Event
sequences
Fukushima11032011
Event sequencesin Fukushima Dai-ichi and Dai-ni following
the Tohoku-Chihou-Taiheiyou-Oki earthquakeon 11.03.2011

Fukushima
37° 25‘ 26.57“ N, 141° 1‘ 56.87“ E11.03.2011

2|3

Table of Contents
1

Event series at Fukushima Dai-ichi		 4
1.1 Event series in general		 4
1.2 Unit 1			 9		 1.2.1 Reactor		9		
1.2.2 Spent Fuel Pool		 171.3 Unit 2		 18
		1.3.1 Reactor		 18
		1.3.2 Spent Fuel Pool		 221.4 Unit 3		 24
		 1.4.1 Reactor		24
		
1.4.2 Spent Fuel Pool		 271.5 Unit 4		 281.6 Units 5 and 6		 311.7 Common Spent Fuel Pool		 321.8 Dry Storage Cask Facility		 322 Event Sequence at Fukushima Dai-ni		
34
2.1 Event Sequence in General		 342.2 Unit 1			 372.3 Unit 2		 382.4 Unit 3		 392.5 Unit 4		 393 Abbreviations		
42

4 References		44
List of figures
Figure 1 View of Fukushima Dai-ichi, before/after; source: TEPCO						4Figure 2 Tsunami warning 11.03.2011, 14:50; source: JMA 							5Figure 3 Estimated tsunami height 11.03.2011, 14:50; source: JMA						5Figure 4 Comparison of height - estimated and measured; source: Ref. 43 					5Figure 5 Sectional view with heights above sea level, Units 1 – 4; source: VGB 					6Figure 6 Failure of cooling water supplies (Dai-ichi); source: University of Japan					7Figure 7 Failure of auxiliary power supply (Dai-ichi); source: TEPCO						8Figure 8 Diagram of the venting system of Unit 1; source: Ref. 43 						13Figure 9 View of Unit 1 after the hydrogen explosion; source TEPCO						14Figure 10 Fire trucks for sea water injection; source: Ref. 44 							15Figure 11 Fill level records, BWR Example Unit 1; source TEPCO						19Figure 12 Venting system of Units 2 and 3; source: Ref. 43 							20Figure 13 Debris in the spent fuel pool, Unit 3; source Ref.18 							27Figure 14 Units 3 and 4 after the hydrogen explosion; source: TEPCO						28Figure 15 H2 overflow from Unit 3 to Unit 4; source: Ref. 43, Fig. IV 5-10						29Figure 16 Spent fuel pool, Unit 4, on 29.04.2011; source: TEPCO						30Figure 17 Overview Fukushima Dai-ni; source: TEPCO							34Figure 18 Flooding as compared between Dai-ichi and Dai-ni; source: TEPCO					
351 | Event series at Fukushima Dai-ichi

1 Event series at Fukushima Dai-ichi
1.1

Event series in general
Before event occurrence, the following unitsof Fukushima Dai-ichi were in power operation	
  Unit 1
BWR/3, 1380 MWth Mark I,
manufacturer: GE (1967 – 1971)
Unit 2
BWR/4, 2381 MWth Mark I,
manufacturer: GE/Toshiba (1969 – 1974)
Unit 3
BWR/4, 2381 MWth Mark I,
manufacturer: Toshiba (1970 – 1976)
on full power operation
Unit 4
BWR/4, 2381 MWth Mark I,
manufacturer: Hitachi (1973 – 1978)
Unit 5
BWR/4, 2381 MWth Mark I,
manufacturer: Toshiba (1972 – 1978)
Unit 6
BWR/5, 3293 MWth Mark II,
manufacturer: GE/Toshiba (1973 – 1979)
undergoing maintenance, inspection orrefitting (Unit 4, exchange of core shroud).
The operator of this unit is TEPCO(Tokyo Electric Power Company).

Figure 1:
View of Fukushima Dai-ichi, before/after;source: TEPCO

	
  

4|5

Japan Meteorological Agency | Tsunami Warnings/Advisories, Tsunami Information

Tsunami Information (Estimated Tsunami arrival timeand Height)
This Tsunami Information was issued in the pastOccurred at 14:46 JST 11 Mar 2011
Japan Meteorological Agency | Tsunami Warnings/Advisories, Tsunami Information
Page 1 of 2
Region name Sanriku Oki
Depthabout 10 km
Magnitude
7.9
Tsunami Information (Estimated Tsunami arrival timeand Height)

Due to the Tohoku-Chihou-Taiheiyou-Oki
earthquake on 11.03.2011 at 14:46 (magnitude MW=9.0 (moment magnitude scale), an
automatic reactor scram occurred in Units
1 – 3 (see Ref. 1). As a result of the earthquake, there was also a loss of off-site power(LOOP) and therefore, there was a loss of theauxiliary power supply. This led to containment isolation in Units 1 – 3 as well as thestart-up of the emergency diesels (see Ref.
1). Due to this circumstance, the plants werein a state of loss of offsite power shortly after the earthquake. In spite of the high magnitude of the earthquake, the plants reacted,
according to TEPCO (see Ref. 26), as per
design.
On 11.03.2011 at 14:49, the Japan Meteorological Agency (JMA) issued a tsunami warning(see image 2). At 14:50, the first alert concerning the arrival and height of the tsunami
was issued (Fukushima Pref. on 11.03.2011 at
15:10 expected a tsunami of a height of 3 m,see image 3; see Ref. 27). Later, JMA corrected the height of the tsunami (see Fig. 4).

Tsunami Forecast
Region

PACIFIC COASTOF AOMORI PREF.

Estimated Tsunami Arrival

Estimated Tsunami

Tsunami Forecast Region
Time
HeightFigure2: Tsunami warning
11.03.2011, 14:50;
IWATE PREF.
(*1)
3m
MIYAGI PREF. JMA
15:00 JST 11 Mar
6m
source:
FUKUSHIMA PREF.
15:10 JST 11 Mar
3m
CENTRAL PART OF PACIFIC COAST OF
15:30 JST 11 Mar
HOKKAIDO
PACIFIC COAST OF AOMORI PREF.
15:30 JST 11 Mar
IBARAKI PREF.
15:30 JST 11 MarKUJUKURI AND SOTOBO AREA, CHIBA
15:20 JST 11 Mar
PREF.
Tsunami Forecast Region
IZU ISLANDS
15:20 JST 11 Mar
EASTERN PART OF PACIFIC COAST OF
15:30 JST 11 Mar
IWATE
PREF.
HOKKAIDO
WESTERN PART OF PACIFIC COAST OF
15:40 JST 11 Mar
MIYAGI PREF.
HOKKAIDOJAPANSEA COAST OF PREF.
AOMORI PREF.
16:10 JST 11 Mar
FUKUSHIMAUCHIBO AREA, CHIBA PREF.
15:20 JST 11 Mar
CENTRAL
PART OF PACIFIC 16:00
COAST
OF
OGASAWARAISLANDS
JST 11MarSAGAMI BAY AND MIURA PENINSULA
15:30 JST 11 Mar
HOKKAIDO
SHIZUOKA PREF.
15:30 JST 11 Mar
PACIFIC COAST OF AOMORI16:10
PREF.
PACIFIC COAST OF AICHI PREF.
JST 11 Mar
SOUTHERNPARTPREF.
OF MIE PREF.
16:00 JST 11 Mar
IBARAKI
WAKAYAMA PREF.
16:10 JST 11 MarKUJUKURIAND
SOTOBOAREA,
CHIBA
TOKUSHIMA PREF.
16:40 JST11 Mar
KOCHI PREF.
16:30 JST 11 Mar
PREF.
MIYAZAKI PREF.
17:00 JST 11 MarIZU ISLANDSTANEGASHIMA AND YAKUSHIMA AREA
17:10 JST 11 Mar
AMAMI
ISLANDS AND
TOKARA
JST 11OFMar
EASTERNPART
OF ISLANDSPACIFIC 17:10
COAST

1m
1m
2m
m
Estimated2 Tsunami
Arrival
1 Timem
0.5 m(*1)
0.5 11m Mar
15:00 JST0.5 11m Mar
15:10 JST
0.5 m

Estimated Tsunami
Height
3m
6m
3m

0.5 m
15:30 JST11 Mar1m

15:30 JST11 Mar
0.5 m
0.5 11m Mar
15:30 JST

1m
2m

0.5 m
0.5 m

0.5 m
0.5 m
15:20 JST11 Mar
0.5 m
0.5 m
15:20 JST11 Mar
0.5 m
0.5 m

2m
1m

15:30 JST 11 Mar
0.5 m
HOKKAIDO
TsunamiInformation
WESTERN
PART OFNUMBER
PACIFIC 1COAST OF
15:40 JST 11 Mar
0.5 m(EstimatedTsunami
ArrivalTime
andHeight)
HOKKAIDOJAPANSEA COASTOF AOMORI
PREF.
16:10 JST 11 Mar
0.5 m
Issuedat 14:50
JST 11 Mar
2011
UCHIBO3:AREA,
CHIBA PREF.
15:2011.03.2011,
JST 11 Mar 14:50;
0.5 m
FigureEstimatedtsunami height
Estimated Tsunami Arrival Time and Height
OGASAWARA ISLANDS
16:00 JST 11 Mar
0.5 m
source:JMA
SAGAMI BAY AND MIURA PENINSULA
15:30 JST 11 Mar
0.5 m
SHIZUOKA PREF.
15:30 JST 11 Mar
0.5 m
Table III-1-2 Comparison of issuing times, arrival
timesCOAST
and OFheights
estimated tsunami
andobserved one.
PACIFICAICHI forPREF.
16:10 JST11 Mar0.5 m
http://www.jma.go.jp/en/tsunami/info_04_20110311145026.html
10.06.2011
SOUTHERN PART OF MIE PREF.
16:00 JST 11 Mar
0.5 m
Estimated Tsunami Arrival Time and HeightWAKAYAMA PREF.
Observed Tsunami
Timeand Height of Initial and Maximum
Tsunami16:10Arrival
JST 11Mar
0.5 m
TOKUSHIMA
PREF.JST 11 Mar
16:40 JST 11 Mar
0.5 m
Issued at 14:49* JST 11 Mar
Updated at 15:14 JST 11 Mar
Updated at 15:30*
Initial Tsunami
Maximum Height Tsunami（3 minutes after the earthquake） （28 minutes after the earthquake） （44minutes
after the earthquake）
KOCHI
PREF.
16:30 JST 11 Mar
0.5 m
MIYAZAKI
17:00 JST 11 Mar
0.5 m
EstimatedPREF.
Estimated
Estimated
Observed
Observed
Estimated
Estimated
Estimated
Observed Time
Observed Time
Tsunami Arrival AND YAKUSHIMA AREA
Tsunami Arrival
Tsunami Arrival
TANEGASHIMA
17:10Tsunami
JST 11Height
Mar
0.5 m
Tsunami Height
Tsunami Height
Tsunami Height
Tsunami Height
Time
Time
Time
AMAMI
ISLANDS AND TOKARA ISLANDS
17:10 JST 11 Mar
0.5 m
15：30

1m

Arrival of tsunami
confirmed

3m

8m

Hachinohe 15：22( - ) 0.8m

Hachinohe 16：57

4.2m or higher

III-24

Ofunato 14：46( - ) 0.2m

Ofunato 15：18

Arrival of tsunami
Issuedat 14:50
JST 11 Mar
2011
10m or higher
Ayukawa 14：46( + ) 0.1m

Ayukawa 15：26

8.6m or higher

6m

Arrival of tsunami
Estimated
Tsunami
Arrival SomaTime
and Height
10m or higher
14：55( + ) 0.3m
confirmed

Soma 15：51

9.3m or higher

10m or higher

Oarai 15：15( + ) 1.8m

Oarai 16：52

4.2m

10m or higher

Choshi 15：13( + ) 0.5m

Choshi 17：22

2.4m

Arrival of tsunami
inferred3m

Arrival of tsunami
confirmed

6m

MIYAGI PREF.

