor depressurization
functionIssues to be addressed○

Loss of function due to inundation
Power supply from adjacent plant not able to be
Difficulty in arriving at site

Tie-in work progressed slowly

No capacity to supply electric power

Auxiliary cooling function

Factor Analysis

××△(Operated
temporarily)
×Conrol rod drive
hydraulic system (AM)×

Main steam safety relief
valve(relief valvefunction/automatic×

Loss of function due to inundation
Loss of function due to loss of AC power source
Loss of function due to inundation
Loss of function due to loss of AC power source
Loss of function due to loss of DC power source
Loss of function due to loss of auxiliary coolingseawater system
Loss of function due to loss of AC power source
Loss of function due to loss of DC power source(Controlled by isolation condenser)

Earthquake damaged Okuma line 1L receiving-line circuit① Reliability of switchyard circuit breakers
breaker at switchyard for Units 1 & 2② Inundation of emergency diesel generators
Inundation of emergency diesel generator by tsunami
Inundation of 6.9kV high-voltage power supply
Inundation of 6.9kV high-voltage power supply panels by③
panelstsunamiInundation of 6.9kV high-voltage power supply panels bySame as ③
tsunamiInundation of 480V low-voltage power supply
Inundation of 480V low-voltage power supply panels by④
panelstsunami⑤ Inundation of 125V DC power supply panels
Inundation of 125V DC power supply panels by tsunami
Tsunami caused loss of station power supply at adjacent plant ⑥ Inundation of power supply panels at adjacent⑦ Furnishing alternative electricity from outside
Difficulty in arriving due to road damage and traffic
Connection not possible due to inundation of Unit 1 AC powerSame as ③ and ④
supply panels
Difficulty with work laying cable （weight, distance and work
⑧ Laying of cable for alternative power supply
environment）
Intermittant aftershocks and continuously issued tsunami－
warnings
Communication
means
during loss of AC power
Loss of communication means due to loss of AC power supply⑨
supplyAutomatic shutdown of power generating car and damage to－cable due to explosion at Unit 1 reactor building
Inundation of containment cooling service water
Inundation of containment cooling service water system⑩
system pumps by tsunami
pumps by tsunami
Same as ③
Inundation of 6.9kV high-voltage power supply panels by
Inundation of auxiliary cooling seawater pumps by tsunami ⑪ Inundation of auxiliary cooling seawater pumps
Same as ③
Inundation of 6.9kV high-voltage power supply panels bySame as ⑤
Inundation of 125V DC power supply panels by tsunami
Inundation of auxiliary cooling seawater pumps by tsunami
Same as ⑪－

Loss of function due to loss of DC power source
Inundation of 125V DC power supply panels by tsunami
Functionality deteriorated due to loss of DC power Inundation of 125V DC power supply panels by tsunami
source (remote operation not possible)
Isolated due to transmission of isolation signal
Operation of pipe rupture detection circuit due to loss of DC(inner isolation valve halfway open due to loss of AC
power supply
power supply)
Inundation of 480V low voltage power supply panels by
Loss of function due to loss of AC power source
tsunamiElectric motor pumps unusable due to loss ofEarthquake damaged Okuma line 1L receiving-line circuit
external power source
breaker
Mechanical failure (presumed)
Shutdown of diesel driven fire protection pump
Loss of function due to loss of AC power source
Slow progress made in operation to restore power
supplyLoss of function due to loss of AC power source
Loss of function due to loss of DC power source
Loss of function due to loss of auxiliary coolingseawater system

Inundation of 480V low voltage power supply panels by【Non-recoverable】Automatic shutdown of power generating
car and damage to cable due to explosion at Unit 1 reactor
Inundation of 480V low voltage power supply panels by
Inundation of 125V DC power supply panels by tsunami
Inundation of auxiliary cooling seawater pumps by tsunami

Loss of function due to loss of DC power source

Inundation of 125V DC power supply panels by tsunami

11

Same as ③
Same as ⑤

Inundation of 6.9kV high-voltage power supply panels by
Inundation of 125V DC power supply panels by tsunami－
Same as ⑤
Same as ⑤⑫

Suitablity of design of isolation signal to handle
loss of DC power supply
Same as ④
Same as ①－
Same as ④－
Same as ④
Same as ⑤
Same as ⑪
Same as ⑤

Safety Functions
Cooling

Low-pressure core cooling
function

Related Equipment, etc.

