Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, Japan

FACT AND CAUSE OF
FUKUSHIMA NUCLEAR POWER PLANTS
ACCIDENT
Hideki NARIAI
Professor Emeritus, University of Tsukuba
Ibaraki, Japan, hdnariai@mail1.accsnet.ne.jp

ABSTRACT: The Fukushima Daiichi Nuclear Power Station was attacked by the big
earthquake and the large tsunami on March 11, 2011. The operating three nuclear power
plants became the severe accident with the meltdown of nuclear fuels. The fact and causeof the accident is reviewed focusing on the sequence of the accident and the damage ofthe important components, on the basis of the information from the reports by theGovernment, the Tokyo Electric Power Company, the Academic Societies and others.
The countermeasure for the reactor safety is also discussed.
Key Words: Fukushima Daiichi nuclear power station, loss of AC power, loss ofultimate heat sink, severe accident, meltdown of nuclear fuels

INTRODUCTION
A big earthquake attacked north-eastern Japan at 14:46 on March 11, 2011. After about 40 minutes to
one hour from the first earthquake, seven waves of large tsunami attacked the coast of Tohoku andKanto area, resulting the disaster named as the Great East Japan Earthquake. The earthquake andtsunami affected the Fukushima Daiichi Nuclear Power Station (NPS) of the Tokyo Electric Power
Company (TEPCO), resulting the severe accident with the meltdown of nuclear fuels for operating
three reactors at Fukushima Daiichi NPS. The operating nuclear power plants were scrammed bydetecting the earthquake, though the earthquake caused all off-site AC power loss for the NPS. The
emergency diesel generators started to operate in order to supply the AC power and to remove theresidual heat of the nuclear fuels. However, the tsunami attacked the NPS, causing the loss of theemergency diesel generator power and the loss of the residual heat removal functions. The emergency
cooling systems started. However, they did not work for so long time, and the fuels became to heat up
and melt down, resulting the severe accident. The hydrogen was generated by the Zirconium-Water
reaction and was accumulated in the pressure vessel and in the containment vessel with the steam andthe radioactive materials. The containment vent was carried out for protecting the destruction of thecontainment vessel. Then the hydrogen explosion occurred at the reactor buildings of Units 1, 3 and 4.
The radioactive materials were discharged in atmosphere. The residents around the Nuclear Power
Station were evacuated. It will take long time to settle completely the accident and now on the way.
Present paper reviews the damage of the important components, the sequence of the accident, theactions for the settlement of the accident, the lessons learned and the actions for the lessons, and122

discusses the essential points for the nuclear safety in Japan.

MAIN INFORMATION SOURCES ON THE ACCIDENT
After the accident, the information about the status of nuclear plants was mainly from the TV andnews papers. However it was very hard to know the detailed and exact status from them for several
days after the occurrence of the accident. We could estimate the hard struggle against the severe
situation of the plants by the operators and the employees at the Fukushima NPS.
On April 17, TEPCO announced the Roadmap for the settlement of the accident, in which theStep 1 target for 3 months and Step 2 target for 3 to 6 months after the Step 1 were indicated (TEPCO
2011). Further TEPCO disclosed the details of the plant data of three reactors at the accident andreported to the Nuclear and Industrial Safety Agency (NISA) on May 16 (TEPCO 2011).
The Japanese Government presented the Report on “The Accident at TEPCO's Fukushima
Nuclear Power Stations” to the IAEA Ministerial Conference on Nuclear Safety from June 20(Jap.Gov. 2011). The report included the detailed data of the accident by TEPCO and the data andanalyses by the Japan Nuclear Energy Safety Organization (JNES), the Japan Atomic Energy Agency(JAEA), and other organizations, along with the lessons learned from the accident.
On July 19, TEPCO announced that the Step 1 target of the Roadmap was accomplished with 3
months, and showed the activities for the Step 2 (TEPCO 2011).
On September 11, the Japanese Government presented the Additional Report of the Japanese
Government to the IAEA, in which the actions for the lessons learned from the accident by theGovernment were presented (METI 2011). It also explained the activities to strengthen the standards
and guidelines, additional safety assessment efforts for the NPSs, and the reform of the regulatory
bodies, in which the NISA will be departed from the Ministry of Economy, Trade and Industry(METI), and the Nuclear Regulatory Agency (tentative name) will be established.
At the end of October, the Japan Nuclear Technology Institute (JANTI) reported the Review ofAccident at Tokyo Electric Power Company Incorporated's Fukushima Daiichi Nuclear Power Station
and Proposal of Countermeasures (Draft), in which the cause of accident was analyzed and the actions
to be done by the electric companies was proposed (JANTI 2011).
On December 2, the TEPCO released the Fukushima Nuclear Accidents Investigation Report(interim), in which the detailed explanation of the actions at the accident is included (TEPCO 2011).
On December 16, the Government and TEPCO announced the accomplishment of the Step 2
target of the Roadmap, in which the cold shutdown condition was reached (TEPCO 2011). The next
step action was to start the middle and long term Roadmap toward the decommissioning of the Units 1
to 4. The new R&D is necessary for the accomplishment of the decommissioning.
In order to make clear the fundamental cause of the accident, the Japanese Government
established the Investigation Committee on the Accident at the Fukushima Nuclear Power Stations ofTokyo Electric Power Company on June. The Committee released the interim report on December 26,
2011, in which the behavior and actions by the operators and other relevant persons at the accident inFukushima Daiichi NPS were explained. The final report will be open at the summer of 2012.
Along with them, the urgent meetings and symposiums were held frequently by the Science
Council of Japan and by many academic societies. Further the articles, papers, and explanations on theaccident were presented in the journal of academic societies and others.