15：00

6m

Arrival of tsunami
confirmed

10m or higher

3m

Arrival of tsunami
confirmed

15：10

Arrival of tsunami
confirmed

Tsunami Information NUMBER 1
Kamaishi 14：45( - ) 0.1m
Kamaishi 15：21
Arrival of tsunami Tsunami Arrival Time and Height)(Estimated
10m or higher
Miyako 14：48( + ) 0.2m
Miyako 15：26

4.1m or higher
8.5m or higher
8.0m or higher

IWATE PREF.

FUKUSHIMA PREF.

This Tsunami Information was issued in the pastOccurred at 14:46 JST 11 Mar 2011
Region name Sanriku Oki
Depthabout 10 km
Magnitude
7.9

confirmed

confirmed

IBARAKI PREF.

15：30

2m

15：30

4m

Arrival of tsunami
inferredKUJUKURI AND
SOTOBO AREA,
CHIBA PREF.

15：20

2m

15：20

3m

Arrival of tsunami
confirmed

http://www.jma.go.jp/en/tsunami/info_04_20110311145026.html

Reference: JMA (Tsunami Information: Estimated Tsunami arrival time and Height (Issued at 14:50* JST, 11 March 2011))

Figure 4: Comparisonof height - estimated and measured;[Online]. http://www.jma.go.jp/jp/tsunami/info_04_20110311145026.html
source: Ref.JMA
43(Tsunami Information: Estimated Tsunami arrival time and Height (Updated at 15:14 JST, 11 March 2011))[Online]. http://www.jma.go.jp/jp/tsunami/info_04_20110311151439.html
JMA (Tsunami Information: Estimated Tsunami arrival time and Height (Updated at 15:31* JST, 11 March 2011))[Online]. http://www.jma.go.jp/jp/tsunami/info_04_20110311153109.html
JMA (The 2011 off the Pacific coast of Tohoku Earthquake ~14th report~)*Note）Announced time of tsunami warning presented on[Online]. http://www.jma.go.jp/jma/press/1103/13a/kaisetsu201103130900.pdf
this table is slightly different from that on prompt
JMA (Observed values of Tsunami records at Miyako and Ofunato)
reports on JMA web site.
[Online]. http://www.jma.go.jp/jma/press/1103/23b/stn03231400.pdf

1 | Event series at Fukushima Dai-ichi

On 11.03.2011 at 15:27, the first tsunami
reached the plant site. At 15:35, parts of theplant site were flooded by the second tsunami (see Ref. 44). This led to the destruction of the infrastructure and the equipment
as well as to the loss of the ultimate heatsink (see Ref. 1) and a common-cause failure(CCF) of the auxiliary service water supply
and also the destruction of the circulatingwater structure. Furthermore, a commoncause failure (CCF) of the diesel generatorsand thus, the failure of safety-related electrical power-consumers was a direct result of
the flooding. The diesel generators of Units
1 to 5 are all arranged on the lowest level ofthe turbine building. The failure of the watercooled emergency diesels would also haveoccurred indirectly by the loss of cooling(circulating water structure).

The failure of the power source and subsequently, the failure of the diesel generators
sets led to a total loss of AC power and thus
to a station blackout (SBO).
On 11.03.2011 at 15:42, a state of emergency
was declared at Fukushima Dai-ichi (see Ref.
44).
By means of photographs dated 11.03.2011
at approximately 15:57 it can be seen thatthe water had already withdrawn to a large
extent from the plant site (see Ref. 31). Thismeans that the flooding lasted for about 22
minutes.

Meerwasserpumpe

Tsunamihöhe

Auslegung

Notstromdiesel
Figure 5:
Sectional view with heights above sea level, Units 1 – 4;
source: VGB

Schutzwall

6|7

In 2002, the tsunami protection wall of
Fukushima Dai-ichi had been raised from theoriginal height of 3.1 m (design tsunami: Chile
1960 magnitude 9.3 m, height in Japan 3.2m) to the maximum wave height of 5.7 m. It
should be noted that the foundation level is
set to 10 m resulting in an additional 4.3 m
safety margin for the equipment located atthat height.
After data evaluation of the on-site analysis,
TEPCO reported that the RPV as well as theprimary containments of Units 1 – 3 weredamaged due to the accident (see Ref. 33
and Ref. 42). On 23.03.2011, the temperaturein the primary containment of Unit 1 was still
approximately 400° C (see Ref. 33).
Annotation 1
A diverse power source (off-site supply, external emergency power system, etc.) conducted via different cable routes could haveminimised the consequences of the accident.
Annotation 2
A diverse water supply (wells, reservoirs,etc.) could have minimised the consequencesof the earthquake.
Annotation 3
A secured physical separation within thecirculating water structure could have minimised the consequences of the earthquake.

There is no direct reference to the design
within the framework of these reviews. However, the design and backfitting of the plantcan be indirectly questioned when looking at
various examples of “Operating Experience”(OE), for example in the IRS-reports (for example 7342 or 7788) in connection with the
OSART missions or via the Significant Ope­rating Experience Reports (SOER) about important operating experiences of the member companies in connection with the WANO
missions. WANO member companies holdthemselves accountable to specifically review the recommendations of WANO – SOER
and to survey the actions where appropriate, derived from this review, while doing thepeer reviews. This is an obligation within theoperating company. However, there is no obligation to inform the public. It is slightly different within OSART since there is no formal
obligation for the plant to implement all possible actions derived from the peer reviews interms of safety. Within the frame-work of theperiodic safety review, the state of the plantshould be reviewed every 10 years. There arealso different international procedures andstandards. These three instruments together
could enhance the safety level of nuclear
plants within the framework of an obligatory,
continuous process.

Annotation 4
A better flood protection of the emergency
diesels and associated cooling systems could
have minimised the consequences of theearthquake.
Annotation 5
The reviews conducted within the frameworkof the OSART- and WANO-missions shouldhave detected the vulnerabilities in the plant.

Figure 6:
Failure of cooling water supplies (Dai-ichi);source: University of Japan

1 | Event series at Fukushima Dai-ichi

Figure 7:
Failure of auxiliary power supply (Dai-ichi);source: TEPCO

8|9

1.2

Unit 1

1.2.1

Reactor

Following the tripping of the reactor (SCRAM)
and the loss of offsite power (LOOP) at 14:47
in Unit 1 (see Ref. 44), residual heat was removed in accordance with the specifications
using an isolation condenser (IC, Ref. 2),starting automatically at 14:52 (see Ref. 44).
Reactor core isolation cooling (RCIC) via
the high-pressure coolant injection system(HPCI) was available for coolant make-up
at decreasing RPV level (Ref. 44). Due tothe high temperatures transients (a pressure drop from 70 to approx. 45 bar) approximately 10 minutes after the onset of theevent, the isolation condenser was switched
off manually at 15:03 (see Ref. 44) in order toensure that the maximum cooldown gradientfrom Δt > 55 K/h met the prescribed standards. A train (Train A) of the IC was used for
RPV pressure control. According to TEPCO,
there was no loss-of coolant accident(LOCA) as a direct result of the earthquake.
(see Ref. 26). This assumption was probably
based on the severe drop in pressure.
As a consequence of taking the isolation
condensers out of operation, there was anincrease in pressure, with reactor pressure
limitation at 60-70bar. The RPV was left on
the high pressure path (hot shutdown) (Ref.
44). Therefore, the plants operated in thehigh pressure range and the required coolant
injection took place via the HPCI. The reactor
water level did not, however, reach the automatic actuation criteria (LL 148cm below thelower edge steam separator, see Ref. 43).
On 11.03.2011 at 15:35, the second tsunami
damaged the circulating water intake, leading to the loss of the ultimate heat sink (mechanically). Additionally, there was a failurein the emergency power diesels (flooding),

which lead to an electrical failure of the
trains necessary for residual heat discharge
to the ultimate heat sink (sea water cooling system), and at 15:37 a station blackout(SBO) occurred (see Ref. 43). Thus there
was only the torus available for taking up theresidual heat. (The isolation condenser had
been taken out of operation manually andcontinued to be operated manually before
the tsunami-induced flooding).
As a result of the simultaneous loss of theemergency power supply from Units 1 and
2 (diesel and interconnections), the standby
from these units was also unavailable (seeSection IV-39, Ref. 43).
Due to the unavailability of the signals in thecontrol room, Tepco assumed a loss of function of HPCI. The signal of the valve positionof the IC was also no longer showing (Ref. 44).
Owing to the continuing decay heat, the lost
isolation condenser and the failed coolant
injection, the RPV level began to drop, resulting in the uncovery of the fuel assemblies.
The pressure was probably limited by safety
relief valve (SRV) discharging into the pressure suppression pool (torus). More detailed
information, however, is not currently available. This led to a temperature increase in thetorus as a result of the decay heat yet to be
removed, and as boiling point was reached,
there was a pressure increase in the torus
and drywell. Whether there was an automatic depressurisation of the RPV due to the
low coolant level is currently unknown. Dueto the disturbance or failure of the battery
supply it is assumed that there was a fault in
the instrumentation and control functions.
(Failure of the strip chart recorder of thecore monitoring system).

1 | Event series at Fukushima Dai-ichi

Annotation 6
It has to be clarified whether manual operation in the first 30 seconds was appropriate
or not. The manual operation ‘deactivationof the isolation condensers’ caused the interruption of RPV pressure relief. The plant
returned to the high pressure path. The aim
in a radiological emergency must be to bring
the plants into a state of low pressure where
it is flooded and thereby able to be cooled.
As the alert warning was of an estimated
3-m-high tidal wave (bearing in mind that the
Fukushima Dai-ichi power plant is locatedon the border to the Maiyagi region, which
had an alert of a 6-m-high tidal wave (seeRef. 27), there was no direct need for quick
RPV pressure relief and RPV flooding. This
would surely have lead to a less severe event
sequence. The tsunami warnings of JMA were
updated at 15:14 and 15:30 (see Fig. 4).
The connection to the power supply vehicle forbattery support did not work straight away;
access was made difficult by debris (see Ref. 6).
Annotation 7
It was not possible to carry out emergency
actions immediately. The necessary power
supply vehicles were available and ready for
operation. One reason for the delay could be
due to damaged switchgears. However, a lackof adequate emergency training could also
be a reason as TEPCO arranged for training
in this area for all the plants shortly after theaccident (e.g. Dai-ni, see Ref. 41).
On 11.03.2011 at 15:50, the power supply of
the instrumentation was lost (see Ref. 44).
On 11.03.2011 at 16:36, for approximately 10
minutes, there was a temporary failure in themonitoring of the RPV level (see Ref. 28).
At 17:07, monitoring of the RPV level was lostcompletely (see Ref. 44).