Core spray system

Make up water condensate
system (AM)Containment vessel coolingsystem (AM)Fire protection system (AM)
Alternative coolant injection
function(low pressure)

Loss of Function or State of Deteriorated Function
No capacity to inject coolant due to inablity todepressurize reactor
Loss of function due to loss of AC power source×
Loss of function due to loss of DC power source
Loss of function due to loss of containment cooling
service water system
No capacity to inject coolant due to inablity to×
Loss of function due to loss of AC power source
No capacity to inject coolant due to inablity todepressurize reactor
Loss of function due to loss of AC power source×
Loss of function due to loss of DC power source
Loss of function due to loss of containment cooling
service water system
No capacity to inject coolant due to inablity to×depressurize reactor
Shutdown of diesel driven fire protection pumpNo capacity to inject coolant due to inablity todepressurize reactor
Difficulty in securing coolant flow required

Fire protection vehicle(support during Chuestsu-oki
earthquake)△(Insufficient
flow, and work
progressed
Work progressed slowly
slowly)12

Factor Analysis
Inundation of 125V DC power supply panels by tsunami

Issues to be addressed
Same as ⑤

Inundation of 6.9kV high-voltage power supply panels by
Inundation of 125V DC power supply panels by tsunami
Inundation of containment vessel cooling seawater pumps by
tsunamiInundation of 125V DC power supply panels by tsunami
Inundation of 480V low voltage power supply panels by
Inundation of 125V DC power supply panels by tsunami

Same as ③
Same as ⑤

Inundation of 6.9kV high-voltage power supply panels by
Inundation of 125V DC power supply panels by tsunami
Inundation of containment vessel cooling seawater pumps by
tsunamiInundation of 125V DC power supply panels by tsunami

Same as ③
Same as ⑤

Mechanical failure (presumed)
Inundation of 125V DC power supply panels by tsunami

Same as ⑩
Same as ⑤
Same as ④
Same as ⑤

Same as ⑩
Same as ⑤－
Same as ⑤

Difficulty in ensuring capacity of fire control cisterns used as⑬ Depletion of fresh water supply
water sources
Obstructions resulting from earthquake andObstructions resulting from earthquake and tsunami⑭
tsunamiWorsened working environment due to rise inHigh dose debris and evacuation of site due to explosion at⑮
dosage and hydrogen explosion
Unit 1 reactor building
Seawater injection hose damaged and relaid due to explosion
Same as ⑮
at Unit 1 reactor building
Intermittant aftershocks and continuously issued tsunami－
warnings
Same as ⑨
Loss of communication means due to loss of AC power supply

2.3

Summary of Analysis of Causes
The number one cause as to why the accident at Fukushima Daiichi
escalated was that the loss of power sources for driving safety systemequipment brought about a secondary loss of a number of safety systemequipment functions.
With regard to power sources, if electric power generation shuts down, poweris first supplied from an external power source. The system is designed so that,
if the external power source loses functionality, an emergency DG supplies
power to safety system equipment. This time, functionality of the external
power source was lost due to the earthquake, and then tsunami submerged
emergency DGs and inundated electric power panels. Because access to powersource connections was lost due to inundation of the panels, rapid restoration
was not possible, which led to the accident escalating.
The essence of the problem is that preparations assuming a contingency
brought about by tsunami strike were ineffectual. What was necessary to
prevent an accident from escalating was that provisions be made to implement
profound measures for ensuring safety where the preparations undertaken
assuming an escalating accident scenario would be expressed as: “If a tsunami
floods the site, electric power panels and various important safety equipments
will be submerged. In response, necessary countermeasures will be identified
and the required equipment provided for in advance. Furthermore, all needed
preparations will be made beforehand on the chance that such equipment does
not function.”
The fact that external power sources, emergency DGs, electric power supply
panels and other equipment related to power supply lost functionality, and that
preparations were ineffectual for a situation in which function of such facilities
was lost due to a tsunami are considered to have been significant factors
leading to escalation of the accident.
The issues deduced in 2.2 have been arranged and consolidated into thefollowing five categories.
・ Preparations for earthquakes and tsunami (natural hazards)
・ Preparation of power sources
・ Measures to counter heat sink loss
・ Hydrogen countermeasures
・ Preparations for emergency situations
Category
Deduced issues