NUCLEAR POWER PLANTS AT FUKUSHIMA DAIICHI NPSFukushima Daiichi Nuclear Power Station is one of the oldest NPS in Japan. It has the 6 BWR Plants
and the Unit 1 was started commercial operation on March 1971. By the way, the oldest commercial
nuclear power plant in Japan was the carbon dioxide gas cooled reactor of the Tokai Nuclear Power
Station of the Japan Atomic Power Company (JAPCO). The plant was introduced from the United
Kingdom and started commercial operation in 1966. After the second plant, Japanese Electric

123Companies introduced the Light Water Reactors (LWR) from USA. They were the Boiling Water
Reactor (BWR) by General Electrics and the Pressurized Water Reactors (PWR) by Westinghouse.
The first commercial BWR in Japan is Tsuruga NPS Unit 1 of JAPCO and started commercial
operation on March 1970. The first commercial PWR is Mihama NPS Unit 1 of Kansai Electric Power
Company and started commercial operation on November 1970. These two plants were located at the
Fukui Prefecture, and a part of the electricity was transported to the Osaka Exposition in Osaka. At
present, 54 plants with 30 BWR and 24 PWR exist in Japan.
All of the 6 nuclear power plants at Fukushima Daiichi NPS were BWR. The electric output,
reactor model, pressure containment model and others were indicated in Table 1. The plants were
improved as time passed. The Unit 1 is BWR 3 with 460 MW electric output and Mark-1 containment
vessel, though the Tsuruga Unit 1 is BWR 2 with 360 MW electric output and Mark-1 containment
vessel. The Units 2 to 5 are BWR4 with 784 MW electric output and Mark-1 containment vessel. The
Unit 6 is BWR 5 with 1100 MW electric output and Mark-2 containment vessel. The last Unit 6
started commercial operation on October 1979. Figure 1 shows the main installation of Mark-1 type
BWR.
Table 1 Specification of nuclear power plants at Fukushima Daiichi NPS
Plant Number

Unit 1

Electric Output (MWe)Unit 2

Unit 3460

Unit 4 Unit 5784

Unit 6
1100

Commercial Operation

1971.03 1974.07 1976.03 1978.1 1978.04 1979.1

Reactor Model

BWR3

BWR4

Containment Vessel Model

Mark-1IC

BWR5Mark-2

RCIC

Emergency Core Cooling System

HPCI

HPCS

LPCS
HPCS:High Pressure Core Spray System, LPCS:Low Pressure Core Spray System
A Reactor building

3 CraneB Turbine building

4 Nuclear reactor

1 Condenser

5 Drywell of containment vessel

2 Turbine generator

6 Suppression chamber of containment vessel

Fig. 1

Installation of Mark-1 type BWR (Unit 2)

124(Hatamura 2011)

Emergency cooling systems of Units 1, 2 and 3
The emergency cooling systems were equipped in BWR providing for the cooling of nuclear fuels at
the accident. In Unit 1 (BWR 3), the Isolation Condenser (IC) and the High Pressure Coolant Injection
System (HPCI) were equipped. In Units 2 and 3 (BWR 4), the Reactor Core Isolation Cooling System(RCIC) and the HPCI were equipped. At the accident with the loss of normal fuel cooling function, theIC works to condense the steam in pressure vessel and to supply the generated water back in reactor
vessel for the cooling of the nuclear fuels. Both the RCIC and the HPCI, which work by the steam
driven pump using the high pressure steam in pressure vessel, inject the water from the Condensate
Storage Tank or from the Suppression Pool for the cooling of the fuels. They work without AC
power, though the DC power is required to operate the valves.
Further, for the cooling of the reactor core under low pressure condition, the Core Spray System(CS) and the Shutdown Cooling System (SHC) were equipped for Unit 1, and the CS and the
Residual Heat Removal System (RHR) were equipped for Units 2 and 3. However the AC power isneeded to operate them and they could not work at the accident.
Plant status of the 6 plants before the earthquake
On March 11, 2011, the three plants (Units 1, 2 and 3) were under operation, and the other three plants(Units 4, 5 and 6) were under shutdown for periodic inspection. The fuels of Unit 4 were transferred tothe spent fuel pool since exchange of the shroud was underway during the periodic inspection. The
fuels of Units 5 and 6 were in the pressure vessel.

OCCURENCE OF THE EARTHQUAKEAt 14:46 on March 11, 2011, the big earthquake occurred at north-eastern Japan. The earthquake waswith the scale of moment magnitude 9.0, fourth largest earthquake ever recorded in the world history.
The earthquake occurred as the result of faulting on the boundary between the Pacific Plate and North
American Plate. The epicenter was about 130 km southeast of Oshika Peninsula with a depth ofapproximately 24 km. The size of the faulting zone was about 400 km long, and approximately 200
km wide. Figure 2 shows the epicenter of the earthquake and the location of the 5 nuclear power
stations in the afflicted area of the disaster.

Nuclear power plants at the afflicted area
In the afflicted area of the disaster, 15 BWR plants have been constructed. They are from the north, 1
plant at the Higasidori NPS of Tohoku Electric Power Co., 3 plants at the Onagawa NPS of Tohoku
Electric Power Co., 6 plants at the Fukushima Daiichi NPS of TEPCO, 4 plants at the Fukushima
Daini NPS of TEPCO, and 1 plant at the Tokai Daini NPS of JAPCO. Most of the plants have beenbrought to the cold shutdown condition within several days. However, the tsunami caused the reactor
accident with the meltdown of fuels for three plants at the Fukushima Daiichi NPS.
Affect of the earthquake on the Fukushima Daiichi NPS
The three operating plants (Units 1, 2 and 3) were shut down automatically by detecting theearthquake at 14:46 on March 11. However, all external electric power for Units 1 to 6 was lost by theearthquake. Then emergency diesel power generators started and the decay heat of nuclear fuels wasremoved by the cooling system until the tsunami attack. The emergency cooling systems started
automatically or manually. They were the IC for Unit 1 and the RCIC for Units 2 and 3.