Annotation 8
The reason for the failure of the RPV levelmeasurement could be due to the problems
with the power supply. If the RPV levelmonitor had functioned again, the low levelshould have been noticed. It is not known
whether a quick manual depressurisation of
the RPV was executed. As the RPV level atthis point must have already been low, pressure relief should have been carried out according to the procedures. Emergency procedures should be in place for the case thatelectrical actuation of the safety relief valves
SRV is no longer possible. Pressure relief isnecessary for the emergency measure ‘external water supply’.
On 11.03.2011 at 17:12, the emergency teamdiscussed an alternative water supply to
the RPV via fire protection lines of the firefighting system into the core spray system.
At 17:30, a diesel-powered fire pump wasavailable as stand-by, the associated valvesof the core spray system were set manually.
RPV injection is possible from a pressure of
approximately 6.9 bar (see Ref. 44).
On 11.03.2011 at 18:18h to 18:25, one train (A)of the isolation condenser was opened(see Ref. 44).
On 11.03.2011 at 20:07 the RPV pressure wasmeasured locally as being 69 bar(see Ref. 44).
On 11.03.2011 at 20:49, lighting in the main
control room was temporarily re-established(see Ref. 44).
On 11.03.2011 at 21:00, stand-by water supply(to the RPV) with more fire-fighting pumps
was established (see Ref. 6).

10 | 11

On 11th March 2011 at 21:19h, the DC supply
to the RPV level measurement was temporarily available, at the point in time the level
display showed +200mm above the top of
active fuel (see Ref. 44).

(flooding of emergency diesels, damage tothe circulating water building). Moreover,contrary to the measure of containment
venting, it was probably the operator’s responsibility to execute these actions.

On 11.03.2011 at 21:30, one train (train A) of
the isolation condenser was opened (see Ref.
44). It is not completely clarified if this measure was sufficiently effective. According to
the JAIF Earthquake Report dated 18.08.2011
the IC was manually shut down approx. 5hours after the event without the lead engineer knowing (see Ref. 47).

On 11.03.2011 at 21:51, entering the reactor
building was prohibited due to high local
dose rates. At 23:00, the local dose rate wasmeasured at 1.2 mSv/h in the turbine hallroom at the entrance to the reactor building(double door, northern side, 1st floor; Ref. 44).

On 11.03.2011, TEPCO assumed that at 21:40
the level reached top of active fuel and thatat around 22:30h, first core damage would
have to be expected (see Ref. 6). Based on
this assumption, the government ordered the
2-km evacuation zone around Unit 1 of theDai-ichi plant on 11.03.2011 at 20:50. (see Ref.
3). According to new assessments (see Ref.
33), the level reached the top of active fuelas early as 3h after SCRAM at approx. 17:50
and the bot-tom of reactor core 4.5h after
SCRAM at approx. 19:20.
On 11.03.2011 at 21:33, as a result of theevents in Unit 1, the government ordered theestablishment of a 3-km evacuation zone andadvised the population within a 10-km zone
around the Dai-ichi plant to stay indoors(see Ref. 3).
Annotation 9
Why the injection of water into the RPV withan additional coolant could not be initiatedhas not been completely clarified. At thispoint in time (approximately 5hrs after theplant was flooded), all the necessary supplies had already failed. In addition, a quick
repair of the power supply and water coolingsystems was not possible for the time being

On 11.03.2011 around midnight and in theearly morning of 12.03.2011, several power
supply vehicles from different power plants
arrived (see Ref. 44). As a result of traffic
jams and the damaged roads and infrastructure, a delay was caused in the arrival of several high- and low-voltage supply vehicles.
Attempts to transport the machines witharmy helicopters failed due the weight of theauxiliary equipment (see Ref. 44).

1 | Event series at Fukushima Dai-ichi

On 11.03.2011 at 00:49, the pressure build-up
began in the primary containment (according to Ref. 44 the pressure in the drywell(DW) at this point in time was >6 barabs(11.03.2011 at 23:50). This took place approximately 7 hours after the flooding by thetsunami. At this point in time, the decay heat
was still approximately 0.8 % of the thermal
rated power of 1380 MW. There was no RPV
cooling. The decay heat was probably led
directly into the pressure suppression pool
(torus) via the pressure relief valves. As heatremoval from the torus to the ultimate heatsink of the sea was no longer a possibility,
the pressure in the primary containment(drywell) rose continually. Also, as no coolant was injected into the RPV, the RPV level
dropped.
The oxidation of the covered zirconium fuelrod cladding tubes of the fuel elements invapour atmosphere resulted in the forming of hydrogen (Zr + 2 H2O → ZrO2 + 2 H2).
During the equalisation of pressure, the hydrogen was transported via the torus into
the drywell and accumulated in the area ofthe drywell head. Due to leakages, probably
furthered by the high drywell pressure, theaccumulation of hydrogen (H2) took place inthe reactor building (see also Ref. 43). The
pressure in the drywell rose above the designpressure of 50-60 psig (approximately 3.454.14 bar, see Ref. 34). Presumably the drywellhead failed. However, there is currently noconclusive proof of this.
On 12.03.2011 at 01:48 there was a failure of
the diesel-powered fire pump (on stand-by).
After refilling and changing of the starter
battery, the re-start remained unsuccessful.
The connection of fire engines to the fire
water system was prepared. Owing to thedestruction of a vehicle as a result of the tsunami and owing to the impossibility to drivealong the connecting roads to Units 5 and 6,
there was only one vehicle available at first(see Ref. 44).
The extreme high pressure in the primarycontainment that had reached approximately
double the design pressure made the pressure relief from the primary containment(venting) necessary. This is necessary in order to prevent an uncontrolled overpressurisation failure in the primary containment.
On 12.03.2011 at 01:30, clearance for containment venting was requested from the prime
minister, NISA and METI. Permission wasgiven at 03:00 (see Ref. 44), with conditions
on the part of METI and the task force headquarters with regard to the evacuation.
At a press conference at 03:06 h on the partof TEPCO, the preparation of filtered venting
was announced (see Ref. 3). According todiagrams from NISA and JAIF, the pressure in
the primary containment was already above
design pressure and had reached levels between 6 and 8.4 bar (DW-pressure is confirmed at 8.4 bar at 02:30, see Ref. 44).
On 12.03.2011 at 05:00, it was ordered thatfull face masks with active carbon filters had
to be worn by the staff in the control room.
Due to the rising local dose rate in the control room of Unit 1, the control room staff
was relocated to Unit 2 (see Ref. 44).
On 12.03.2011 at 05:44, the government decreed a 10 km evacuation zone around thepower plant (see Ref. 3).
On 12.03.2011 at 08:03, the plant director
announced the time set for the start of theventing for 09:00.

12 | 13

On 12.03.2011 at 09:04, 2 operators were sent
to manually open the main valve upstreamof the rupture disk (MO) to a degree of 25%. This took place at 9:15. At 9:24, a second
group was sent to open the “small” valve forpressure relief of the torus (AO90). Due tothe high local dose rate (LDR), this work was
stopped at approximately 9:30. In the scopeof this preparatory work for the venting, one
worker obtained a radiation dose of > 100
mSV (see Ref. 3). The manual opening on site
was interrupted because of the high LDR. It
was tried to open the valves from the control
room with compressed air. Between 10:17 and11:15, it was tried to open valve AO90 (seeRef. 44). Thereupon on 12.03.2011 at 14:00,
a compressor for the pressure relief of thetorus was connected in order to open the“large” valve AO72 (see Ref. 44).
According to Ref. 4 pressure relief was successfully carried out on 12.03.2011 at 14:30.
According to Ref. 44, the pressure of thedrywell decreased from 7.5 bar to 5.8 bar.
The reason for the relatively long time difference between the preparation and thestarting of the venting is partially technical(preparing, obtaining materials, exercising)as well as administrative (evacuation, communication, clearance).
Annotation 10
The venting system at the GE plants is designed as an “active” system. Auxiliary power
is needed to open the valves. Due to theeffects of the flooding, auxiliary power nolonger existed. Due to the high activity (thisalso applies for the loss-of-coolant within thedrywell), manual opening became problematic. Presumably, there were no measures forthe reduction of radiation exposure for theon-site options to open the venting valvesmanually. Apparently, a sufficiently dimensioned recombiner for the case of a core

meltdown did not exist. Even if it had existed,(electrical) auxiliary power would presumably
have been required.

Figure IV-2-13 Overview of PCV Venting Facility (Unit 1)
Figure 8:
Diagram of the venting system of Unit 1;
source: Ref. 43

According to TEPCO (see Ref. 44), up until
12.03.2011, 14:53, approximately 80,000 litres offreshwater were injected into the reactor via thefire extinguishing line and a mobile pump. This
volume does not cover the refill volume required
for residual-heat removal.

Figure IV-2-14 Overview of PCV Venting Facility (Units 2 and 3)

IV-27



1 | Event series at Fukushima Dai-ichi

On 12.03.2011 around 15:30, the power supplyof the emergency borating system (highpressure system) was established via theconnection of a power supply vehicle to thehigh voltage distribution of Unit 2. At 15:36,
the preparation of water injection via theemergency borating system was completed(see Ref. 44).
On 12.03.2011 at 15:36, a hydrogen combustion occurred in the upper part of the reactor building, damaging the building structure. At this moment it is not clear how the
combustion of the hydrogen in the reactor
building occurred. It is assumed that an aftershock (see Ref. 5) triggered sparks that
led to the ignition of the hydrogen. Anotheroption for the ignition source might havebeen the re-established power supply (shortcircuit, overvoltage) of the emergency borating system.
The detection of caesium on the plant after
venting (12.30.2011 at 14:49, see Ref. 1) as
well as the increase of the dose rate after
the explosion are evidence of the fuel damage or of core meltdown. Currently, TEPCO
assumes that at approximately 6:50 on
12.03.2011 already, nearly all fuel assemblies
were affected by the core meltdown (seeRef. 10). According to the latest assumptions(see Ref. 33), all fuel assemblies melted 16
hours after SCRAM. The melt gathered in thelower part of the RPV.
On 12.03.2011 at 18:25, the evacuation of a
20-km radius around the plant and the instruction to shelter indoors within a 30-km
radius was issued as a consequence of thehydrogen explosion and the increase of thelocal dose rate (see Ref. 1). According to Ref.

Figure 9:
View of Unit 1 after the hydrogen explosion;
source TEPCO

1, on 25.03.2011 the government advised the
population within a 30-km radius around theplant to evacuate; this was however, theirown decision.
On 12.03.2011 at 19:04, injection of sea waterby means of three fire trucks connected inseries (see Figure 10) began.

14 | 15

On 12.03.2011 at 20:45, the injection of borated sea water was started by means of thefire-fighting pipes in the RPV (see Ref. 3).
Currently, it is not clear why there was a relatively long time lag between the readinessfor injection and the start of the injection. It
is believed that this time lag was due to administration and that sea water injection wasstarted on 12.03.2011 at 20:05, after the issue
of a directive on 12.03.2011 at 19:55 (see Ref.
3) and its release by the Nuclear and Industrial Safety Agency (NISA). Due to insights
gained from Ref. 43 and Ref. 44, sea water
injection was conducted without a regulatory
directive. According to Ref. 43, this is attributed to insufficient communication. Currently, it is unknown whether sea water injection
is part of the Severe Accident Management
Guidelines (SAMG).
Annotation 11
The long time lags concerning the ‘water
injection’ and delay during ‘venting’ cannot
be fully explained. If this was due to the longdecision-making process, it is to be checked
how decision-making is regulated in ‘extreme
situations’. At what point are the employees
on site allowed to make their own decisions?
How can it be ensured that decisions are
made in sufficient time?
On 20.03.2011, the auxiliary power supply
could be restored via Unit 2 (see Ref. 40).
Permanent lighting in the control room wasrestored on 24.03.2011 at 11:30 (see Ref. 12).
From the point of view of defence in depth,
there are the following essential events in
Unit 1:•

The need for manual interventions in safety systems 10 min after the occurrence ofthe event needs to be questioned. The 30
min-criterion is the international standard(from the TMI-event of 1979, no operator
intervention before 30 minutes).