13CategoryPreparations forearthquakes andtsunamiDeduced issues
・ Loss of function of external power sources due
to earthquake or tsunami
・ Loss of function of switching stations due totsunami・ Loss of function of seawater facilities due totsunami・ Inundation of seawater pump rooms
・ Loss of function of emergency DG due totsunami・ Infiltration of seawater through building
openings
Preparation of power ・ Incoming lines for external power sources atsources
one location
・ Submersion of power supply equipment due totsunami・ Depletion of DC power supplies
・ Securing storage battery capacity
・ Furnishing of power source externally (power
generating cars, etc.)
・ Responding to secure supply of AC power・ Laying of cable for alternative power sources
Measures to counter ・ Inundation of seawater pumps
heat sink loss
・ Not all equipment is damaged at once in theface of a tsunami
・ In preparation for damage by tsunami, meansare provided that are not impacted by tsunami
・ Transmitting a signal to isolate the isolation
condenser in response to a loss of DC powersource・ Securing materials and equipment to respondto a loss of driving source for valves
・ Development of procedures and training
・ Depletion of fresh water supplies
・ Maintaining function of fire protection pipes
against the occurrence of earthquake ortsunami・ Furnishing fuel for fire protection vehicles
・ Enhancing driving source for pressure vent line
valves・ Inoperability of ruptured disks at low pressure
14Category

Deduced issues
・ Securing compressed air for maintaining
containment vessel vent valves in an open state

Hydrogencountermeasures

・ Enhancing seals on penetrations andcontainment vessel gaskets
・ Circulation of hydrogen gas into and aroundbuildings from pressure vent lines
・ Circulation of hydrogen gas into and between
units having a common exhaust stack・ Retention of hydrogen inside reactor building
・ Hydrogen detection
Preparations for・ Securing personnel for radiation control
emergency situations ・ Tsunami protection measures for radiation
control equipment
・ Obstructions resulting from earthquakes
and/or tsunami
・ Communication means when AC power sources
are lost
・ Materials and equipment for supporting night
and outdoor work
・ Materials and equipment for securing
accessibility of roads
・ Securing multiple routes within the station
grounds
・ Interchange system between electric power
operators
・ Worsening environment due to hydrogen
explosions and rising doses
For some of the aforementioned issues, such as enhancing seals on
penetrations and containment vessel gaskets, securing storage battery capacity,and transmitting a signal to isolate the isolation condenser in response to a lossof DC power source, it has been determined, having taken into account thefeasibility and effectiveness of measures, that these issues be covered bymeasures to address other issues and not measures directly responding to such
issues.

3. Proposed Measures
After examining the extracted issues intently, the facts were arranged once
again in line with the course along which the same accident progressed and15

countermeasures based on the issues were deduced.
All measures deduced have been arranged in a diagram and are shown in
Diagram 2.4-1.
Also, as shown in Table 2.4-1, safety functions, that is measures directly
related to power supplies, heat sinks (coolant injection and cooling), measures
to prevent a hydrogen explosion, maintaining the water level in the spent fuelpool (SFP) as well as ensuring other functions have been arranged inaccordance with the deep protection approach, and high priority measures havebeen selected from among all the countermeasures. From the standpoint ofpreventing a recurrence of the accident (giving depth to preparations assuming
a scenario in which an accident escalates), even more measures have been put
in place in the fourth tier.
The measures have been selected in combination, giving consideration to thesafety of the whole plant, and various combinations of these measures areconceivable depending on the conditions at respective power stations andstrategies for ensuring safety.
It is necessary to have effective measures sufficient to prevent and mitigate
severe accidents by putting in place measures in a multitiered manner so thatsafety is able to be ensured, even if the measures readied, which are a
combination of the various steps detailed here plus those taken into accountsince the design and construction stages, lose functionality in succession.
Keeping the aforementioned point in mind, the following measures are thosewhich are particularly recommend for adoption by all companies.
.
－Power sources
・ Installation of seawalls or tide embankments for important safety
equipment
・ Preventing against inundation of sections where important safety
equipment is installed
・ Preventing against inundation including improving seals around
penetrations and air inlets and other openings in proportion to the heightof inundation
・ Development of watertight locations for deployment of power receiving
transformers and switchyards or measures to prevent against inundationof equipment
・ Development of watertight locations for deployment of DC power supply
equipment or measures to prevent against inundation of equipment
16

