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Why Did Severe Accidents Not Occur
at the Fukushima Daini, Onagawa and Tokai Daini NPSs?
Japan Atomic Industrial Forum, Inc.
Seismic vibrations and tsunamis
caused by the off the Pacific coast of
Tohoku Earthquake on March 11, 2011,
hit not only the Fukushima Daiichi
Nuclear Power Station (NPS) owned by
the Tokyo Electric Power Co., Inc.
(TEPCO), but the Fukushima Daiini NPS
of TEPCO, the Onagawa NPS of the
Tohoku Electric Power Company, and
the Tokai Daini NPS of the Japan Atomic
Power Company (JAPC). While
comparisons are difficult because of
differences in seismic vibration strength
and height of tsunamis among these
NPSs, nevertheless, while severe
Source: Japan Meteorological Agency
accidents (SAs) involving core
meltdowns occurred at Units 1, 2 and 3 at the Fukushima Daiichi NPS, SAs were
prevented at Fukushima Daini, Onagawa and Tokai Daini. How each reactor at
Fukushima Daini, Onagawa and Tokai Daini was affected by the earthquake and tsunami,
and the kinds of safety measures that prevented SAs, are examined below.
I. Ground Motion
As for seismic vibrations recorded at NPPs, there were some values above the
maximum response acceleration for the Design Basis Seismic Ground Motion Ss at the
Fukushima Daiichi and Onagawa NPSs, but, otherwise, all were generally within the
maximum response acceleration values, and, even when exceeding them, did not do so
radically. The fact, however, that some did exceed the expected maximum itself means
the design base estimation was not sufficient, posing issues and serving as a lesson.
Nevertheless, based on information obtained so far, there was no serious effect on facilities
or equipment, which suggests that margins provided during seismic designing (actual proof
strength) greatly contributed to ensuring safety.
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Regarding off-site power systems, they were heavily damaged by ground motion at
most reactors, while as expected from their designs. At Fukushima Daiichi, all seven lines
were lost, two of four (one source was under periodic inspection) were lost at Fukushima
Daini, four of five at Onagawa and all three at Tokai Daini. If the off-site power had been
received, accident management (AM) measures would mostly have been operable; and if
all power sources had’nt been lost, restoration activities would have been greatly facilitated.
Reliability of power sources as a whole should be improved, including that of external
sources. (On this point, the Nuclear Regulation Authority (NRA) has called on all
operators to improve reliability of external power sources, as much as they can reasonably
achieve, as a voluntary effort under the new regulatory standards.)
II. Tsunami
As for tsunami – in circumstances where there had traditionally been no clear
regulatory standards – each plant had renewed its expectations for the potential height of a
tsunami whenever there was new knowledge or experience, and had implemented
necessary measures. Review of measures occurred specifically whenever filing the
application of the reactor establishment permit; when the Japan Society of Civil Engineers
(JSCE) released a method of evaluating tsunami height in 2002; and, at Fukushima Daiichi,
Fukushima Daini and Tokai Daini, in 2007 when Fukushima and Ibaraki prefectures made
their own predictions on tsunami heights as part of their disaster preparedness measures.
In response to these, companies also strengthened tsunami-protection measures – raising
locations where pumps were located, making heat exchanger buildings watertight and
more.
Actual tsunami height, however, at Fukushima Daiichi was 13.1 meters, far higher than
the assumed 6.1 meters. Damage to equipment important to safety and other problems
were caused by tsunami. (On this point, the NRA demands strict tsunami measures in its
new regulatory standards.)
At Fukushima Daini, actual tsunamis were 7 ~ 8 meters in contrast to the assumed 5.2
meters; at Onagawa, actual 13 meters against assumed 13.6 meters; and at Tokai Daini,
actual 5.5 meters against assumed 6.61 meters.
III. Extent of Damage
Including Fukushima Daiichi, damage to each facility of each NPS by the earthquake
and tsunami is listed in the table below. The situation of damage at each NPS is
2
summarized here.
Fukushima Daiichi
Units 1 - 3 were in rated power output operation, and Units 4 - 6 had been shut down.