Figure 10:Fire trucks for sea water injection;
source: Ref. 44

1 | Event series at Fukushima Dai-ichi

•

•
•

•

Loss of offsite power and sustained ‘highpressure mode’ (the goal is to get theplant into shutdown cooling mode andnot to remain ‘hot standby’). A standard
simulator training program includes theloss of offsite power and also the station
blackout scenario. We currently do notknow what kind of training programs andcurricula exist in nuclear power plants inJapan. Furthermore, it is unknown to us
whether TEPCO has plant-specific simulators for its nuclear power plants.
Flooding by the tsunami – beyonddesign-basis event.
After the flooding of the plant, essential
and correct emergency actions weretaken or prepared. Shortly after the flooding, the battery supply was ensured andthe injection into the RPV was prepared
6 hours after the event happened. There
were many problems during the carryingout of the emergency actions taken toconnect the ‘power supply vehicles’. The
injection into the RPV could not be sufficiently carried out before the top of active fuel was reached although the loss of
the ultimate heat sink (nuclear auxiliary
service water) and the total loss of theemergency diesels must have been known
to every-one at this point in time.
Pressure build-up in the primary containment occurred due to the lack of coolant
injection and the heat removal that was
still required as a result of the decay heatand metal oxidation. Again, as in the caseof the water injection, there were delays
until the venting was carried out.

This means that 3 technical and organisational barriers in the framework of defence
in depth for the prevention or minimisationof accident consequences failed after theflooding the plant. Currently, we do not know
to what extent cross-functional actions thatare due to the ‘multi-unit plant’ (shift, emergency team, maintenance) may have been
crucial. The indepth analysis was performedin the framework of an interim report about
the Fukushima events (see Ref. 47).

16 | 17

1.2.2

Spent Fuel Pool

Caused by the station blackout, loss of cooling for the spent fuel pool occurred in Unit
1. Following the hydrogen combustion eventon 12.03.2011 at 15:36 in the reactor building,the spent fuel pool was in direct communication to the outside environment. The lackof cooling caused an increase in the watertemperature and a drop in the water level inthe spent fuel pool due to evaporation. The
last fuel exchange for Unit 1 took place from27.09.2010 during the refuelling outage of2010.
On 31.03.2011 from 13:03 to 16:04, freshwater was injected for the first time into thespent fuel pool by a truck-mounted concrete
pump.
Nothing is known about the condition of the
392 FA and the spent fuel pool. It is also not
clear whether parts of the loading machineor parts of the crane fell into the pool after
the explosion or earthquake. Owing to thehigh masses of debris (Fig. 9), no conclusionon the present condition is possible.

1 | Event series at Fukushima Dai-ichi

1.3

Unit 2

1.3.1

ReactorAfter the reactor SCRAM at 14:47 and theblackout caused by the loss of power, residual heatremoval was carried out, according to
the design basis after manual actuation of theturbinedriven Reactor Core Isolation Cooling
System (RCIC). This is a system operated by
the steam generated in the reactor and does
not need any AC power supply. Due to thesignal ‘reactor level high’, the RCIC systemshut down automatically and repeatedly. A
water injection from the torus and from thecold condensate tank rendered the continuation of this operation mode possible until
14.03.2011 at about 12:00 (see Ref. 43). Thenat 13:25, the loss of the reactor cooling function was reported (see Ref. 7).

On 12.03.2011 at 02:55, it was confirmed that
the RCIC was in operation, and priority ofpressure relief was concentrated on Unit 1.
According to Ref. 43, the coolant injection
from the condensate tank was changed to injection from the torus. On 12.03.2011 at 17:30,
the plant director ordered to start preparations for pressure relief (see Ref. 44).
On 13.03.2011 at 08:10, the main valve upstream of the rupture disk (MO271) opened
up to 25 % (see Fig. 12). At 11:00, the ‘large’
valve (AO205) upstream of the torus was
opened for pressure relief, thus pressure reliefof the primary containment was actuated upstream of the rupture membrane. According
to Ref. 44, the actuation pressure amounted
to 4.27 bar (over-pressure), thus making it
5.28 barabs.

On 11.03.2011 at 15:35, the tsunami caused thedestruction of the cooling water inlet andthus the loss of the ultimate heat sink (mechanically). Moreover, the loss of the emergency power supply (by the flooding) caused
the loss of the trains necessary for residualheat removal to the ultimate heat sink (seawater cooling) as well as the station blackout(SBO). This meant that only the torus wasavailable for residual-heat removal.

Caused by the failure or loss of the batteries, afailure in the I&C functions must be assumed(loss of the reactor core monitoring records).
Moreover, particular attention must be paid
to the RPV level records. As these are differential-pressure measurements, evaporations
or sub-cooled boiling in the measuring pillars
could be assumed. At present it is unknown
whether diverse measurements (like reactor
core internal temperature) were available.

On 11.03.2011 at 15:39, 4 minutes after thesecond tsunami wave, the RCIC was manuallyactuated (see Ref. 44).

It is essential to note that according to theinformation available, water was injected
into the reactor pressure vessel (RPV). The
amount of the water injected was insufficient.

On 11.03.2011 at 20:49, lighting in the main
control room was temporarily re-established.
At 21:50, the coolant level in the RPV wasmeasured as +3400 mm above the top of
active fuel, then at 23:25 the pressure in thedrywell was 1.41 barabs (see Ref. 44).

18 | 19

Annotation 12
Problems with the signal displays duringtransients are not new. This was the reason
to partly upgrade the in-core instrumentation of the reactor core internal temperature
measurement. It is essential to know whether
the accident management instrumentation
was available and provided the necessary
quality of the information.

Shutdown zone
water level gauge+10000 mm
Narrowband
water level
gauge

Narrowband
water level
gauge+1500 mm+1500 mm

Fuel zone
water level gauge+5000 mm
+1730 mm+3430 mm

0 mm

On 14.03.2011 at 11:01, the explosion in Unit
3 caused the closure of the ‘large’ valve(AO205) upstream of the torus for the pressure relief as it had lost its actuation (pressurised air). Moreover, the explosion caused aloss of the prepared feed water injection line
and damaged the necessary fire extinguishing vehicle (see Ref. 43 and Ref. 44). The
drywell pressure amounted at that time to
4.5 barabs (3.5 bar overpressure).
On 14.03.2011 at 12:30, the wetwell pressure (in the torus) amounted to 4.86 barabs,
the temperature in the torus amounting to149.3 °C (see Ref. 44).

+1700 mm-1500 mm

0 mm

-3000 mm

Reactor water level gauges, Unit 1

Reactordruckgefäss

Kondensationsgefäss

H-h

On 14.03.2011, in order to avoid a hydrogen
explosion, preventive openings were made in
the roof of the reactor building (see Ref. 3).
After the loss of reactor cooling and coolant
supply at 13:25, it was estimated that coolant
level would reach ‘top of active fuel’ at 16:30
on 14.03.2011. It was in fact reached at 17:17(see Ref. 44).

Hh

Barton-Zelle

On 14.03.2011 at 16:34, after RPV pressure relief, arrangements for sea water injection into
the RPV began (see Ref. 1).
Due to the RPV pressure relief at 18:03 (60.7bar), RPV pressure decreased until about
19:03 up to 6.3 bar (see Ref. 44), while the
RPV level dropped to -3700 mm below the

Figure 11:
Fill level records, BWR Example Unit 1;
source TEPCO

1 | Event series at Fukushima Dai-ichi

Figure 12:
Venting system of Units 2 and 3;
source: Ref. 43

top of active fuel at 18:22 (see Ref. 43), and asignificant pressure increase occurred in the
primary containment (drywell).
On 14.03.2011 from 19:20 until 19:54, the lossof the sea water injection to the RPV occurred due to a lack of diesel fuel of the fire
extinguishing vehicle (see Ref. 44). In thetime from 13:25 until 19:54, no coolant injection to the RPV took place for about 6 hours.
According to the assessment of the IAEA, thecoolant level in the RPV probably reached
the core bottom level (bottom of active fuel –BAF) about 76 hours after the tsunami(see Ref. 46).

On 14.03.2011 at 21:00, the ‘small’ valve(AO206) upstream of the torus was opened
to relieve pressure (see Ref. 44). In the period between 21:20 and 22:50, RPV pressure
relief was again carried out by safety relief
valves (SRVs). The drywell pressure was lower
at 22:50 than the actuation pressure of the
rupture disk (4.27 bar as overpressure). The
drywell pressure at that moment was stablewithin the range of 3 to 4 barabs (see Ref. 44).
On 15.03.2011 at 00:02, the ‘small’ valve(AO208) upstream of the drywell was opened
for some minutes. According to Ref. 44 it was
confirmed that the rupture disc was not open.

20 | 21

The drywell pressure remained at 7.5 barabs(see Ref. 44). Another attempt to relievepressure at 03:00 was unsuccessful.
On 15.03.2011 at 06:14, a hydrogen explosion
occurred within the reactor building. Due tothe falling pressure in the torus afterwards,
the explosion is assumed to have occurred in
the area of the torus (see Ref. 3).
The root cause of the explosion is assumed
to be the insufficient coolant supply, resulting in the fuel tubes oxidation in the reactor
core. The released hydrogen was transportedby the RPV pressure relief into the torus. The
question if the torus had already been damaged by the harsh condensation (hot steam
injection into the already boiling water), or
if the torus was damaged by the explosion
cannot be clarified at the moment. It must be
assumed that still more hydrogen was forced
into the torus due to the high pressure in
the drywell and the rup-ture disk remaining closed or the valve (MO271) upstream of
the rupture disk remaining closed (see Fig.
12). Should the valve MO271 have remained
closed, a failure of the pipe, caused by the
overpressure with following hydrogen combustion, could have been the cause of the
explosion.
On 15.03.2011 at 11:25, the drywell pressureamounted to 1.55 barabs (see Ref. 44).
On 20.03.2011 at 15:46, it became possible tore-establish the off-site power supply(see Ref. 35).
On 20.04.2011, an inspection of the site by
robots found a high temperature and humidity within the reactor building (see Ref. 29),
which could have been caused by the damage of the torus (see Ref. 3).

Annotation 13
The insufficient coolant replenishment suggests insufficient coolant injection. The question whether the necessary water injection
amounts were specified in the Severe AccidentManagement Guidelines (SAMG) or how they
were verified is yet unknown. Moreover, it is .
Annotation 14
The long delay prior to ‘venting’ is notclearly explainable. If it was due to the longdecision-making process, it must be checked
how the decisions are made in ‘emergency’situations. At what moment can the personnel make their own decision? When are theauthorisations by other persons responsible
necessary? How is it ensured that decisions
will be made in due time?