・ Deployment of power generating cars or large capacity power supplies (gasturbines and diesel generators), and development of procedures for
emergencies
・ Development of routes for recharging DC power supplies by means of
backup power sources
・ Improving reliability through measures to prevent against inundation ofequipment for interchanging power sources between units (severe accident
measure)－Heat sinks (coolant injection and cooling)・ Bulkheads around seawater pumps and other inundation protection
measures・ Deployment of spare parts for seawater pump motors
・ Deployment of mobile seawater pumps
・ Preventing against inundation of sections where the emergency corecooling systems and other safety system equipment are installed
・ Measures to prevent against inundation of equipment for severe accident
measures・ Backup power supplies, standby air cylinders and other preparations for
SRV drives (BWR)・ Ensuring core cooling via SG by means of the main steam relief valve(PWR)
・ Deployment of backup power sources and driving sources for venting
operations (BWR)・ Improving reliability of existing coolant injection systems throughdeployment of backup power generating cars or large capacity power
supplies・ Movable pumps, hoses, etc. not dependant on existing equipment
・ Securing water supplies
・ Releasing heat into the atmosphere through containment vessel venting(BWR)－Measures to protect against hydrogen explosion
・ Releasing and reducing retained hydrogen・ Measures to prevent against inflow from pressure strengthened venting
lines
・ Preventing hydrogen from circulating around between units having acommon exhaust stack
Since the accident at the Fukushima Daiichi Nuclear Power Station, all sortsof measures have been adopted by companies, and these which are shown in
17

Table 2.4-1 along with the approach at a typical plant. At the present stage, it
is able to be confirmed that necessary measures have been adopted at eachstage from the standpoint of deep protection, and a substantial improvement insafety has already been achieved.

18―Preparations Against Earthquakes and Tsunami 1、6、7、9 , Power Supply Arrangements 3、5 , Responding to Loss of Heat Sink (BWR) 2、4、8、10、11―2【Injection of coolant into reactor】①Preparation of backup air cylinders, etc. and backup power supplies for driving SRV【Loss of seawater cooling】

1【Protection against building inundation】②Release of heat into the atmosphere by venting containment vessel①Measures to prevent inundation including seal performance at penetrations and openings such3

【Station black out and loss of DC power supply】【Containment vessel venting】

as air supply openings and exterior doors to buildings in proportion to the height of any①Deployment of power generating cars or large capacity power supplies (gas turbine or diesel③Deployment of temporary power supplies for monitoring instrumentation for venting operations, and deployment

inundation

generators), and development of procedures for emergencies

of venting operation backup power supplies and driving sources②Measures to prevent inundation of sections where important safety equipment is installed②Development of watertight sections for deployment of power receiving transformers and④Optimization of conditions for venting operation and timing of commencement③Deployment of portable discharge pumps

switchyards as well as measures to guard against inundation of equipment⑤Enhancement of alternative containment vessel spraying④Taking into account the wave force on building exterior doors directly impacted by tsunami③Watertight sections for deployment of DC power supply equipment and measures to prevent
against inundation of equipment
Reactor building【Injection of coolant into reactor】

11⑫Measures to prevent inundation of sections where equipment for countering severe accidents orExternal power source
Ordinary (275kV) power
supplyMaincontrolroom