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Fig. 2

Epicenter of the Earthquake and the location of five nuclear power stations

Cause of the external AC power loss at Fukushima Daiichi NPS
Now the cause of the external (off-site) AC power loss will be explained. The electric power was
supplied through 6 lines for Units 1 to 6. For Units 1 and 2, the electricity was transmitted from
Shin-Fukushima Transforming Station through Okuma Nos. 1 & 2 lines to the normal high voltage
switchboard of Units 1 and 2 via the switchyards for Units 1 and 2. Further TEPCO nuclear line fromTohoku Electric Power Co. connected as a reserve to the normal high voltage switchboard of Unit 1.
Due to the earthquake, several parts of the circuit breakers at the switchyards for Units 1 and 2 weredamaged, resulting the actuation of the circuit breakers at the Shin-Fukushima Transforming Station.
As for the TEPCO nuclear line, the connecting cables to metal-clad switch gear of Unit 1 weredamaged and failed. As for Units 3 and 4, the Okuma Nos. 3 and 4 lines connected to the normalhigh voltage switchboard of Units 3 and 4. The line cables touched the tower, resulting the shortcircuit and failed. Further the normal high voltage switchboard was inundated later. These resulted theactuation of the circuit breakers at the Shin-Fukushima Transforming Station. As for Units 5 and 6, theYonomori Nos. 1 and 2 transmission lines connected to the normal high voltage switchboard of Units
5 and 6. By the earthquake, the line cables touched each other, resulting the actuation of the circuit
breakers at the Shin-Fukushima Transforming Station. Further one tower of transmission line
connecting to the switchyards for Units 5 and 6 was collapsed. These resulted the loss of all external(off-site) power supplies to Units 1 to 6.

ATTACK BY THE TSUNAMI
At about 40 minutes to one hour later of the first earthquake, seven waves of large tsunami attacked
the coast of Tohoku-Kanto area. At the Fukushima Daiichi NPS, the first wave of tsunami reached at
15:27, and the second large tsunami at 15:35 on March 11. The tsunami with 15 m high was getting
over the 10 m high sea wall and entered into the reactor and turbine buildings area. The cooling sea
water pumps at the sea side with 4.1 m high were inundated. The emergency diesel power generators,

126metal-clad switch gear, power center, and so on at the lower level of the turbine buildings were also
inundated.
Function loss of major machines and components
Emergency diesel power generators
The number of emergency diesel power generators (DG) at Fukushima Daiichi NPS were 13. They
supply the power to each Unit through matal-clad switch gear (M/C). Each Unit had 2 (A and B) DG
except Unit 6 which had 3. Among them, 10 DG were sea water cooled and 3 DG (Unit 2B, Unit 4B
and Unit 6B) were air cooled. By the tsunami attack, all sea water cooled DG lost the function, though
3 air cooled alive. For Unit 1, DG 1A and 1B located at the first basement of the turbine building(T/B) inundated and lost the function. For Unit 2, DG 2A located at the first basement of T/Binundated and lost the function. DG 2B located at the first floor of the common spent fuel poolbuilding was alive, though the M/C inundated and the function was lost. For Unit 3, DG 3A and 3B
located at the first basement of T/B was inundated and lost the function. For Unit 4, DG 4A was underperiodic inspection. DG 4B located at the first floor of the common spent fuel pool building was alive,
though the M/C inundated and the function was lost. For Unit 5, DG 5A and 5B located at the first
basement of T/B was alive, though the connected components inundated and the function was lost. For
Unit 6, DG 6A and DG 6C located at the first basement of the reactor building was alive, though thesea water pump necessary to cool the DG was inundated and the function was lost. DG 6B located at
the first floor of diesel generator building was alive and the function to supply power was alive.
Metal-clad switch gear (M/C) and power center (P/C)
M/C is the 6900 V switch board for high voltage circuit power, and P/C is the 480V switch board forlow voltage circuit power. Number of the M/C and the P/C was 15 each. They are three kinds such as
for normal operation, for emergency and for common. Due to the earthquake, the M/C and the P/C fornormal and for common were lost the function since the external power was lost. By the tsunami, 12
among 15 M/C for emergency were inundated and lost the function, and 9 among 15 P/C for
emergency were inundated and lost the function.
Emergency cooling sea water pump
The emergency cooling sea water pumps are equipped to supply the sea water to the heat exchanger ofContainment Cooling System (CCS) for Unit 1 and Residual Heat Removal System (RHR) for Units 2
to 6. After the tsunami attack, the emergency cooling sea water pump stopped by the loss of AC powerof Units 1 to 5, and the function of the CCS and the RHR was lost.

ACCIDENT SEQUENCE OF UNITS 1 TO 6
Outline of the sequence
The operating Units 1, 2 and 3 were scrammed by detecting the earthquake. Since the external AC
power was lost, emergency DG started to work for cooling the nuclear fuels. The emergency coolingsystems, such as the IC for Unit 1 and the RCIC for Units 2 and 3, started.
However, about 50 minutes later, all the emergency DG stopped, resulting all AC power loss. All
ultimate heat sink were also lost. The emergency cooling systems were only way to cool the nuclear
fuels, though control of the systems should be done by DC battery with short life time. During the fuelcooling, recovery of the ultimate heat sink should urgently be done. However, the aftershock, thetsunami remnants and the destroyed road by the earthquake prevented the work to recover the situationfrom the outside. The batteries did not work for so many days, though operators did the best to prolong
the batteries' life time. Finally heat-up and melt-down of the fuels occurred and the severe accident
resulted in Units 1, 2 and 3.
Units 4, 5 and 6 were in periodic inspection outage. Unit 4 was on the way to replace the reactor