1 | Event series at Fukushima Dai-ichi

1.3.2

Spent Fuel Pool

The station blackout (SBO) caused a loss of
coolant supply for the spent fuel pool at Unit
2. Due to the decay heat of the fuel, watertemperature in the spent fuel pool increased
and the level dropped. The last fuel exchange
for Unit 2 took place starting on 18.11.2010
during the refuelling outage of 2010.
On 20.03.2011 beginning at 15:05, sea water was injected for the first time into thespent fuel pool via the coolant line. From
29.03.2011, this was changed to fresh-water
injection (see Ref. 3).
On 16.04.2011, water samples were taken fromthe spent fuel pool of Unit 2. The samples
indicated nuclides of I-131, Cs-134 and Cs-137(see Ref. 30). It is yet unknown if the presenceof these nuclides can be put down to the damage of the fuel. Nothing is yet known about
possible damage of the spent fuel pool or.

Annotation 15
The insufficient auxiliary water supply leads
to the conclusion that the injection amounts
were insufficient. It is unknown whether theappropriate water injection amounts are
specified in the Severe Accident Management
Guidelines (SAMG) and how they were verified. Moreover, it is unclear if sufficient material for the necessary accident management
was in place at the site. It is unclear whether
the necessary measuring displays (like pooltemperature, coolant level) were available.

Fukushima
37° 25‘ 26.57“ N, 141° 1‘ 56.87“ E11.03.2011

1 | Event series at Fukushima Dai-ichi

1.4

Unit 3

1.4.1

ReactorAfter the reactor scram (SCRAM) at 14:47, themain turbine was manually shut down (seeRef. 44). Due to the station blackout causedby the loss of off-site power, residual-heatremoval from Unit 3 was carried out as per
design by the manually actuated reactor coreisolation cooling system (RCIC) and then by
the high-pressure coolant injection system(HPCI). Actuated by the signal “reactor levelhigh“, the system RCIC repeatedly shut down
automatically.
On 11.03.2011 at 15:35, the tsunami caused
the destruction of the cooling water inlet
and thus the loss of the ultimate heat sink(mechanically). Moreover, the loss of theemergency power supply (due to flooding)caused the loss of necessary residual-heatremoval trains to the ultimate heat sink (seawater cooling) as well as the station blackout(SBO) at 15:38 (see Ref. 43). This meant that
only the torus was available for residual-heatremoval.
On 11.03.2011 at 16:03, the RCIC was manuallyactuated. On 11.03.2011 at 21:58, lighting in themain control room was temporarily re-established. On 12.03.2011 at 11:36, the RCIC shut
down automatically; while the HPCI started
up automatically at 12:35, actuated by the lowlevel in the RPV (see Ref. 44).
On 12.03.2011 in the time between 12:30
and 19:00, the reactor pressure decreasedby more than 60 bar and became stable atabout 10 bar (see Ref. 43). It is assumed that
due to the strong pressure decrease, a freshsteam leak occurred in the HPCI system(see Ref. 43).

Due to the residual-heat removal to the torus
and after the loss of the function ‘pressure
relief to the torus’, the pressure in the primary containment increased. TEPCO began
preparations for the pressure relief from theprimary containment (see Ref. 4).
On 12.03.2011 at 17:30, the plant director gave
the instruction to begin arrangements forpressure relief (see Ref. 44).
On 13.03.2011 at 02:42, the HPCI shut down
and at 05:10, TEPCO announced that all injection functions had been lost (see Ref. 8). In
the time between 02:30 and 04:00, the pressure in the RPV increased from about 10 bar
up to about 70 bar. The assumption of a freshsteam leak in the HPCI system was confirmed
by a new pressure increase in the RPV.
From then on, pressure relief was only via the
SRVs. At the moment of the loss of the HPCI,
no level signals were available. At 03:51, DC
power was re-established for the instrumentation, with the coolant level being signalled-1600 mm below the top of active fuel – TAF)
(see Ref. 43).
TEPCO estimated that the RPV coolant level
would reach the top of active fuel at 04:15 on
13.03.2011 (see Ref. 44). According to Ref. 45,
the level dropped below the top of active fuelat about 08:00.
On 13.03.2011 at 05:15, the plant director gave
the instruction to start arrangements forpressure relief up to the rupture disk (see Ref.
44). At 05:23, the ‘large’ valve AO205 wasto be opened for pressure relief in the torus;
however, while the necessary auxiliary energy(pressurised air) was not available. Following
the switch-over of the pressurised air tank,
it became possible to open the valve(see Ref. 44).

24 | 25

On 13.03.2011 at 07:39, the containment
spray function was actuated (see Ref. 44).
At 08:35, the main valve MO271 upstreamof the rupture disk opened as per design by
15 % (see Ref. 44). On 13.03.2011 at 08:41, thearrangements for the pressure relief of the
primary containment (drywell) were completed (see Ref. 36 and Ref. 44).
On 13.03.2011 at 09:20, after the manuallyactuated RPV pressure relief (at 09:08), theinjection of borated freshwater (until about
12:20) and then injection of freshwater by thefire extinguishing line was initiated (at 13:12)(see Ref. 3 and Ref. 44). In the time between
02:42 and 09:20, i. e. for about 7 hours, no
coolant injection into the RPV was in place(see Ref. 46).
On 13.03.2011 at 09:24, the drywell pressure dropped from 6.37 barabs (at 09:10) to
5.4 barabs, with TEPCO assuming successful
venting (the rupture disk opened). Due tothe pressurised-air leakages, valve AO205
closed, while after the switch-over of thepressurised-air tank, the valve could be reopened at 12:30. Due to the continuous relief
by SRVs and the resulting high room temperature and strong vibrations in the torus,
the valve could not be interlocked manually(see Ref. 44).
On 13.03.2011 at 15:28, a dose rate of 12
mSv/h was measured in the control room,
the operators were evacuated to Unit 4 (seeRef. 44).
On 13.03.2011 at 17:52, a temporary compressor was connected to the pressurised-air
supply. The compressor was inserted into
the reactor building by a crane via the material airlock. Due to the decreasing pressurein the drywell (20:10), TEPCO assumes thatthe ‘big’ venting valve of the torus (AO205)
could be opened by the compressor. After arenewed loss of the pressurised-air supply,
the valve closed, and pressure in the dry-wellincreased from 2.65 bar (on 14.03.2011 at
02:00) to up to 3.15 bar (at 03:00)(see Ref. 44).
On 14.03.2011, in the time between 01:10 and
03:20 (see Ref. 3) the sea water supply had
to be discontinued owing to the low level inthe interim tank.
Despite the later continuing water injection,
the level in the RPV dropped and the pressure increased. This was possibly due to an
insufficient amount of water injected. Due tothe high activity level in the drywell the coredamage possibility was estimated as 30 %(see Ref. 43).
Annotation 16
The insufficient water supply led to theconclusion that the injection amounts wereinsufficient. It is unknown whether the appropriate water injection amounts are specified
in the Severe Accident Management Guidelines (SAMG) and how they have been verified. Moreover, it is unclear whether sufficientmaterial for the necessary accident management was in place at the site.

1 | Event series at Fukushima Dai-ichi

Annotation 17
The question whether the accident instrumentation was still available or whether thenecessary quality of information was ensured
cannot be clarified conclusively.
On 14.03.2011 at 05:20, preparations for thefurther pressure relief of the primary containment began (see Ref. 36).
On 14.03.2011 at 06:10, the ‘small’ valve was
opened to relieve the pressure in the torus(AO206). After a renewed loss of pressurised-air supply the valve closed, several attempts to open the valve were not successful(see Ref. 44). The pressure in the drywellamounted at 06:10 to 4.6 barabs, at 06:50
to 5.3 barabs (see Ref. 12) and at 09:05 to
4.9 barabs (see Ref. 43).
RPV pressure was periodically relieved by the
Safety Relief Valves (SRVs) which brought
about uncovery of the fuel and fuel cladding
tube oxidation with subsequent hydrogen
release.
Also, in this case, it is assumed that the hydrogen – via leakages from the drywell(e.g drywell head flange, penetrations etc.),
favoured in turn by the high pressure in thedrywell – accumulated in the reactor building.
On 14.03.2011 at 11:01 (see Ref. 3), a hydrogen
explosion occurred in the reactor building
which caused severe damage of the structures of Unit 3, the equipment of Unit 3 (thecrane in the reactor building) as well as theexternal shell of Unit 4. It is still unclear whatcaused the combustion of the hydrogen accumulated in the reactor building.

11 persons were injured in the explosion (ofwhich 4 were members of TEPCO staff, 3were contract workers and 4 members of themilitary staff). Caused by the blast wave or
flying debris, fire extinguishing vehicles weredamaged and pipelines and hoses that weremeant to supply sea water for injection weredisconnected. Since then, debris with very
high dose rates has been making the work
even more difficult. Moreover, the explosion
impaired the work at Unit 2 (like pressurisedair supply, venting see Section 1.3.1, Ref. 44).
On 14.03.2011 at 11:25, the reactor pressure
increased according to Ref. 43, to 1.85 bar,the drywell pressure amounting to 3.6 barand the pressure in the torus amounting to
3.8 bar.

26 | 27

1.4.2

Spent Fuel Pool

The station blackout (SBO) also caused
a loss of the coolant supply for the spent
fuel pool at Unit 3. The last refuelling for
Unit 3 during the 2010 outage started on
23.09.2010. Due to the decay heat of thefuel assemblies, the water temperature in
the spent fuel pool increased and the level
dropped. On 16.03.2011 at 09:48, the water inthe spent fuel pool was replenished for the
first time by sea water dropped from a helicopter and later injected by water cannons(see Ref. 3).
Due to the severe damage of the reactor
building and the resulting debris in the spent
fuel pool (Fig. 13, see Ref. 18), mechanicaldamage of the fuel in the spent fuel poolmust be assumed. The increase in the ambient dose rate (up to about 11 mSv/h) at
the main gate or the western gate after thesteam releases from the spent fuel pool on
16.03.2011 and 21.03.2011 (see Ref. 19) also
indicates damage to the spent fuel. Nuclide
analyses of 08/09.05.2011 indicate strong
concentration of radionuclides (mainly
Cs-134/137) in the spent fuel pool, as compared to the reference value of 02.03.2011(see Ref. 37). This also indicates damage tothe spent fuel.

Annotation 18
The insufficient auxiliary water supply led tothe conclusion that the injection amounts
were insufficient. It is unknown whether theappropriate water injection amounts are
specified in the Severe Accident Management Guidelines (SAMG) and how they havebeen verified. Moreover, it is unclear if sufficient material for the necessary accident
management was in place at the site. It is
unclear whether the necessary measurement
displays (e.g. pool temperature, water level)
were available.

Figure 13:
Debris in the spent fuel pool, Unit 3;
source Ref.18

1 | Event series at Fukushima Dai-ichi

1.5

Unit 4

When the earthquake and the later Station Blackout (SBO) occurred, Unit 4 was inoutage (since 29.11.2010) for an impending
exchange of the core shroud. The RPV had
been completely unloaded for the next coreshroud exchange. For this, the entire fuel wasstored in the fuel pool. The decontaminationof the primary circuit had been completed.

tion of the analysis team that the earthquake
caused the damage to the fuel pool was
refuted according to Ref. 9, in which TEPCO
confirms the integrity of the spent fuel pool.