Spent fuel pool

External power source
Reserve (154kV) power
supplyPrimary containment
vessel

④Development of routes for recharging DC power supplies by means of backup power sources⑤Improvement of reliability by means of measures to address inundation of power supply
interchange equipment between units

Electric panel

emergency core cooling systems are installed
Steam

Spent fuel coolingsystem (ordinary use)

Generator【Injection of coolant into reactor】

Reactorpressure
vessel10⑩Coolant injection using portable motorized pumps and hoses not dependent on existing power

Fuel
Reactorisolation
coolingpump

Low-pressure
injectionsystempump

sources

Feedwater
pump
Control rods Recirculation pump

⑪Securing sources of water (including seawater as a last resort)

Condenser
Condenser

Water

OutletRecirculation pump4

【Injection of coolant into reactor】

Station internal
power supply(Power panel for
driving safetysystems)⑥ Improvement of reliability of currently installed coolant injection systems through

Coolant
Coolant(seawater)(seawater)

deployment of backup power generating cars or large capacity power supplies【Loss of seawater cooling】

Emergency diesel
generator(backup power supply)

Filteredwater storagetank

Dieselpump

fire

Condensate
storage tankprotection

Outlet(seawater)

9【Assumption that earthquake/tsunami will occur】⑦Power supplies for providing backup for minimum required function of seawater pumps

Suppression poolFire protection vehicle

Seawater pump for coolingemergency diesel generator

5【Station black out and loss of DC power supply】⑥ Watertight sections for deployment of emergency D/G, power supply⑩Formulation of standard seismic ground motion Ss takes into account interlocking
Seawallof multiple seismic centers

equipment and emergency D/G cooling equipment and measures to guard

Sea water

against inundation of such equipment⑪Revision of conditions configured for cases when tsunami height is assessed

8【Loss of seawater cooling】⑧ Deployment of seawater pump motor backup and development ofinterchange procedures to be employed after loss of function【Measures to protect against inundation of seawater systems】

7⑦Floodwalls and other steps to prevent inundation around seawater pumps⑧Deployment of spare parts for motors⑨Deployment of materials for washing and drying seawater pump motors⑨Deployment of mobile seawater pumps and installation of pumps with waterproof
specifications

Diagram 2.4-1 Typical Measures 1/3
19【Preventing tsunami from flooding the site】

6⑤ Installation of seawalls or tide embankments at locations necessary for protection ofimportant safety equipment⑥Protection against tsunami turning tanks and other objects into flotsam and hindering any
emergency response. Deployment of heavy machinery for removing flotsam.

―Hydrogen Countermeasures 14, Preparations for Emergencies 12、15、16、17、18、20、21, Ensuring Soundness of Spent Fuel 13、19 ―【Radiation control/work management】

12①Diversification of power supplies for radioactivity measuring instruments (battery-type), securing

13【Ensuring soundness of spent fuel】

long-term power supplies, provisions placed on high ground, and company- external interchanging①Enhancing status monitoring and supply of electricity from emergency power sources to②Securing mobile WBC at other facilities

water temperature meters and water level meters for spent fuel pool

③Securing protective equipment and gear

②Securing alternative cooling function

14【Hydrogen countermeasures】

15【MCR air conditioning and shielding】①Measures for releasing and reducing retained hydrogen⑥ Securing power supply and developing procedures for②Installation of hydrogen gas detectors

operating MCR ventilation systems during emergencies③ Measures for preventing hydrogen from circulating into branch pipes from

⑦MCR radiation protection measures④Radiation protection of aseismic important buildings, etc., sharing of site dose rate information,
containment vessel venting exhaust pipes
promotion of understanding about emergency exposure treatment④Preventing hydrogen from circulating around between units sharing a stack⑧Securing connection cables and temporary storage battery

External⑤Development of system supporting personnel required for radiation control⑤Training that anticipates a hydrogen explosion

cells for instrumentation

Important aseismic

21【Communication】【Measuring during accidents】⑨Review of instrument power supplies

buildings

3○0 Backup power supplies for independent communication equipment not impacted by earthquake or⑩Selection of parameters required for new instrumentation

tsunami3○2 Deployment of multiple communication devices and clarification of operating methods

and review of instrumentation systems

Reactor building

Emergency response

3○1 Communication equipment able to withstand natural disasters

Maincontrolroom

Spent fuel poolfacility3○3 Deployment of equipment for communication with site workers【Measuring during accidents】