127core shroud and all fuel assemblies had been transferred to the spent fuel pool. As for Unit 5, all fuelassemblies was loaded in the reactor core and the pressure leak test for reactor pressure vessel was
being conducted. As for Unit 6, all fuel assemblies were loaded in the reactor core that was in cold
shutdown condition.
Sequence of the Unit 1
Unit 1 reactor was scrammed by detecting the earthquake, and two emergency DGs started. The two
ICs automatically started to cool the fuels. However, the cooling speed was too fast compared with theoperation manual, and operators stopped it manually. After then, one IC was used to cool the fuels.
The cooling and the stop of the IC was repeated 3 times. The tsunami attacked the NPS at 15:35 andthe emergency DGs and the ultimate heat sink were lost. The IC was stopped when the tsunami
attacked. Then the cooling water could not be supplied to the reactor core, and the water in thepressure vessel began to decrease. The fuels could not be cooled when the water level decreased below
the fuel level, resulting the heat-up of the fuels. The cladding material of the fuel is zircaloy or
zirconium alloy, and the zirconium begins to react severely with water (or steam) at above 900 °C,
generating the hydrogen and the zirconium oxide. The reaction is the exothermic reaction and thereaction rate increases rapidly above 1200 °C, resulting the increase of the fuel temperature. Thus alarge amount of the hydrogen generated and the fuel temperature increased more than the melting
temperature of the zircaloy and reached the melting temperature of the fuels.
Since all of the equipped cooling functions were lost, cooling of the fuels by injection of waterfrom outside was necessary. Injection of water through the fire protection system into the pressure
vessel by using the fire engine began in the morning on March 12. However, it was too late to cool thefuels. Damage of the fuels began at the evening on March 11, almost 4 hours from the tsunami attack.
Considerable amount of the damaged fuels moved to and accumulated at the bottom of the reactor
pressure vessel. The pressure vessel was damaged at about 10 hours from the tsunami attack andpressure in the pressure vessel dropped largely. Then the pressure and the temperature in thecontainment vessel began to increase to the highest pressure at 2:30 on March 12. In order to preventthe rupture of containment vessel, wet well venting from the containment vessel was carried out twice
at 10:17 and 14:30 on March 12. After the second venting, a hydrogen explosion occurred in thereactor building of Unit 1, destroying the upper floor of the reactor building at 15:36 on March 12, just
24 hours later of the tsunami attack. After the hydrogen explosion, sea water injection into the pressure
vessel was again started at 19:04. However, some of the molten fuels may have dropped on the lower
plenum of the pressure vessel on around 20 March, and further they dropped and accumulated on thepedestal of the drywell floor of containment vessel, at almost the end of March. The molten
core-concrete reaction may have occurred there.
Sequence of the Unit 2
Unit 2 reactor was also scrammed by detecting the earthquake, and two emergency DGs started. The
RCIC was also started manually. After the loss of both emergency DG and ultimate heat sink by thetsunami attack, the RCIC was alive for 3 days until at 13:25 on March 14. The operators conducted thefeed and bleed operation, that is, the water was fed into reactor by the RCIC, then the steam pressurerose. The steam was bled through the safety relief valve into the water pool in the suppression
chamber. Water temperature of suppression pool increased to almost saturation temperature andgradually steam could not be condensed. The leakage was estimated to occur probably around the
suppression chamber of containment vessel before noon on March 14. After 6 and half an hour laterfrom the RCIC stop, sea water was injected into the pressure vessel. During the no cooling period,
however, the fuels were exposed to steam and started melting at around 16:30 on March 14, andmolten fuels dropped to lower plenum of the pressure vessel. Further the lower part of the pressure
vessel was damaged at around 21:30 and some of the molten fuel dropped on the pedestal of thecontainment vessel before 24:00 on March 14. The drop of the molten fuel on the pedestal ofcontainment vessel was continued for a while. The containment vent was already prepared on March
13, but it was not carried out. Noise of an explosion occurred at around 6:00 on March 15. There is apossibility that any explosion occurred around the torus room of the containment vessel. However

128

TEPCO reported that the noise might be the sound of the explosion just occurred at Unit 4 at that time.
Sequence of the Unit 3
Unit 3 reactor was also scrammed by detecting the earthquake, and the power was supplied by the
emergency diesel power generators. The RCIC was manually controlled. After the tsunami attack, theRCIC was started manually at 16:03 on March 11, since the DC batteries were alive. However the
RCIC stopped at 11:36 on March 12. After one hour's later, the HPCI started automatically bydetecting the low water level to continue the cooling of the fuels. The operators stopped the HPCI at
2:42 on March 13, since they intended to save the consumption of the batteries. However they could
not start the HPCI again, since the batteries were already consumed. The steam relief valve was open
to decrease rapidly the pressure in pressure vessel at 9:08 and the water was injected through fire
protection system at 9:25 on March 13. During the no cooling period, the fuels exposed and started
melting at around 8:00 on March 13. At around 9:00 on March 13, the upper flange gasket ofcontainment drywell was estimated to be damaged. At the same time the lower part in pressure vessel
was damaged, and the molten core dropped on the pedestal floor of the containment vessel. The ventfrom the containment vessel was carried out several times at 9:20, at 12:30 on March 13 and at 5:20 on
March 14. A hydrogen explosion occurred in the reactor building at 11:02 on March 14, destroying thereactor building.
Sequence of the Unit 4
Unit 4 reactor had been shut down for periodic inspection, with the nuclear fuels having beentransferred to the spent fuel pool. Both the cooling and the water feeding functions for the spent fuelpool were lost by the earthquake and the tsunami. Temperature of the spent fuel pool rose to 84 °C at
4:08 on March 14. At about 6:00 on March 15, an explosion in the reactor building occurred, anddestroyed the upper part of the building. At first, the cause of the explosion attributed to the hydrogengenerated by the over-heating and melting of the fuels in the pool. Injection of water to the pool was
conducted from the helicopter, and by the water discharge car. However it was confirmed that theenough water existed in the spent fuel pool. As the cause of the explosion, an inflow of hydrogen from
Unit 3 is possible, since the exhaust pipe for venting, joins with the exhaust pipe from Unit 4 before
the exhaust stack.
Sequence of the Units 5 & 6
Unit 5 was in periodic inspection outage. At the accident, all fuel assemblies were loaded in thereactor core and the pressure leak test for reactor pressure vessel was being conducted. By the tsunami
attack, all the AC power supply was lost, resulting the loss of the ultimate heat sink. As the cooling
function was lost, the reactor pressure continued to increase. The water level and the pressure wasmaintained by injecting water into the reactor by operating the make-up condensing water pump after
the power was supplied from Unit 6. The water in the spent fuel pool was also supplied. On March 19,
a temporary seawater pump was activated to operate the RHR, bringing the reactor to a cold shutdown
condition at 14:30 on March 20.
Unit 6 was in periodic inspection outage. At the accident, all fuel assemblies were loaded in thereactor core that was in cold shutdown condition. One of the emergency DGs for Unit 6 had been
installed at a relatively high location, and its functions were not lost after the tsunami attack. However,the sea water pump lost its function. The reactor water level and the pressure was controlled byinjecting water into the reactor by operating the make-up condensing water pump. A temporary
seawater pump was activated to operate the RHR on March 19, bringing the reactor to a cold
shutdown condition at 19:27 on March 20.
Cooling of the reactors and spent fuel pool
The residual heat from the nuclear fuels for damaged Units 1 to 3 reactors, for Units 5 and 6 reactors
under outage, and for spent fuel pools of Units 1 to 6 should continuously be removed. Particularly thedamaged molten fuels in reactors and in containment vessels in Units 1 to 3 should be cooled by129