Of the emergency diesel generators (DG)
provided, one DG was shut down for maintenance, the second one was actuated automatically after the earthquake (see Ref. 43).
Following the earthquake, all work and inspections at Unit 4 were suspended (see Ref. 43).
On 11.03.2011 at 15:35, the tsunami caused
the destruction of the cooling water inlet
and thus the loss of the ultimate heat sink(mechanical). Moreover, the loss of the emergency power supply (by the flooding) caused
loss of the necessary residual-heat removal
trains to the ultimate heat sink (sea watercooling) as well as the station blackout (SBO)
at 15:38 (see Ref. 43). Due to the SBO, thecooling of the spent fuel pool failed. The decay heat of 1535 FA (max. pool capacity: 1590
FA – see Ref. 43) brought about a rise in thepool water temperature and a slow drop of
the water level.
On 14.03.2011 at 04:08, according to the Ref. 3,
the water temperature amounted to 84 °C.
On 15.03.2011 at 06:14, an explosion occurredin the reactor building of Unit 4 which caused
damage to the building structures. The root
cause of the explosion is yet disputed. It is
hardly plausible to assume in this case, as it
was with Units 1 to 3 that a hydrogen explosion occurred due to the oxidation of thefuel cladding tubes, taking into consideration
the event progress and the amount of water
present in the spent fuel pool. The assump-

Figure 14:
Units 3 and 4 after the hydrogen explosion;
source: TEPCO

Hypotheses that the hydrogen could be accounted for by other sources, like the ventingof Unit 3 (see Sec. 1.4.1) or surge lines of theventilation of Units 3 and 4 at the bottom of
the stack (see Ref. 10) or the generation ofan explosive mixture from the welding andcutting gas (shroud dismantling) cannot be
proven explicitly (see Ref. 43). More recent
reports assume, however, that the hydrogenfrom Unit 3 could overflow via the common
exhaust air line and the ‚emergency gas treatment plant’ of Unit 4 into the upper part of
the reactor building of Unit 4 (see Fig. 15).
Apparently, the ‘emergency gas treatment
plant’ does not possess any non-return valvesto prevent such overflow (see IV-90/IV-96,
Ref. 43).

28 | 29

Figure 15:
H2 overflow from Unit 3 to Unit 4;
source: Ref. 43, Fig. IV 5-10

At present, it is assumed that oil leakscaused the fires in the reactor building on
15.03.2011 at 09:38 and on 16.03.2011 at 05:45(see Ref. 1 and Ref. 10).
On 21.03.2011 in the time from 06:37 until
08:41, water was for the first time injection
by mobile water cannons in order to cool
down the spent fuel pool and to compensate
the evaporation (see Ref. 3). The water injection was later continued by fire extinguishing pumps and the truck-mounted concrete
pump.

Ref. 21 gives evidence of a only a slightdebris coverage in the spent fuel pool of
Unit 4 due to the hydrogen explosion (seeFig. 16), while no mechanical damage of
FA was discernible. A nuclide analysis from
08/09.05.2011 gives evidence of elevatedconcentration of radionuclides (mainly of
Cs-134/137) in the pool as compared to thereference value of 04.03.2011 (see Ref. 38).
The question whether these nuclides stemfrom the FA damage in Unit 4 or if they camefrom the ingress from the other units cannot
be answered conclusively.

1 | Event series at Fukushima Dai-ichi

Figure 16:
Spent fuel pool, Unit 4, on 29.04.2011;source: TEPCO

Annotation 19
The insufficient auxiliary water supply leads
to the conclusion that the injection amounts
were insufficient. It is unknown whether theappropriate water injection amounts are
specified in the Severe Accident Management
Guidelines (SAMG) and how they have beenverified. Moreover, it is unclear if sufficientmaterial for the necessary accident management was in place at the site. It is also unclear
whether the necessary measurement displays(e.g. pool temperature, water level) wereavailable.

30 | 31

1.6

Units 5 and 6

On 11.03.2011 at 15:35, the second tsunami
caused the destruction of the cooling water
inlet and thus the loss of the ultimate heatsink (mechanical). Moreover, the loss of theemergency DG (by the flooding) caused theloss of the necessary residual-heat removal
trains to the ultimate heat sink (sea water cooling) as well as the station blackout(SBO) at 15:38 (see Ref. 43). Units 5 and 6 of
the Fukushima Dai-ichi NPP were in outage
at the moment of the station blackouts, theRPVs were completely loaded. While Unit 6
was in ‘cold shutdown’ condition Unit 5 wasundergoing RPV leak tests (see Ref. 43).
The third diesel generator (DG) (air-cooled)at Unit 6 was not flooded by the tsunami. As
this DG was not impaired by the failure ofcoolant supply, it remained functional (seeRef. 32).
Due to leak tests (pressure tests) in Unit
5, the pressure in the reactor during SBO
amounted to 72 bar; owing to the decay
heat, the pressure in the RPV rose for a shorttime to about 80 bar. On 12.03.2011 at 06:06,
pressure relief of the RPV was undertaken. In
Unit 6, a rise in RPV pressure was also indicated. The pressure rose more slowly than in
Unit 5 due to the low decay heat caused by
the long shutdown period (see Ref. 43).
On 14.03.2011 at 05:00, the power supply forthe injection from the cold condensate tank
was re-established for Unit 5. The pressure
relief of both Units was carried out by SRVs,
and coolant injection was provided via thecold condensate tank (see Ref. 43).
Caused by diverse tsunami-inflicted failuresof equipment, the coolant function of the
spent fuel pools was lost. The decay heatbrought about a rise in the water temperature and a slow decrease of the water level(see Ref. 1). On 16.03.2011 at about 08:00,
according to Ref. 1, the water temperature inboth spent fuel pools exceeded 60 °C.
On 19.03.2011, openings were made in theroof of the reactor building to prevent a hydrogen explosion (see Ref. 39).
At first the water injection into the RPV andthe spent fuel pool was carried out by thethird DG of Unit 6 (air-cooled DG) via cold
condensate injections (auxiliary coolant) (seeRef. 3). On 19.03.2011, it became possible toput the second DG of Unit 6 in operation
after its repair. The power supply to the residual-heat removal pumps of Unit 5 and Unit
6 could thus be ensured, and the spent fuelpools were cooled again (see Ref. 3).
On 20.03.2011 at 14:30, Unit 5 - and on
20.03.2011 at 19:27, Unit 6, reached the state‘cold shutdown’ (see Ref. 3).
On 22.03.2011 at 19:41, all consumers wereconnected to the off-site power supply(see Ref. 36).

1 | Event series at Fukushima Dai-ichi

1.7

Common Spent Fuel Pool

Caused by the failure of off-site power supply,a failure of cooling occurred in the so-called‘Common Spent Fuel Pool’. The decay heatfrom 6375 fuel assemblies (storage capacity
6840 FA – see Ref. 43) brought about a slow
rise in the pool temperature (on 11.03.2011
up to about 30°C, see Ref. 43; on 19.03.2011
at 09:00 up to 57°C – see Ref. 3), and thewater level decreased. The water injection
by fire extinguishing vehicles on 21.03.2011 in
the time from 10:37 to 15:30 as well as the reestablishing of the spent fuel pool cooling on
24.03.2011 at 18:05 (see Ref. 3) brought about
a continuous decrease in the pool temperature and a stabilisation at < 30 °C.
On 13.05.2011, water samples were taken fromthe Common Spent Fuel Pool to verify thepool integrity. The samples gave evidenceof a rise in activity of Cs-134 and Cs-137, ascompared to the reference values of February 2011 (see Ref. 22). At present, it is being
verified whether the activity rise can be attributed to fuel damage

1.8

Dry Storage Cask Facility

Caused by the station blackout (SBO), a lossof all power systems occurred in the so-called
dry storage cask facility. However, the dry
storage cask facility has natural convection
cooling (see Ref. 11) which ensured that cooling of the FA casks was not impaired.
On 17.03.2011, the operator carried out avisual inspection which did not indicate any
deviations from the normal condition(see Ref. 12).
At present it is unclear whether the checks
described in Ref. 12 were a detailed inspection, nor is it known what the specific resultsof a storage and casks inspection were.

Fukushima
37° 25‘ 26.57“ N, 141° 1‘ 56.87“ E11.03.2011

2 | Event Sequence at Fukushima Dai-ni

2 Event Sequence at Fukushima Dai-ni
2.1

The operator of these plants is also TEPCO(Tokyo Electric Power Company).
Before the event, the power units of the
Fukushima Dai-ni NPP had been in power
operation:
Unit 1
BWR/5, 3293 MWth Mark II,
Manufacturer: Toshiba (1975 – 1982)
Unit 2
BWR/5 advanced, 3293 MWth Mark II,
Manufacturer: Hitachi (1979 – 1984)
Unit 3
BWR/5 advanced, 3293 MWth Mark II,
Manufacturer: Toshiba (1980 – 1985)
Unit 4
BWR/5 advanced, 3293 MWth Mark II,
Manufacturer: Hitachi (1980 – 1987)
on full power operation
The operator of this unit is TEPCO(Tokyo Electric Power Company).

Figure 17:
Overview Fukushima Dai-ni;
source: TEPCO

34 | 35

Caused by the Tohoku-Chihou-Taiheiyou-Oki
earthquake on 11.03.2011 at 14:46 (magnitude9), automatic reactor SCRAM occurred atUnits 1 to 4 (see Ref. 1).
Contrary to Fukushima Dai-ichi, no station
black out was caused by the earthquake (seeRef. 14). However, it can be assumed that ashort voltage disconnection or voltage peaks
occurred, for there were some actuations of
the emergency diesel generators, and containment isolation occurred.
The tsunami also caused partial flooding andequipment damage (auxiliary service waterof Units 1, 2 and 4) at the Dai-ni site. According to TEPCO (information of 30.3.2011),
two emergency DGs of Unit 1 failed irreparably. The DGs of Unit 2 could, despite theflooding, be put back into standby operation

Figure 18:
Flooding as compared between Dai-ichi and Dai-ni;
source: TEPCO

on 14.03.2011 after repair and restoration of
the cooling function. 2 DGs of Unit 3 and 1
DG of Unit 4 remained operable. The other
DGs of Unit 4 could be put back into standby
operation on 14.03.2011 after restoration of
the cooling function (see Ref. 43).
At this site, the damage turned out to be lesssevere owing to the siting (located 2 m higher) and the slightly more favourable layout of
the buildings. (see Fig. 18).

2 | Event Sequence at Fukushima Dai-ni

Annotation 20
The question of whether accident management procedures were available for coolingthe diesels in the case of a loss of off-site
power and how they are specified in the
Severe Accident Management Guidelines(SAMG) or how they were verified is unclear.
Moreover, it is not completely clear whether
sufficient material for the necessary accident management was in place at the site.
Presumably, the necessary measurement
displays (e.g. pool temperature, water level)
were available owing to the off-site power
supply.
Annotation 21
A diversified water supply (wells, pools)
could have mitigated the accident consequences.
Annotation 22
A reliable physical separation within the coolant pump structures could have protected
the nuclear auxiliary service water systems
better.
Annotation 23
The reviews which had taken place under
the OSART and WANO missions should also
have shown up the deficiencies of the plant.
Here, no direct reference to the design can be
identified in the reviews. However, the design
and backfitting of the plant can be indirectly
questioned when looking at its ‘Operating Experience’ (OE) - e.g. during OSART missions
by IRS-Reports (e.g. 7342 or 7788) or during
WANO missions by the “Significant Operating
Experience Reports” (SOER) – describing relevant operating experience of the companies
involved. The WANO members have committed themselves to consider specifically therecommendations of WANO–SOER and to review the respective measures in Peer Reviews.