Primary containment
vessel【Environmental monitoring】⑪Measures for facilities and instrument measuring range taking account of

3○4 Review the equipment required for interchanging during emergencies and the number of suchsevere accidents
Steam

Spent fuel coolingsystem (ordinary use)equipment required⑫Development of instruments for severe accidents (environmental resistance,
Generator

3○5Review required support personnel and develop routes for deployment

Reactorpressure
vessel3○6 Securing alternative power supplies for permanent monitoring posts, etc. and reinforcing

Low-pressure
injectionsystempump

3○8 Clarification of alternative methods for permanent monitoring posts, etc.

diversity of measuring methods)

Fuel
Reactorisolation
coolingpump

transmission lines
3○7 Enhancing durability of permanent monitoring posts, etc.

Condenser

Water【Communication】

Feedwater
pump
Control rods Recirculation pump

OutletRecirculation pump

Coolant(seawater)

17⑬Deployment of equipment for communications between workers and main
control room, etc. and proficiency in such operation

3○9 System for cooperation and support for environmental monitoring implemented by local

Emergency diesel
generator(backup power supply)

communities
Suppression pool

4○0 Clarification of content of information to be disclosed to general public, etc. and methods

Fire protection vehicle【Preparations for dealing with disasters, and cooperative framework during emergencies】

Dieselfire
protection pump4
○1 Diversification of means for transporting materials, equipment and personnel when access routes

16

Filteredwater storagetank

Condensate
storage tank
Outlet(seawater)

are blocked【Training】

4○2 Securing necessary materials and equipment

Seawater pump for coolingemergency diesel generator

4○3 Redevelopment of inter-operator (power company) interchange system

Seawall【Emergency response facility】

Sea water

20

2○4 Securing means of contacting the site, main control room and means of monitoring plant parameters
2○5 Radiation protection measures⑭Consolidation of required knowledge and skills as well as implementation of training⑮On-site activity training for severe accident measures⑯Implementation of training simulating night work, having no means of communication,
presence of heavy riggings, and obstacles hindering access to site

2
○2 Structures and siting conditions able to withstand earthquakes, tsunami
2○3 Securing emergency power supply independent of plant

18【Ensuring soundness of spent fuel】

19⑰ Highly effective training including uninformed training, real-time and other suchcomponents③ Securing water supplies, injection routes and injection of coolant by fire
⑱Understanding of impact that activities, which are implemented for responding during
protection vehicles, etc.
an accident, have on how events progress

2○6 Ensuring habitability
2○7 Securing emergency materials and equipment⑲Utilization of simulators

2○8 Scrutinizing and securing documentation necessary for emergency countermeasures【Preparations for dealing with disasters, and cooperative framework during emergencies】【Organization/command and control】⑳Deployment of heavy machinery to remove debris

2○9 Clarification of timing of decisions and preparations for rapid operations through early external coordination

2○1 Cooperation in developing and introducing remotely operable equipment for nuclear

Diagram 2.4-1 Typical Measures 2/320

disaster prevention and development of cooperative frameworks

―

Responding to Loss of Heat Sink (PWR) 1、2、3 ―【Injection of coolant into reactor】

2②Securing alternative means of injecting coolant into steam generator by【Injection of coolant into reactor】

1

using portable electric-power pumps, hoses, etc.