injecting the water until the cold shutdown condition, in order to decrease the hydrogen generation andalso to prevent the release of the radioactive materials in atmosphere. The temperature in the damaged
reactor pressure vessels decreased gradually by continuous water injection.
However, the injected water became the contaminated water with the radioactive materials, andleaked to outside of the containment vessel through the damaged part. The contaminated water
accumulated at the lower part of the turbine buildings and the pits, and some of them leaked in the sea.
A large amount of the accumulated water was a big problem. The circulating water injection coolingsystem was constructed in order to reuse the accumulated contaminated water for the injection coolingafter doing the decontamination and the desalination. The accumulated water was decreased byoperating the system. Figure 3 shows the concept of the system.

Fig. 3 Circulating injection cooling system (TEPCO 2011)

SEQUENCE OF OTHER 9 PLANTS AT AFFLICTED AREA
Now the situation of other 9 nuclear power plants at the NPSs of the afflicted area will shortly bereviewed. Among 9 plants, eight plants were under operation and one plant was under periodic
inspection. The important functions are (1) external (off-site) AC power, (2) emergency diesel power
generator (DG), and (3) residual heat removal system (RHR). Whether they were alive or not, then the
result changed.
The first is the case that the off-site AC power was alive. They were 6 operating plants. Amongthem, the RHR or other auxiliary cooling system and the RCIC et al of 3 plants were alive, and theplants could reach cold shutdown condition on March 12. They were Onagawa Units 2 and 3, andFukushima Daini Unit 3. The RHR of other 3 plants was lost by the tsunami. However the temporary
cable for the RHR system was installed within several days. The 3 plants reached cold shutdown
condition on March 14 or 15. They were Fukushima Daini Units 1, 2 and 4.
The second is the case that the off-site AC power was lost. They were 3 plants with 2 plants
under operation and 1 plant under periodic inspection. For operating 2 plants, both the emergency DG
and the RHR were alive, and they reached cold shutdown condition on March 12 and 15. They were
Onagawa Unit 1 and Tokai Daini Unit. As for one plant under periodic inspection, emergency DG wasalive and the fuels were outside the reactor. It was not so severe situation. That was Higashidori Unit.
Considering the above situations and also the situation of Fukushima Daiichi Units 5 and 6 inwhich the off-site AC power and the RHR were lost and the emergency DG was alive, we are sure thatthe plants could reach the cold shutdown condition, when either external AC power or emergency DG
was alive.

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RELEASE OF RADIOACTIVE MATERIALS AND EVACUATION OF THE RESIDENTS
Release of the radioactive materials
The radiation dose rate around the Fukushima Daiichi NPS increased many times within the first 2
weeks after the occurrence of the accident. This indicates that the radioactive materials released in
atmosphere. The release of the radioactive materials were mainly by the venting from the containment
vessel, by the explosion of the reactor buildings, and also by the leak through the damaged opening of
the containment vessel just after the operation of the safety relief valve or after the water injection
over the hot damaged fuels. The radiation dose rate decreased gradually after the end of March. Figure
4 shows the change of the radiation dose rate at the NPS with the estimated causes of the increase(METI 2011).

Fig. 4 Changes in dose rates at Fukushima Daiichi (Monitoring car) (METI 2011)
The radiation dose rate at the NPS started to increase at around 4:00 on March 12. This may
indicate any damage of the reactor pressure vessel and containment vessel in Unit 1, since the drywellpressure of Unit 1 started to increase rapidly at around 1:00 on March 12. The radiation dose rateincreased further by the wetwell venting at 10:17 and at 14:30 on March 12, and also by the leakthrough any damaged part at lower location of the containment vessel. Radiation dose rate further
increased at around 20:00 on March 12. This may be due to the generation of large amount of steam,hydrogen, and radioactive materials by the sea water injection into the reactor pressure vessel at 19:04
on March 12.
In order to decrease the pressure in pressure vessel of Unit 2 after the stop of the RCIC, theoperator opened the safety relief valve at around 18:00 on March 14. Then the steam, hydrogen andradioactive materials in pressure vessel moved to the suppression pool. As any damage may exist at
the suppression chamber, as explained in sequence of the Unit 2, the radioactive materials leaked fromsuppression chamber to outside. This increased the radiation dose rate at around 22:00 on March 14.
After the noise of explosion at around 6:00 on March 15, the radiation dose rate increased at the NPS.
If the hydrogen explosion occurred at around the torus room and made any crack there, then theradioactive materials may be discharged into atmosphere through the crack. The radiation dose rateincreased at around morning to noon of March 15. This corresponds to the time that the damaged core131