This is a voluntary commitment on the part ofthe operators, while there is no public transparency. As for OSART, the matter is treateddifferently; here, however, there is no commitment of all countries to implement via the
IAEA the necessary and generally accepted
measures for safety. Under the periodic safety reviews, the plant safety status has to be
re-viewed every decade. Here, there are also
different approaches and criteria used within
the international framework. Taken together
and with the commitment to implement themin a steady process, these three instruments
could enhance safety worldwide.

36 | 37

2.2

Unit 1After the reactor SCRAM, residual-heat removal was carried out as per design by thereactor core isolation cooling system (RCIC).
The off-site power supply was in place.
On 11.03.2011 at about 15:38, the tsunami
caused a partial destruction of the coolant
inlet, hence the loss of the ultimate heat sink(mechanical).
Due to a rise in the pressure of the primarycontainment, an automatic actuation of theemergency core cooling system (ECCS)
occurred. TEPCO assumes that there was aleakage of coolant to the primary containment (see Ref. 13). After the RCIC switchedoff (12.03.2011 at 03:48), the coolant was supplied from the cold condensate tank.
On 12.03.2011 at 05:22, the pressure suppression function became unavailable due toboiling in the condensate tank (see Ref. 14).
The assumption of a coolant leakage to theprimary containment could not be confirmed.
On 12.03.2011 at 07:10, spraying into the drywell began (see Ref. 43)
On 12.03.2011 at 07:45, after the signal ‘lossof pressure relief function’, the government
ordered the evacuation from the 3 km-zone
advised the population in the area of 10 km
around Dai-ni to shelter. On 12.03.2011 at
17:39, the evacuation zone was extended toan area of 10 km around Dai-ni (see Ref. 3).
On 12.03.2011 at 08:19, an alert signal indicated that one control rod was not completely
inserted. The signal was reversed at 10:43, allcontrol rods were inserted properly (see Ref. 14).
On 12.03.2011 in the time between 09:43 and18:00, TEPCO prepared pressure relief in the
primary containment (see Ref. 14).

On 14.03.2011 at 01:24, after the emergency
coolant systems were re-established byre-placement of the actuators of the auxiliary service water pumps, the cool-downof the reactor was resumed. The respective
switchgears were damaged irreparably, so
the power supply had to be established via
temporary cables (see Ref. 43).
On 14.03.2011 at 10:15, the temperature in thetorus dropped to below 100 °C (see Ref. 43).
With the auxiliary coolant supply re-established, pressure relief in the primary containment was no longer necessary (see Ref. 17).
On 14.03.2011 at 17:00, the reactor reached‘cold shutdown’ condition (see Ref. 15).
On 15.03.2011 in the time between 15:20 and
16:25, a failure of the power supply of theauxiliary service water pump caused a loss of
emergency coolant (see Ref. 16).
Annotation 24
As no loss of power occurred and the nuclear auxiliary service water supply could
be re-established, no activity was released.
Also, it should be asked here how emergency
coolant supply procedures are specified in
the Severe Accident Management Guidelines(SAMG). Also it is unclear whether sufficientmaterial for emergency procedures was inplace at the site.

2 | Event Sequence at Fukushima Dai-ni

2.3

Unit 2After the reactor SCRAM, residual-heat removal was carried out as per design by thereactor core isolation cooling system (RCIC).
The off-site power supply was in place.
On 11.03.2011 at about 15:38, the tsunami
caused a partial destruction of the coolant
inlet, hence the loss of the ultimate heat sink(mechanical).
On 12.03.2011 at 04:50, the RCIC switched off,the coolant supply into the RPV was carriedout from the cold condensate tank (see Ref.
14).
On 12.03.2011 at 05:32, the pressure relief
function was no longer available due to boiling in the condensate tank (see Ref. 14).
On 12.03.2011 in the time between 10:33 and22:58, TEPCO prepared pressure relief in the
primary containment (see Ref. 14).
On 14.03.2011 at 07:13, after the emergency
core cooling system was re-established following the replacement of the actuatorsof the auxiliary service water pumps, thecool-down of the reactor was resumed. The
respective switchgears were damaged irreparably, so the power supply had to be established via temporary cables (see Ref. 43).
On 14.03.2011 at 15:52, the temperature in thetorus dropped to below 100 °C (see Ref. 43).
With the auxiliary coolant supply re-established, pressure relief in the primary containment was no longer necessary (see Ref. 17).
On 14.03.2011 at 18:00, the reactor reached‘cold shutdown’ condition (see Ref. 15).

Annotation 25
As no loss of power occurred and the nuclear
auxiliary service water supply could be reestablished, no activity was released. Also, itshould be asked here how emergency coolant supply procedures are specified in the
Severe Accident Management Guidelines(SAMG). Also it is unclear whether sufficientmaterial for emergency procedures was inplace at the site.

38 | 39

2.4

Unit 3After the reactor SCRAM, residual-heat removal was carried out as per design by thereactor core isolation cooling system or theauxiliary service water supply from the cold
condensate tank.
On 11.03.2011 at about 15:38, the tsunami
caused a partial destruction of the coolant
inlet, hence the loss of the ultimate heat sink(mechanical) while train B of the residualheat removal system (RHR) remained operable (see Ref. 43).
On 12.03.2011 in the time between 12:08 and12:13, TEPCO prepared preventive pressure
relief in the primary containment (see Ref.
14).
On 12.03.2011 at 12:15, the reactor reached‘cold shutdown’ condition (see Ref. 14).

2.5

Unit 4After the reactor SCRAM, residual-heat removal was carried out as per design by thereactor core isolation cooling system (RCIC).
The off-site power supply was in place.
On 11.03.2011 at about 15:38, the tsunami
caused a partial destruction of the coolant
inlet, hence the loss of the ultimate heat sink(mechanical).
On 12.03.2011 (exact time unknown), the RCIC
switched off, the coolant supply into the RPV
was carried out from the cold condensate tank(see Ref. 14).
On 12.03.2011 at 06:07, the pressure relief
function was no longer available due to boiling of the condensate tank (see Ref. 14). On
12.03.2011 at 07:35, spraying in the drywellbegan (see Ref. 43).
On 12.03.2011 in the time between 11:44 and11:52, TEPCO prepared pressure relief in the
primary containment (see Ref. 14).
On 12.03.2011 at 12:43, there was an alert
signal indicating that one control rod was notcompletely inserted. The signal was reversedby another signal; all control rods inserted
properly (see Ref. 23).

2 | Event Sequence at Fukushima Dai-ni

On 14.03.2011 at 15:42, after the emergency
core cooling system was re-established following the replacement of the actuators o theauxiliary service water pumps, the cool-downof the reactor was resumed. The respective
switchgears were damaged irreparably, so thepower supply had to be established via temporary cables (see Ref. 43). With the auxiliary
service water supply re-established, pressure relief of the primary containment was nolonger carried out (see Ref. 16).
On 15.03.2011 at 07:15, the reactor reached‘cold shutdown’ condition (see Ref. 43).

Annotation 26
As no loss of power occurred and the nuclear auxiliary service water supply could
be re-established, no activity was released.
Also, it should be asked here how emergency
coolant supply procedures are specified in
the Severe Accident Management Guidelines(SAMG). Also it is unclear whether sufficientmaterial for emergency procedures was inplace at the site.
On 15.03.2011 in the time between 20:05 and21:25, there was an interruption in the supplyof emergency coolant due to a failure in the
power supply of the auxiliary service water
pump (see Ref. 16).

Fukushima
37° 25‘ 26.57“ N, 141° 1‘ 56.87“ E11.03.2011

3 | Abbreviations

3 Abbreviations
AC

Alternate Current

BAF

Bottom of Active Fuel

FA

Fuel Assembly

BWR

Boiling Water Reactor

CCF

Common Cause FailureDC

Direct Current

DG

Diesel Generator

DW

Drywell

ECCS

Emergency Core Cooling System

GE

General Electric Company

HPCI

High Pressure Coolant Injection

IAEA

International Atomic Energy Agency

IC

Isolation Condenser

IRS

Incident Reporting System

JAIFJapan Atomic Industrial Forum Inc.

JMAJapan Meteorological Agency

JST

Japan Standard Time

LOCA

Loss of Coolant Accident

LL

Low Low – Actuation Value

LOOP

Loss of Off-Site Power

METI

Ministry of Economy, Trade and Industry

NISA

Nuclear and Industrial Safety Agency

ADR

Ambient Dose Rate

OE

Operating Experience

OSART

Operational Safety Team

RCIC

Reactor Core Isolation Cooling

RPV

Reactor Pressure Vessel

RHR

Residual Heat Removal

SAMG

Severe Accident Management Guidelines

42 | 43

SCRAM

Safety Control Rod Axe Man

SOER

Significant Operating Experience Reports

SRV

Safety Relief Valve

SBO

Station Blackout

TAF

Top of Active FuelTEPCOTokyo Electric Power Company

USNRC

United States Nuclear Regulatory Commission

VGB

Verband der Grosskessel-Besitzer

WANO

World Association of Nuclear Operators

4 | References

4 References

1

Sequence of Developments at Nuclear
Power Stations Affected by the Earthquake Japan Atomic Industrial Forum Inc.
(JAIF)2011-03-18 http://www.jaif.or.jp/
english/aij/member/2011/2011-03-18c.pdf

2 Boiling Water Reactor, GE BWR/4, Technology Advanced Manual, Chapter 6.0,
BWR Differences USNRC Technical Training Centre Rev. 1195 http://pbadupws.nrc.
gov/docs/ML0230/ML023010606.pdf
3 Seismic Damage Information (80th release) Nuclear and Industrial Safety Agency (NISA) 2011-04-08 http://www.nisa.
meti.go.jp/english/files/en20110411-1.html
4 Press Release (Mar 12, 2011), Impact to
TEPCO’s Facilities due to Miyagiken-Oki
Earthquake (as of 3PM), Tokyo ElectricPower Company (TEPCO) 2011-03-12
http://www.tepco.co.jp/en/press/
corp-com/release/11031222-e.html
5 Press Release (Mar 12, 2011), White smoke
around the Fukushima Daiichi Nuclear
Power Station Unit 1, Tokyo Electric Power
Company (TEPCO) 2011-03-12 http://www.
tepco.co.jp/en/press/corp-com/release/1031225-e.html
6 Current Status of Units 1 to 4 at Fukushima Daiichi NPS (as of noon, March 17,2011), Japan Atomic Industrial Forum Inc.
(JAIF) 2011-03-18 http://www.jaif.or.jp/
english/aij/member/2011/2011-03-18a.pdf