①Securing coolant injection means through deployment of backup power③Reactor cooling through steam generator by using the main steam relief

generating cars or large capacity power supplies

External
powersourceRefueling
water
tank

Conde
nsate
tank

To fire service
water system
Motor-driven
fire
servicewater pump

Diesel fireservice waterpump

valve (including reserve air cylinders, etc.)
Primary
containment vesselContainment vessel spray

Safety injection tankControl rods

Main steam

To steam turbine

Drive steam
Main condensate
Reactorauxiliarybuilding

Containment
spray pump

Motor-driven feedwater pump

Dieselgenerators
panel

High pressure
injection pump

Reactor core

Primary
coolant pumpResidual heatremoval pump

Spent fuelpool

Diesel generator
Containment
vessel sump

Turbine auxiliary feedwater pump

Reactor auxiliary coolant injection pump
Refuelingwater tank

Seawater pump
To sea3【Injection of coolant into reactor】④Securing sources of water (including seawater as a last resort)

Diagram 2.4-1 Typical Measures 3/3
21

CondensatetankTable 2.4-1 Checklist Arranged from the Standpoint of Deep Protection (Status of Commitment at Typical Plant)

Multiple Protection Levels

First Tier
（Preventing abnormalities from occurring）

Objective Preventing abnormal operation and accidents

Essential means

Power sources

Core

Heat sinks(Coolant
injection andcooling)

Conservative design and construction
High quality in operation

Preventing accidents from occurring/
Mitigating the effects of accidents
Second Tier
Third Tier
Fourth Tier
(Preventing abnormalities from spreading and (Preventing the extraordinary release of(Preventing severe accidents from occurring)developing into an accident)
radioactive material)
Controlling abnormal operation and detecting
Controlling accidents to within design standards Preventing an accident from progressing
failures
Control, confining and protection systems

・Based on past records, the aseismatic class is ・Emergency DG automatic startup from
configured using a sufficiently conservative
detection of low voltage signal on high-voltage
design premise and the site height is designed power source bus
by estimating twice the margin necessary
・Along with the automatic startup of・Inundation through building air supply,
emergency DG, related equipment starts up
exhaust and other openings due to tsunami not sequentiallytaken into account・When a plant's power generation shuts down,
electric power is supplied by an external power
Prior to the source
disaster ・Redundant emergency DGs are set up, and if
the external power source fails, backup isprovided by the emergency DG
・Restoration is assumed in a short period even
if the power shuts down
・The diversity and multiplicity of powersources is ensured by deploying a variety of
power sources across multiple systems using
external power systems, emergency DG, and DCb tt i○Seawalls and bulwarks are installed forequipment important for safety●Preventing against inundation of sections
where important safety equipment is installed○Preventing against inundation including
improving seals around penetrations and air
inlets and other openings in proportion to the
Proposed height of inundation
measures ●Development of watertight locations for
deployment of power receiving transformers andswitchyards and measures to prevent against
inundation of equipment●Development of watertight locations for
installation of DC power supply equipment andmeasures to prevent against inundation ofequipment
・In response to transient events, capability to・Based on past records, the aseismatic classand site height are designed using a sufficiently respond through operations even if ECCS andother safety system equipment cannot be hoped
conservative design premisefor・Heat removal systems are installed which
・Due to inundation of seawater pumps andhave redundancy
・Multiplicity or diversity and independence are power supply panels, the simultaneous loss offunction of all units is not assumed
ensured for heat removal systems
Prior to the・When the seawater pump is tripped, thedisaster
backup automatically starts up
・Detectors having to do with the safety of thereactor are designed so that they can be tested
even during operation of the reactor andmaintenance of soundness and multiplicity canbe confirmed○Bulkheads around seawater pumps or othermeasures to prevent inundation●Preventing against inundation of sections
where the emergency core cooling systems andother safety system equipment are installed
Proposedmeasures

Prior to thedisasterHydrogen

ECCS and accident procedures
・On receipt of the automatic startup signal for
the ECCS pump, the emergency DG
automatically starts up despite supply from anexternal power source
・Power supply is secured by DC batteries andemergency DG which have redundancy

Supplementary means and severe accident
measures

Fourth Tier
(Mitigating the impact of a severe accident)
Mitigating the impact of SA
Supplementary means and severe accident
measures including protecting the PCV・Power source interchange developed between
・The restoration of AC power source isanticipated at an early stage (8 hours) and long- units as a severe accident measures. However,loss of power sources simultaneously at multiple
term power outage is not assumed
units not assumed.
・After the failure of AC power sources, corecooling is done using DC-driven euipment andthe recovery of AC power source is expected
・Development of recovery procedures●Deployment of power generating cars or large ○ Improving reliability through measures tocapacity power supplies (gas turbines and diesel prevent against inundation of equipment for
generators), and development of procedures for interchanging power sources between units(severe accident measure)emergencies●Development of routes for recharging DC
power supplies by means of backup powersources●Deployment of portable drainage pumps, etc.