Units 1 - 3 all scrammed at the off the Pacific coast of Tohoku Earthquake. As a result of
damage to off-site power facilities and the transmission towers collapse caused by the
earthquake, off-site powers were lost at all units. All emergency diesel generators (D/Gs)
except one air-cooled D/G at Unit 6 lost function as they were covered with water by
tsunami; cooling systems lost function and power panels were submerged. At Units 1, 2
and 4, all direct-current (DC) power sources were also lost to tsunami. Emergency
seawater pumps for cooling reactors lost their ability to remove residual heat due to
tsunami. Consequently, Units 1 - 3 suffered meltdowns of their reactor cores. At Unit 4,
no nuclear fuel was in the reactor, but there was an explosion of hydrogen that had flowed
in from Unit 3. Unit 5 and 6 had been shut down and time was available to take action.
With its available air-cooled D/G, Unit 6 managed to avoid an SA. Additionally, as part of
AM measures, a system was in place to power source cross-ties from Unit 6 to Units 5, so
Unit 5 also avoided an SA.
Fukushima Daini
Units 1 - 4 were in rated power output operation. All four scrammed when the
earthquake occurred. The subsequent huge tsunami recorded a runup height of O.P.1 +
about 15 meters at the south side of Unit 1 and the seismic isolated building . In particular,
the building housing the seawater heat exchanger located at a place on the premises at
O.P. + 4 meters was heavily damaged. As a result, emergency seawater pumps for
cooling the reactors at Units 1, 2 and 4 (excluding Unit 3) lost heat-removal to tsunami.
Later the temperature in the pressure suppression pool, to which the heat in the reactors
was supposed to escape, rose above 100℃, resulting in a “nuclear emergency situation”,
an event corresponding to Article 15 of the Nuclear Disaster Act (loss of pressure
suppression).
As for off-site power, although off-site power facilities were damaged, it was possible to
obtain electricity from one of the four lines. The D/Gs were covered with water and cooling
system function was lost, resulting in loss of function of all three D/Gs at Units 1 and 2, one
of three D/Gs at Unit 3, and two of three D/Gs at Unit 4. DC power sources remained
sound.
1
O.P. for Fukushima Daiichi and Daini: Onahama Port construction site point of reference
(0.727 meters below Tokyo-bay Mean Sea Level(T.P.))
3
Under the nuclear disaster prevention team of the NPS deployed at the seismic
isolated building, a cold shutdown state at all NPPs was achieved at 7:15 a.m. on March 15,
four days after the earthquake. During the time when warnings of huge tsunami had been
issued, the extent of equipment/facility damage was confirmed by plant walk-down;
logistics were established toward recovery; equipment was procured in cooperation with
the head office nuclear disaster prevention unit; and station staff and cooperating
companies made strenuous efforts to replace seawater pump motors and lay temporary
installed power-source cables.
Onagawa
Units 1 and 3 were in rated power output operation and Unit 2 was in the process of
start-up. All of them scrammed when the off the Pacific coast of Tohoku Earthquake
occurred.
Following the earthquake, power was provided from off-site, one of the five lines was
maintained, D/Gs were all sound, and so were DC power sources.
Subsequent tsunami did not reach main buildings in the site. As functions to cool the
reactors and the spent fuel pool were also sound, the reactors shut down in stable condition,
ensuring the safety of the NPS. D/Gs for Units 1 and 3 were all sound, but, at Unit 2,
function of a component cooling water system was partially lost to tsunami and two of three
D/Gs lost function. One D/G was sound. At 1:17 a.m. on March 12, a cold shutdown
state was achieved at the three units.
Tokai Daini
The unit was in rated power output operation and the reactor scrammed when the
earthquake occurred. The function of off-site power facilities was lost, and all three off-site
power lines were lost, but three D/Gs were usable. As one of the D/G powered seawater
cooling pumps stopped automatically as a result of flooding due to subsequent tsunami, the
D/G had to be manually stopped and one residual heat removal (RHR) system went out of
use. But functions of other important safety related equipment were maintained. Power
to charge emergency storage batteries (power sources for the Reactor Core Isolation
Cooling (RCIC) system) was provided from separate power source systems, maintaining
the RCIC function. DC power sources were sound. Consequently, a cold shutdown
state was achieved at 0:40 a.m. on March 15.
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IV. Primary Factors to Ensure Safety Functions
Fukushima Daini
At Units 1, 2 and 4, which lost RHR function to tsunami, cooling of reactors was
maintained according to the operating procedural manual for emergencies by injecting
water using the RCIC system. Later when reactor pressure decreased, based on AM
measures, alternative water injection using make up water condensate (MUWC) system
started to adjust the water level of the reactors.
Procurement requests for motors, vehicle-mounted high-voltage power sources,
mobile transformers and cables necessary for recovery work were sent urgently to the
whose specifications matched requirements were transported via all possible means, air
and land, by the Self-Defense Forces to Fukushima Daini.