7 Press Release (Mar 14, 2011), Occurrenceof a Specific Incident (Failure of reactor
cooling function) Stipulated in Article 15,
Clause 1 of the Act on Special Measures
Concerning Nuclear Emergency Preparedness, Tokyo Electric Power Company(TEPCO) 2011-03-14 http://www.tepco.co.
jp/en/press/corp-com/release/11031403-e.html
8 Press Release (Mar 13, 2011), Occurrenceof a Specific Incident Stipulated in Article
15, Clause 1 of the Act on Special Measures
Concerning Nuclear Emergency Preparedness Tokyo Electric Power Company(TEPCO) 2011-03-13 http://www.tepco.co.
jp/en/press/corp-com/release/1031303-e.html
9 Earthquake Report No.66: 20:00, April 28,
Japan Atomic Industrial Forum Inc. (JAIF)
2011-04-28 http://www.jaif.or.jp/
english/newsimages/pdf/
ENGNEWS011303986888P.pdf
10 Earthquake Report No.83: 18:00, May 16,
Japan Atomic Industrial Forum Inc. (JAIF)
2011-05-16 http://www.jaif.or.jp/english/
newsimages/pdf/
ENGNEWS011305536094P.pdf
11 Integrity Inspection of Dry Storage Casks
and Spent Fuel at Fukushima Daiichi, ISSF
2010 Session 6, Tokyo Electric Power
Company (TEPCO) 2010-11-16
12 Press Release (Apr 18,2011), Status of TEPCO’s Facilities and its services after the
Tohoku-Chihou-Taiheiyou-Oki Earthquake(as of 4:00PM, April 18th), Past Progress
Report Tokyo Electric Power Company(TEPCO) 2011-04-18 http://www.tepco.co.
jp/en/press/corp-com/release/1041804-e.html

44 | 45

13 Press Release (Mar 11,2011), Occurrence ofa Specific Incident Stipulated in Article 10,
Clause 1 of the Act on Special Measures
Concerning Nuclear Emergency Preparedness(Fukushima Dai-ni, Tokyo ElectricPower Company (TEPCO) 2011-03-11
http://www.tepco.co.jp/en/press/corpcom/release/11031104-e.html

19 Japan’s Nuclear Accident – Update
Ministry of Economy, Trade and Industry(METI) 2011-05-02

14 Press Release (Mar 12,2011), Plant Statusof Fukushima Dai-ni Nuclear Power Station (as of 11pm March 12th), Tokyo Electric Power Company (TEPCO) 2011-03-12
http://www.tepco.co.jp/en/press/corpcom/release/11031233-e.html

21 Status of the Spent Fuel Pool of Unit 4 of
Fukushima Daiichi Nuclear Power Station,
Tokyo Electric Power Company (TEPCO)
2011-05-08 http://www.tepco.co.jp/en/
news/110311/index-e.html
22 Press Release (May 15,2011), Results of nuclide analyses of radioactive materials in water taken in common spent fuel pool of Fukushima Daiichi Nuclear Power Station, Tokyo
Electric Power Company (TEPCO) 2011-05-15
http://www.tepco.co.jp/en/press/corp-com/
release/11051510-e.html

15 Press Release (Mar 15,2011), Plant Statusof Fukushima Dai-ni Nuclear Power Station (as of 7:15 am Mar 15th), Tokyo Electric Power Company (TEPCO) 2011-03-15
http://www.tepco.co.jp/en/press/corpcom/release/11031501-e.html
16 Press Release (Mar 16,2011), Impact to
TEPCO’s Facilities due to Tohoku-Taiheiyou-Oki Earthquake (as of 2:00PM), Tokyo
Electric Power Company (TEPCO) 201103-16 http://www.tepco.co.jp/en/press/
corp-com/release/11031608-e.html
17 Press Release (Mar 18,2011), Status of
TEPCO’s Facilities and its services after
To-hoku-Taiheiyou-Oki Earthquake (as of
10:00AM), Tokyo Electric Power Company(TEPCO) 2011-03-18 http://www.tepco.co.
jp/en/press/corp-com/release/1031802-e.html
18 Status of the Spent Fuel Pool of Unit 3 ofFukushima Daiichi Nuclear Power Station,(video on May 8th, 2011), Tokyo ElectricPower Company (TEPCO) 2011-05-08

20 Earthquake Report No.84: 18:00, May 17, Japan
Atomic Industrial Forum Inc. (JAIF) 2011-05-17
http://www.jaif.or.jp/english/news_images/
pdf/ENGNEWS011305623138P.pdf

23 Press Release (Mar 13,2011), Plant Status of
Fukushima Dai-ni Nuclear Power Station (asof 8pm March 13th), Tokyo Electric Power
Company (TEPCO) 2011-03-13 http://www.
tepco.co.jp/en/press/corp-com/release/1031311-e.html
24 Press Release (Mar 12,2011), Plant Status of
Fukushima Dai-ni Nuclear Power Station (asof 8pm March 12th), Tokyo Electric Power
Company (TEPCO) 2011-03-12 http://www.
tepco.co.jp/en/press/corp-com/release/1031230-e.html
25 Press Release (Apr 01,2011), Plant Status of
Fukushima Dai-ni Nuclear Power Station (asof 4:00 pm April 1st), Tokyo Electric Power
Company (TEPCO) 2011-04-01 http://www.
tepco.co.jp/en/press/corp-com/release/1040104-e.html

4 | References

26 Earthquake Report No.91: 18:00, May 24,
Japan Atomic Industrial Forum Inc. (JAIF)
2011-05-24 http://www.jaif.or.jp/english/
newsimages/pdf/
ENGNEWS011306229916P.pdf
27 Tsunami Information (Estimated Tsunami
arrival time and Height), Japan Meteorological Agency 2011-03-11, 14:49 http://
www.jma.go.jp/en/tsunami/
info0420110311145026.html
28 Press Release (Mar 11,2011), Occurrence ofa Specific Incident Stipulated in Article 15,
Clause 1 of the Act on Special Measures
Concerning Nuclear Emergency Preparedness(Fukushima Daiichi), Tokyo ElectricPower Company (TEPCO) 2011-03-11
http://www.tepco.co.jp/en/press/corpcom/release/11031103-e.html
29 Earthquake Report No.58: 20:00, April 20,
Japan Atomic Industrial Forum Inc. (JAIF)
2011-04-20 http://www.jaif.or.jp/english/
news_images/pdf/
ENGNEWS011303298461P.pdf
30 Press Release (Apr 20,2011), Status of
TEPCO’s Facilities and its services after
the Tohoku-Chihou-Taiheiyou-Oki
Earthquake(as of 9:00AM, April 20th),
Tokyo Electric Power Company (TEPCO)
2011-04-20 http://www.tepco.co.jp/en/press/corp-com/release/11042001-e.html
31 Pictures of Tsunami that hit the Fukushima
Daiichi Nuclear Power Station (pictured on
March 11, 2011), Place: North side of Radiation Waste Treatment Facility taken from4th floor, Tokyo Electric Power Company(TEPCO) 2011-05-19 http://www.tepco.co.
jp/en/news/110311/index-e.html

32 Press Release (May 16,2011), Submission ofreport to the Ministry of Economy, Tradeand Industry on the damage of electric
plants inside and outside of Fukushima
Daiichi NPS and the situation of recovering offsite power supply through emergency measures since the Tohoku-Taiheiyou-Oki Earthquake, Report regarding
Collection of Reports pursuant to the Provisions of Article 106, Paragraph 3 of theElectricity Business Act, esp. Appendix 5,
Damages to electric plants at Fukushima
Dai-chi Nuclear Power Station, Tokyo Electric Power Company (TEPCO) 2011-05-16
http://www.tepco.co.jp/en/press/corpcom/release/11051613-e.html
33 Reactor Core Status of Fukushima Dai-ichi
Nuclear Power Station Unit 1, Tokyo Electric Power Company (TEPCO) 2011-05-15
http://www.tepco.co.jp/en/press/corpcom/
release/betu11e/images/110515e10.pdf
34 Mark I Containment Report, Nuclear Energy Institute (NEI) 2011-03-19 http://www.
nei.org/filefolder/Report
BWRMarkIContainment031920112.pdf
35 Press Release (Mar 20,2011), Plant Statusof Fukushima Daiichi Nuclear Power Station (as of 6:00 PM Mar 20th), Tokyo Electric Power Company (TEPCO) 2011-03-20
http://www.tepco.co.jp/en/press/corpcom/release/11032009-e.html
36 Information on Status of Nuclear Power
Plants in Fukushima Japan Atomic Industrial Forum Inc. (JAIF) 2011-06-01 http://
www.jaif.or.jp/english/news_images/pdf/
ENGNEWS01_1306898792P.pdf

46 | 47

37 Press Release (May 10,2011), Results of
Nuclide Analyses of (Radioactive Materials in the) Water in Spent Fuel Pool of
Unit 3 at Fukushima Daiichi Nuclear
Power Station, Tokyo Electric Power Company (TEPCO) 2011-05-10 http://www.tepco.co.jp/en/press/corp-com/release/1051009-e.html
38 Press Release (May 09,2011), Results ofanalysis of spent fuel pool water of Fukushima Daiichi Nuclear Power Plant Unit 4(2nd continued report), Tokyo ElectricPower Company (TEPCO) 2011-05-09
http://www.tepco.co.jp/en/press/corpcom/release/11050905-e.html.
39 Informationen zur Lage in Japan: Stand
05.04.2011, 12:30 Uhr (MESZ), Gesellschaft für Anlagen- und Reaktorsicherheit(GRS) mbH 2011-04-05 http://fukushima.
grs.de/kommunikation-medien//japanstatusmeldung-stand-05-04-2011-1230
40 Status of nuclear power plants in Fukushima as of 21:00 March 22 (Estimated by
JAIF), Japan Atomic Industrial Forum Inc.
(JAIF) 2011-03-22 http://www.jaif.or.jp/
english/newsimages/pdf/
ENGNEWS011300796691P.pdf
41 Press Release (May 20,2011), Report to
Nuclear and Industrial Safety Agency on
emergency countermeasures for safety at
Fukushima Dai-ni Nuclear Power Station,
Tokyo Electric Power Company (TEPCO)
2011-05-20 http://www.tepco.co.jp/en/press/corp-com/release/11052017-e.html
42 Status of cores at Units 2 and 3 in Fukushima Dai-ichi Nuclear Power Station,
Tokyo Electric Power Company (TEPCO)
2011-05-23

43 Report of Japanese Government to the
IAEA Ministerial Conference on Nuclear
Safety - The Accident at TEPCO’s Fukushima Nuclear Power Stations - June 7,2011 http://www.kantei.go.jp/foreign/kan/
topics/201106/iaeahoukokushoe.html
44 Press Release (Jun 18,2011), First Response at Tohoku Chihou Taiheiyou Oki
Earth-quake in Fukushima Daiichi Nuclear
Power Station, Tokyo Electric Power
Company (TEPCO) 2011-06-18 http://
www.tepco.co.jp/en/press/corp-com/
release/11061805-e.html
45 Recommendations for enhancing reactor
safety in the 21st century, The near-term
task force review of insights from the
Fukushima Dai-ichi accident, UnitedStates Nuclear Regulatory Commission(USNRC) July 12, 2011 http://pbadupws.
nrc.gov/docs/ML1118/ML111861807.pdf
46 IAEA International fact finding expert
mission of the Fukushima Dai-ichi NPP accident following the great east Japan
earthquake and tsunami, International
Atomic Energy Agency (IAEA) June 2,
2011 http://www.pub.iaea.org/MTCD/
meetings/PDFplus/2011/cn200/
documentation/cn200Final-FukushimaMissionReport.pdf
47 Earthquake Report No.177: 12:00, August
18, Japan Atomic Industrial Forum Inc.
(JAIF) 2011-08-18 http://www.jaif.or.jp/
english/newsimages/pdf/
ENGNEWS011313640580P.pdf

Issued by
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Industriestrasse 19
CH-5200 Brugg
Phone +41 (0)56 460 84 00
Fax+41 (0)56 460 84 99
info@ensi.ch
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Published 26.08.2011