・Development of connections to fire protection
・ECCS and other safety systems are installed ・Severe accident measures (accident
vehicles or other fire fighteing system pipes
management) were developed. Existingalong multiple systems
・Even when external power sources cannot be equipment used as alternative coolingequipment as necessaryused, power is supplied by the emergency DG
・Inundation or loss of power source for a long
・Systems (IC,HPCI,RCIC） are installed
period not assumed
which function on power supplied from DC
batteries even when the AC power source fails ・No different alternative types of drive sources
for valve types
for a short time・Ensuring diversity for driving sources
through the installation of steam-driven
equipment
・Although there are the HPCI and RCIC forwhich seawater cooling systems for cooling oil
and bearings are not needed, their functions failif DC power is lost
○Deployment of spare parts for seawater pump ●Releasing heat into the atmosphere through● Improving reliability of existing coolant
containment vessel venting （guaranteeing
injection systems through deployment of backup motors
power generating cars or large capacity power ●Deployment of washing and drying materials soundness of containment vessel）(BWR)○Deployment of backup power sources andfor seawater pump motors
supplies
driving sources for venting operations (BWR)●Deployment of mobile seawater pumps○ Deployment of backup power sources and○Preventing against inundation of equipment
driving sources for venting operations (BWR)● Backup power supplies, standby air cylinders for severe accident measures●Movable pumps, hoses, etc. not dependant on
and other preparations for SRV drives (BWR)
existing equipment●Securing water supplies (including seawater asa final means)
・Releasing heat into the atmosphere throughcontainment vessel venting (BWR)● Ensuring core cooling via SG by means of themain steam relief valve (PWR)
・ Installation of combustible gas concentration ・ Gas circulating between units not taken into ・If core is damaged, hydrogen may leak fromcontainment vessel.
control systems to control the concentration of accounthydrogen and oxygen which is generated when
there is a cooling failure accident
・ Preventing hydrogen from being generated bycooling the core using the ECCS, etc.
・By filling the PCV with nitrogen, an inert
environment is created which controls theconcentrations of hydrogen and oxygen inside
the containment vessel when there is anaccident environment to keep it below the
flammability threshold● Releasing and reducing retained hydrogen● Preventing hydrogen from circulting frompressure strengthened venting lines●Preventing hydrogen from circulating aroundbetween units sharing an exhaust stack

Proposedmeasures● Releasing and reducing retained hydrogen≪Description of measures at typical plant≫
Guide to column for typlical measures：●⇒Items for which implementation is completed or which have been taken into account initially due to factors characteristic of the site○⇒Items being implemented or such capability is being planned

22

In Closing
In this report, the nuclear power industry, which is comprised of Japan’s electric
power companies and manufacturers conducted an examination focused mainly on
analyzing factors as to why a core meltdown was not able to be prevented after
tsunami struck the Fukushima Daiichi Nuclear Power Station and on formulating
measures to counter such a situation.
The causes of the accident were that tsunami strikes inundated station electric
installations, particularly the emergency AC and DC power supply panels, whichcaused cooling, control and monitoring functions to be lost, that time was required
to establish means for cooling the core which were not dependent on permanentelectric power sources, and that the core was not able to be maintained in asubmerged state.
In this review, in keeping with the progression of the accident, we focused onwhether the functions expected at each stage worked or not, analyzed causes andfactors, and deduced issues and countermeasures. We looked at all measures incombination with existing equipment and countermeasures, arranged themeasures from the standpoint of deep protection having the capability to ensure
safety, and deduced those with high priority to be multitiered measures.
In addition, we have confirmed that safety has been substantially improved by
the emergency safety measures already implemented by each company.
As new information about the accident is obtained, we plan to conduct further
studies and review this report.

23