In particular, the seismic isolated building established in the wake of
Niigata-Chuetsu-Oki Earthquake in July 2007 served as the core for recovery efforts, and
its functions of communication, storage and more are believed to have contributed to
smooth responses to the situation and to achieving states of cold shutdown – bringing the
reactors under control.
Onagawa
Regarding the height of the ground, from the beginning of construction of Unit 1 there
was common recognition that measures against tsunami were a key issue, and an internal
committee including outside specialists/experts in the areas of civil engineering and
geophysics was formed at the Tohoku Electric Power Company, to fully address it. At the
initial evaluation, height of tsunami was assumed to be around three meters at the NPS site.
The internal committee, however, agreed that (1) the height of the ground is itself an
anti-tsunami measure, and (2) the height of the ground was O.P.2 + 15 meters. Based on
this, it was decided that important outside civil engineering architectural structures and the
first floors of major buildings were at O.P. + 15 meters and the height of the ground was at
O.P. + 14.8 meters. Thereafter, when the application of the reactor establishment permit
for Units 2 and 3 were filed, and whenever new technology to evaluate tsunami was
developed by JSCE, assessments of tsunami were made based on the latest findings, and
it was confirmed that tsunami height was lower than the height of the site ground. As a
result of diastrophism by the earthquake this time, the NPS premises subsided by one
meter to O.P. + about 13.8 meters, but the tsunami height was 13 meters, and tsunami
2
O.P. for Onagawa: Onagawa NPS Port construction site point of reference (0.74 meters
below T.P.)
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were never higher than the ground height where there were main structures.
Tokai Daini
Work on countermeasures based on new knowledge/findings was underway when the
off the Pacific coast of Tohoku Earthquake occurred.
Reflecting the scale of tsunami used for the “predicted tsunami inundated coastal area”
released by Ibaraki Prefecture in 2007, a tsunami evaluation was carried out, producing the
result of H.P.3 + 6.61 meters, based on which sidewalls of an emergency seawater pump
room were raised to H.P. + 7 meters. Work to seal walls and portions where piping
penetrated through walls, to prevent flooding, was ongoing. While some of the equipment
did not escape inundation, the preventive work contributed to protecting important safety
related equipment from tsunami subsequent to the earthquake.
V. Conclusion
At Fukushima Daini, Onagawa and Tokai Daini, SAs due to the earthquake were
avoided. Major factors in this were the following:
Importance of Power Sources
Station black out (SBO) did not occur. At Fukushima Daini, one of four off-site power
lines survived and at Onagawa, one of five lines. At Tokai Daini, although all three off-site
power lines were lost, two of three emergency D/Gs remained sound.
AM Measures
At Fukushima Daini, although emergency seawater pumps for reactor cooling systems
for three units (i.e., except Unit 3) were lost, AC and DC power sources were sound,
enabling injection of water by the RCIC system. Later, when reactor pressure decreased,
the water level of the reactors was maintained and core meltdowns were avoided by
MUWC system, based on AM measures.
Meanwhile, although AM measures were in place also at Fukushima Daiichi, SBO and
loss of functions to remove residual heat following the earthquake and tsunami resulted in
delays in reducing reactor pressure and injecting water into the reactors, and led to core
damage. In particular, loss of DC power sources and functions to remove heat, and
inundation of power panels by tsunami hindered all AM measures.
3
H.P.: Hitachi Port construction site point of reference (0.89 meters below T.P.)
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Seismic Isolated Building
The seismic isolated building at Fukushima Daini had been newly constructed based
on findings from Niigata-Chuetsu-Oki Earthquake and operation of it began in July 2010,
only about eight months before the off the Pacific coast of Tohoku Earthquake. It served
as the core base in recovery efforts, playing a key role in achieving states of cold shutdown.
At Fukushima Daiichi also, the seismic isolated building was the base for on-site
response activities. Without the building, responding to the accident could not have
continued.
The seismic isolated buildings at Onagawa and Tokai Daini were under construction
when the earthquake occurred and were not operated as emergency headquarters.
Preparedness for Tsunami
At Onagawa and Tokai Daini, ground height with margins and work to prevent
inundation based on the latest knowledge/expertise resulted in maintaining safety against
tsunami strikes estimated by the method of JSCE.
At the same time, at Fukushima Daiichi and Daini, measures based on new knowledge
were not effective because the tsunami scale was substantially greater than was estimated
using the JSCEs’ method.