dropped to the lower plenum in pressure vessel of Unit 2.
The radiation dose rate was increased at around 9:00 on March 13. This is estimated that theradioactive materials leaked through the damaged flange gasket at the upper part of the drywell in Unit
3, by the wetwell venting from containment vessel and also by the sea water injection into pressure
vessel. On March 16, several very high radiation dose rates were measured, which are estimated to beby the wetwell venting. The reason is that the suppression chamber became full of the water injectedfrom outside at that time, and the wetwell venting resulted the release of the highly contaminatedwater and steam directly into atmosphere.
Evacuation of the residents
TEPCO recognized that the injection of water via the emergency core cooling system was not certain
at Units 1 and 2, and notified the Government of the state of Nuclear Emergency at the evening on
March 11. The Prime Minister declared a state of nuclear emergency, and established the Nuclear
Emergency Response Headquarters and the local Nuclear Emergency Response Headquarters. The
Prime Minister instructed to evacuate the residents within 3 km radius from NPS, and to stay in housewithin 10 km at 21:30 on March 11. According to the escalation of the events, the evacuation area was
expanded to evacuate the residents within 10 km at 5:44 on March 12 and within 20 km at 18:25 from
NPS. The area to stay in house was also set as within 20 to 30 km on March 15.
On April, the government changed the area and settled the restricted area within 20 km from the
NPS. The deliberate evacuation area and the emergency evacuation preparation area were newly
settled beyond 20 km from NPS. The deliberate evacuation area is the area in which the accumulated
dose may reach 20 mSv in one year. Figure 5 shows the integrated dose in one year after the accident(METI 2011). The high dose area expands to the north-west direction of the NPS which was reflecting
the wind direction and the rain fall at the day that the radioactive materials discharged. In restricted
area, the residents can temporarily access to their home.

[Unit: mSv]
Fig. 530 km line

20 km line

Estimated Integrated dose for one year up to March 11, 2012 (METI 2011)132

ACTIONS FOR THE SETTLEMENT OF THE ACCIDENT AND TOWARDS
THE DECOMMISSIONING OF THE DAMAGED REACTORS
Actions for the settlement of the accident
TEPCO announced the “Roadmap towards Restoration from the accident in Fukushima Daiichi
Nuclear Power Station” on April 17. The basic policy was for the reactors and the spent fuel pools to
be stable condition, and for the release of radioactive materials to be mitigating. Further every effort
should be done for evacuees to return to their home and for all citizens to secure a sound life. Two
steps were set as the target, as step 1 with the time line target of 3 months and step 2 with the time line
target of 3 to 6 months after achieving step 1. The target of the step 1 was the radiation dose in steadydecline and that of step 2 was the release of radioactive materials under control and radiation dosebeing significantly held down. TEPCO announced the accomplishment of the step 1 target on July 19
and that of the step 2 target on December 16, 2011. After the accomplishment of the step 2 target, themiddle and long term Roadmap towards the decommissioning of the four damaged reactors should bemade and now on the way.
Activities during Step 1
The major items conducted during the step 1 were (1) the continuous fuel cooling by minimum water
injection, (2) the transfer of accumulated water in the basement of the turbine buildings into temporary
tanks and the installation of the circulation type water purification system, and (3) the design of thecover for destroyed reactor buildings.
The water was continuously injected in order to cool the reactor core of Units 1, 2 and 3. Largeamount of water was injected into the reactor core, though it was leaking outside of the containment
vessel. The leaked water accumulated in the reactor buildings and turbine buildings. The circulation
type water purification facility was designed in order to reuse the decontaminated water for thecooling of the reactor core. The cover for the destroyed reactor building was also very important for
decreasing the release of radioactive materials.
During the step 1, the temperature at the bottom of reactor pressure vessel decreased to 100 °C
120 °C, and the radiation dose rate decreased to 1.7 mSv/y at the site boundary.
Activities during Step 2
The major items conducted during the step 2 were the continuation of the step 1. The first is thereactor cooling by the circulating injection cooling along with the treatment of high level radioactive
waste water. The operation started on June 27. The second was the spent fuel cooling using the heatexchanger. The third was the mitigation of contamination in the sea through the underground water.
Design of the ground water shielding wall started. The fourth was the installation of the cover over thehighly damaged reactor buildings, in order to prevent further release of the radioactive materials. The
cover of Unit 1 was completed at the end of October 2011. The construction of the cover for Units 3
and 4 has been on the way.
During the step 2, the temperature at the bottom of the reactor pressure vessel decreased
sufficiently below 100 °C, and the reactors were brought to a condition equivalent to the cold
shutdown. The radiation dose rate decreased to 0.1 mSv/y at the site boundary. Basing on these data,
TEPCO announced the accomplishment of the step 2 on December 16, 2011 (TEPCO 2011).
Activities towards the middle and long term roadmap for the decommissioning of the reactors
After the completion of the step 2, the roadmap towards the decommissioning of Units 1 to 4 has to be
made, including the implementation of the on-site work and the R&D towards the decommissioning
such as the removal of fuels from the spent fuel pools. Now the middle term activities for 3 years
until the start of the work for decommissioning has started.

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LESSONS LEARNED AND ACTIONSFOR THE RECONSTRUCTION OF NECLEAR REACTOR SAFETY
Lessons learned and actions
The Japanese Government indicated the lessons learned from the accident in the report to the IAEA
Ministerial Conference on Nuclear Safety on July 2011. Lessons were in 5 categories with 28 items.
Further the Japanese Government presented the additional report to the IAEA on September 2011, in
which the actions for the lessons were explained. The major items relating directly to the cause of theaccident and the actions by the Japanese Government will be explained. Further the activities by the
Atomic Energy Society of Japan, the JANTI and the TEPCO will shortly be explained.
Lessons learned in Japanese Government report
Among five categories in Japanese government report, major items in three categories which are
directly relevant to the cause of the accident will be explained. The first category of the lessons
learned is to strengthen preventive measures against severe accident. This requires for the reactors to
avoid the severe accident under various kinds of big natural disaster, such as earthquake and tsunami.
The reactors and important equipments were not damaged by the big earthquake of this accident.
However, off-site electric power supply system was damaged by the earthquake. For tsunami, thewave height estimated before was not enough to prevent the accident. The current guideline indicatesthe 30 min. loss of AC power. In this disaster, some plants were safely shut down since a part ofpower supply system was alive and the sea water pump was recovered for the RHR. The consideration
for the longer loss of AC power supply was necessary for the reconstruction of the nuclear safety, andit should be included in the safety design guidelines. The loss of emergency DG should also properly
be considered. As for the accident management, the alternative water injection system was not
sufficient under no power source and high radiation environment.
The second category is the enhancement of response measures against severe accidents. As forthe hydrogen explosion, the explosion inside the containment vessel was mainly considered forproviding the countermeasures. Hydrogen explosion outside the containment vessel should also be
considered. The containment venting system should much be enhanced. Further the radiation dose
became very high in the main control room and the operators could not enter the room temporarily.
The environment for the accident response work should be improved.
The fourth category is the reinforcement of safety infrastructure. This is to enforce the safety
regulatory bodies, legal structures, criteria and guidelines and human resources. Several subjects suchas the use of PSA in government examination and such as the consideration of severe accident in theregulatory matter were retarded in Japan. Further the severe accident research should be promoted
much more.
Actions by the Government
According to the lessons, the Government started the investigation for several items. The first is theComprehensive Safety Evaluation, which is similar to the stress test in foreign countries. The Nuclear
Safety Commission (NSC) and the NISA decided to perform the evaluation of existing nuclear power
plants. The evaluation items are for the earthquake and tsunami as the natural disaster, and for thelosses of all AC power and ultimate heat sink as the loss of safety functions. The evaluation will makeclear the potential weakness of the plants by assessing the safety margin for the severe external events.
The second is the revision of safety design guidelines. The NSC started to revise the relevant
guidelines, such as the seismic safety including the tsunami, loss of the duration of power source, thesevere accident, and the nuclear emergency preparedness and response including the evacuation area.
The third is the revise of the regulatory bodies. NISA will be departed from the METI. NISA and
NSC along with a part of Ministry of Education, Culture, Sports, Science and Technology (MEXT),
will become one Agency, tentatively named as the Nuclear Regulatory Agency. It will be settled at the
Ministry of the Environment on April 2012.
The Government organized the Investigation Committee on the Accident at the Fukushima