※ This paper was prepared by the Japan Atomic Industrial Forum, Inc. (JAIF),
based on a Report of “Seminars to investigate the accident at the Fukushima Daiichi
Nuclear Power Station” issued by the Nuclear Safety Division (NSD) of the Atomic
Energy Society of Japan (AESJ) .
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Extent of Damage to Equipment/Facilities by Earthquake and Tsunami at Fukushima, Onagawa and Tokai NPSs
(Note: Where only some of several items of equipment were damaged, this is indicated as “number sound/total number”)
D/G
External power
Site
Unit
Type
Output
VehicleD/C power
(No loss due to
source*1
Seawater cooling
source
BWR3
460
2
BWR4
784
275 kV: ×
Fukushima
Daiichi
3
4
BWR4
BWR4
784
66kV: ×
784
Major fuel damage
source
Emergency
Common
Emergency
Common
× *2
×
×
×
×
×
×
× *2
×
×
×
×
2/3
2/4
× *2
◯→Exhausted
×
×
×
×
×
×
×
×
1/2 (1)*7
1/1 (1)*7
× *2
×
(7 circuits in total)
Partially used
5
BWR4
784
× *2
◯
×
×
×
×
2/7
6
BWR5
1,100
1/3 *2
◯
×
◯
×
◯
×
1
BWR5
1,100
× *3
3/4
×
1/3
◯
1/4
◯
2
BWR5
1,100
× *3
◯
Partially used
×
◯
◯
2/4
◯
◯
(Ensured external
For D/G (H): ◯
2/3 *3
◯
◯
3/4
power source and
◯
For RHR: 1/2
D/Gs)
For D/G: ◯
◯
◯
2/4
◯
◯
1/2
◯
◯
◯
◯
◯
◯
◯
◯
◯
◯
◯
◯
◯
◯
500kV: 1/2
Fukushima
3
BWR5
1,100
66kV: ×
Daini
(4 circuits in total)
4
BWR5
1,100
1
BWR4
524
1/3*3
◯
◯
◯
275kV: 1/4
Onagawa
P/C (low-voltage power panel)
system*6
earthquake)
1
M/C (high-voltage power panel)
mounted power
2
BWR5
825
66kV: ×
For RHR: ×
◯
(Ensured
1/3 *4
◯
power
external
source
For D/G: ×
and
Damaged
Sound
Sound
Sound
For RHR: 1/2
(5 circuits in total)
3
BWR5
825
◯
◯
2/3 *5
◯
D/Gs)
◯
275kV: ×
Tokai Daini
BWR5
1,100
154kV: ×
Ensured by reserve
For D/G: 2/3
(Ensured D/G)
For RHR: ◯
Sound
(3 circuits in total)
*1: At Fukushima Daini, Onagawa and Tokai Daini, external power sources were partially recovered in a day or within a few days.
Use of a 66-kV circuit at Fukushima Daiini was suspended for inspection.
*2: At Units 1 and 4, 1 of 2 D/Gs; a D/G at Unit 5 was not damaged by tsunami water but was lost by indirect factors (inundation of auxiliary cooling system and metal-clad switch gear (M/C) related equipment; D/Gs at Units 2, 4 and 6-B were
air-cooled.
*3: At Unit 1 water came in and reached the D/G through the D/G blower air supply opening, etc., at the Annex attached to the reactor building; at Units 2 – 4, almost no inundation occurred at the Annex attached to the reactor buildings; loss was
by indirect factors (inundation of auxiliary cooling system and metal-clad switch gear (M/C) related equipment)
*4: Cooling system A was sound; auxiliary cooling system B pump and HPCS auxiliary cooling pump were lost due to inundation from seawater pump room opening (a tide gauge)
*5: D/G cooling seawater pump 2C was automatically shut down due to inundation of the seawater pump room; D/G-2C was manually shut down
*6: Function of seawater system was lost (including loss of function of auxiliary cooling pump)
Location of seawater pumps: At Fukushima Daini, in the building that houses the seawater heat exchangers on the coast (inundation from the large-object entrance, etc. (except the building on the south side of Unit 3)); at Onagawa, in the
pump room located in a pit made by digging from the height of the site (inundation in the Annex attached to the reactor building (uncontrolled area) through piping and cable tunnel); at Tokai Daini, in the seawater pump room (inundation from
partially unfinished wall penetrations) located where coastal sidewall was raised for tsunami counter measures
*7: Under work
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