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Nuclear Power Stations. The committee chaired by Professor Hatamura is conducting thecomprehensive investigation for the accident and is expected to make clear the fundamental cause of
the accident. The interim report was already open at the end of December 2011 (Hatamura 2011). The
final report will be accomplished at the summer of 2012.
Actions by the academic society
The Atomic Energy Society of Japan conducted the investigation for the Fukushima Daiichi Nuclear
Power Plant Accident at the Committee for Investigation of Nuclear Safety. It raised 12 items from thelessons learned and proposed the measures for them on May 2011 (Ninokata 2011). The seismic
design, the tsunami, station blackout, loss of ultimate heat sink, accident management, and hydrogen
explosions were included in the 12 items.
Actions by the industries
JANTI reported the Reviews of Accident and Counter-measures for the Cause of the Accident
from the industrial standpoint (JANTI 2011). It deduced the issues and proposed the countermeasuresin 5 categories, such as preparation for earthquake and tsunami, preparation for power source, measurefor heat sink loss, hydrogen countermeasures, and preparation for emergency situations. The importantmeasures were proposed expecting the adoption by all electric companies.
TEPCO analyzed the accident and indicated the issues to be done in the Fukushima Nuclear
Accidents Investigation Report (interim) on December 2, 2011 (TEPCO 2011). It emphasized that thereliable water injection and fuel cooling in pressure vessel was the most important issue in this kind of
accident, and raised several items, such as quick usage of the high pressure water injection system, thepressure reduction method before the loss of high pressure water injection function, stable lowpressure water injection method, reliable venting method from the containment vessel, recovery of thesea water cooling function, and the instrumentation under the accident condition.

ON THE NUCLEAR SAFETY AND THE SEVERE ACCIDENT IN JAPANHistory of the nuclear safety
Here history of the activities for nuclear safety and severe accident in Japan will be reviewed. The
light water reactors (LWR) were developed in USA in the nineteen-fifties to -sixties and introduced inJapan. The first plant Tsuruga NPS Unit 1 started the commercial operation in 1970. In thenineteen-seventies, twenty LWRs started the commercial operation in Japan. However, many initialtroubles, such as the Stress Corrosion Crack, occurred in the LWRs. Japanese industries conductedactive researches supported by the Japanese Government, and solved the problems. These activities
were the technological basis for the Japanese industries, and led to the design and construction of theAdvanced BWR at Kashiwazaki-Kariwa Units 6 and 7 in 1996 and 97, at Hamaoka Unit 5, and at
Shika Unit 2. However, we should remember the lack of the fundamental R & D experience at theinitial stage of designing and developing the LWRs in Japan. This may be one of the fundamental
causes of the Fukushima accident, particularly regarding the actions under such a severe emergency
condition.
For the first 20 years, the design basis accident was the most important issue for the nuclear plantsafety. The safety researches were mainly for the development of and for the confirmation of thesafety functions, such as the effectiveness of the safety injection system. Computer analysis methods
to evaluate the accident behavior were also developed. These results reflected to the development of
new reactors and also to the revision of the safety design guideline of the NSC in 1990.
The TMI accident in USA was occurred in 1979, just half a year later of the NSC establishment
in Japan. This accident was the severe accident with the meltdown of the reactor core. The NSC
extracted the 52 items of lessons learned from the accident. After the accident, Probabilistic Risk
(Safety) Assessment (PRA or PSA) method was developed in Japan, though the internal events werethe main issues. Only external event considered was the air plane clash on the NPS. However, the135severe accident research was not so active in this stage. The Chernobyl accident, the most large andsevere accident, occurred in 1986. Since the reactor type was different and the cause of the accident
was the illegal operation, there were very few for reflecting directly to Western type nuclear plants
including those in Japan. However, the safety culture was recognized as to be very important for
nuclear safety in the world. Further the Japanese Government considered the necessity for the severe
accident researches and for the severe accident countermeasure policy. The severe accident researches
at the Nuclear Power Engineering Center and the investigation for the severe accident countermeasure
at the NSC started in 1987. The NSC determined the accident management (AM) policy as thecountermeasure for the severe accident in 1992. The policy was that the NSC encouraged intensely thelicensees to prepare the AM as a voluntary action, and to enforce it exactly. Though the AM was not
the regulatory matter, the NISA and the NSC reviewed them, and then the AM was actually very close
to the regulatory matter. Further the AM as the countermeasure for the severe accident at that time inJapan was almost at equal level to that of the foreign countries (Hirano 2011). After then the foreigncountries included the severe accident measures into regulatory matter. However the Japanese
regulatory bodies have not encouraged more actions for the severe accident, and the research fund forsevere accident decreased year by year. This may be one of the fundamental causes of the Fukushima
accident.
The regulatory guideline for seismic safety was revised on 2006, and the countermeasure for thetsunami was included in it. According to the revision, the actions for the tsunami were proceeded inevery NPS. An example of the action is to place the sea water pump in water-proof building. The
action for tsunami seems, however, to be retarded and not sufficient in Fukushima NPS. This may be
one of the fundamental causes.
Finally the circumstance of current 20 year's Japanese nuclear was considered. Many issues should
be done earlier. The inclusion of the severe accident measure into regulatory matter. The effort for
the rational regulation as the same level as in USA and in Europe. The actions to raise the plant
capacity factor up to the level in other major countries. The reform of the regulatory system according
to the indication by the IAEA that the regulatory bodies for nuclear are divided in several Ministries inJapan and the regulatory body NISA is in the METI which promotes the nuclear. The retard occurred
particularly for these 20 years, and they say it as the lost 20 years. It occurred that the Government andpeople intensely stuck to the issues which occurred at that time, even though they were not soimportant for the nuclear safety. This accident is a big chance to improve all these issues. We neverrepeat the same mistakes as in former cases in this accident.
For the reconstruction of nuclear safety and severe accident

Basing on the above mentioned lessons, the actions for the reconstruction of nuclear safety should be
done. There are many countermeasures against such severe condition as the Fukushima accident.
Major technical issues are the countermeasure (1) for big natural disaster such as earthquake andtsunami, (2) for station black out by all AC power loss including emergency DG and DC battery, (3)for ultimate heat sink loss, (4) for emergency water injection system and (5) for containment venting,(6) for hydrogen explosion particularly outside of containment vessel and (7) for control room
habitability and instrumentation measurement under emergency situation. The countermeasures should
actually be done by the electric companies and relevant industries. As the urgent action, the NSC andthe NISA is now conducting the Comprehensive Safety Review for existing Nuclear Power Plants.
Besides above mentioned conditions, we have to consider the countermeasures for the severe
natural disaster such as the big volcano eruption, the river flood, et al, and also for the situation such as
the air plane crash, terrorist attack,et al.
As for the severe accident, the NSC issued new framework for preparing against severe accident
on October 23, 2011, in which the former Commission's paper on AM in 1992 was abolished. The
severe accident will become the regulatory matter. The NSC further recommended the safetyassessment pertaining severe accident and the promotion of safety researches. The NSC started torevise the safety design guidelines for relevant issues such as the duration time of all AC power loss,
et al.

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The NSC will also discuss about the safety target, which is now at draft stage. No person died andthere will be no additional increase of cancer death in this accident. These satisfy the current safetytarget. However, we have to consider the effects of accident on the evacuees and on the contaminationof the land including the farmland, et al. how to include these items in safety target is a difficult
problem.
As for the reform of the regulatory system, the NISA and the NSC along with a part of the MEXT
will be one agency. This is what we wanted. We expect further for new Regulatory Agency to conduct
the work on nuclear safety with high technological bases. In today's highly developed science andtechnological society, the role of the high level specialists is very important for constructing andmaintaining the safe and secure society.

CONCLUSIONSThe Fukushima Daiichi Nuclear Power Station was attacked by the Great East Japan Earthquake andthe accompanying Tsunami on March 11, 2011. The operating three nuclear power plants became thesevere accident with meltdown of the nuclear fuels. The sequence of the accident, damage of theimportant components, release of the radioactive materials, lessons learned and actions for the lessons,and discussion on the fundamental cause were reviewed basing on the information from various
reports including TEPCO, Japanese Government and so on.
The direct cause of the accident was the wrong estimation for the duration of off-site AC power
loss and for the height of the tsunami wave. The fundamental cause of the accident was the retard of
the improvement actions for the severe accident, for the safety guidelines, for the regulatory systems,
and for several other issues.
Fukishima accident showed a week point of the BWR. On the other hand, it showed therobustness of the light water reactors, too. Particularly if either of the off-site AC power or the
emergency DG for Units 1 to 4 were alive, then the result of the Fukushima Daiichi accident might bea little different. When we reconsider carefully the function of the nuclear reactors for this kind ofsevere situations, it is possible to provide sufficient countermeasures for any big natural andman-made disasters, and to operate safely the light water reactors.
REFERENCES
Hatamura (2011). “The Interim Report of the Investigation Committee on the Accidents at the
Fukushima Nuclear Power Stations of Tokyo Electric Power Company”,
Home Page of the Investigation Committee on the Accidents at the Fukushima Nuclear Power
Stations of Tokyo Electric Power Company “http://icanps.go.jp/eng/”
Hirano, M. (2011). “The Circumstances of Severe Accident Measure Implementation and “theResidual Risk” ” Journal of the Atomic Energy Society of Japan, Vol. 53, No. 11, 748-754, (inJapanese)
JANTI (2011). “Reviews of Accident at Tokyo Electric Power Company Incorporated's Fukushima
Daiichi Nuclear Power Station and Proposed Countermeasures” Home page of the Japan Nuclear
Technology Institute “http://www.gengikyo.jp/english/index.html”
Jap.Gov. (2011). “Report of Japanese Government to the IAEA Ministerial Conference on Nuclear
Safety – The Accident at TEPCO's Fukushima Nuclear Power Stations -”Home page of Prime Minister of Japan and His Cabinet, ”http://www.kantei.go.jp/foreign/kan/”
METI (2011). “Additional Report of the Japanese Government to the IAEA (Second Report)”
Home page of Ministry of Economy, Trade and Industry“http://www.meti.go.jp/english/earthquake/nuclear/iaea/iaea_110911.html”
Ninokata, H., Okamoto, K. (2011). “Lessons Learned from Fukshima Daiichi Nuclear Power Plant
Accident” Journal of the Atomic Energy Society of Japan, Vol.53, No.8, 540-545, (in Japanese)
TEPCO (2011). Home page of Tokyo Electric Power Company “http://www.tepco.co.jp/index-e.html”

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