Important Report from TEPCO
The Tokyo Electric Power Company, Incorporated, would like to make
comments on subjects that are reported by the newspapers, TV or
websites.
We affirm that the Reactor Building and Spent Fuel Pool of Unit 4 will not collapse in the event of an earthquake
April 26, 2012
The upper part of the Reactor Building of Unit 4 was
damaged due to a hydrogen explosion. We confirmed the items below and
affirm that the building, including its spent fuel pool will not
collapse should another earthquake occur.
1) We measured the distance between the water surface of
the spent fuel pool and the floor surface of the building, and
confirmed that the building has not tilted.
2) Our analyses show that the building, including the spent fuel pool, will not collapse even if an earthquake equivalent (seismic intensity 6) to the Tohoku-Chihou-Taiheiyou-Oki Earthquake occurs in the area.
3) In addition, we have improved the seismic safety margin by 20% by reinforcing the bottom of the spent fuel pool.
4) We will regularly check the reactor building and the spent fuel pool four times per year to confirm their soundness.
2) Our analyses show that the building, including the spent fuel pool, will not collapse even if an earthquake equivalent (seismic intensity 6) to the Tohoku-Chihou-Taiheiyou-Oki Earthquake occurs in the area.
3) In addition, we have improved the seismic safety margin by 20% by reinforcing the bottom of the spent fuel pool.
4) We will regularly check the reactor building and the spent fuel pool four times per year to confirm their soundness.
* Please refer to the attachment for the details of 1) to 4).
Related material and data
- Water
level measurement in order to confirm the soundness of the Unit 4
Reactor Building at Fukushima Daiichi Nuclear Power Station (PDF 18.0KB)
- Submission of Reports concerning the study of the current seismic safety and reinforcement of reactor buildings at Fukushima Daiichi Nuclear Power Station (1)
- Completion
of the installation of the supporting structure at the bottom of the
spent fuel pool at the Unit 4 Reactor Building of Fukushima Daiichi
Nuclear Power Station (PDF 73.6KB)
The scale of the tsunami far exceeded all previously held expectations and knowledge.
April 24, 2012
1.Overview
The scale of the earthquake that caused the tsunami was M9.0, the largest ever recorded in Japan, and the fourth largest ever recorded in the world. The earthquake was caused by an interlocking movement of several regions off-shore of Iwate to Ibaraki prefectures with a length north to south of approximately 500 km, a width from east to west of approximately 200 km, and a maximum slip of more than 50 m. The potential occurrence of an earthquake of this scale was outside the radar of not only TEPCO but also the Central Disaster Prevention Council of the Cabinet Office and the Headquarters for Earthquake Research Promotion (HERP) established in the Ministry of Education, Culture, Sports, Science and Technology.
The scale of the earthquake that caused the tsunami was M9.0, the largest ever recorded in Japan, and the fourth largest ever recorded in the world. The earthquake was caused by an interlocking movement of several regions off-shore of Iwate to Ibaraki prefectures with a length north to south of approximately 500 km, a width from east to west of approximately 200 km, and a maximum slip of more than 50 m. The potential occurrence of an earthquake of this scale was outside the radar of not only TEPCO but also the Central Disaster Prevention Council of the Cabinet Office and the Headquarters for Earthquake Research Promotion (HERP) established in the Ministry of Education, Culture, Sports, Science and Technology.
2. Past Tsunami Evaluations
(Review during Construction)
At the point of construction of Fukushima Daiichi Nuclear Power Station, in adhering to TEPCO's basic policy of always keeping safety first, the plant was designed to be able to withstand even a tsunami equivalent to the maximum tide level ever recorded on the shores of Fukushima. Specifically, this would be the tsunami that followed the Chilean earthquake which was approximately 3 m high at Onahama Port. This standard was written into the application for the establishment permits that were submitted to the government and subsequently approved.
(Review during Construction)
At the point of construction of Fukushima Daiichi Nuclear Power Station, in adhering to TEPCO's basic policy of always keeping safety first, the plant was designed to be able to withstand even a tsunami equivalent to the maximum tide level ever recorded on the shores of Fukushima. Specifically, this would be the tsunami that followed the Chilean earthquake which was approximately 3 m high at Onahama Port. This standard was written into the application for the establishment permits that were submitted to the government and subsequently approved.
(Updating the Evaluation with the Guideline Published by Japan Society of Civil Engineers (JSCE))
Later, in 2002, the JSCE published a guideline called the "Tsunami Assessment Method for Nuclear Power Plants in Japan"
based on the ongoing technological progress. In this assessment,
simulation technology was applied and the results were assumed to be
more conservative. Based on this guideline, TEPCO reevaluated the
tsunami height, which was assessed to be approx. 6 m. In response to the
results, TEPCO has voluntarily implemented measures while reporting
them to the government. This tsunami evaluation technology has been the
standard method for domestic nuclear power plants up to the time of the
accident and is also used for assessing tsunamis at nuclear power plants
all around Japan to report to the government including the ones located
along the Pacific coastline.
Although TEPCO believed that the nuclear power plant safeguards put in place were sufficient per this standard, we deeply regret the accident that occurred on March 11th.
Later, in 2002, the JSCE published a guideline called the "Tsunami Assessment Method for Nuclear Power Plants in Japan"
based on the ongoing technological progress. In this assessment,
simulation technology was applied and the results were assumed to be
more conservative. Based on this guideline, TEPCO reevaluated the
tsunami height, which was assessed to be approx. 6 m. In response to the
results, TEPCO has voluntarily implemented measures while reporting
them to the government. This tsunami evaluation technology has been the
standard method for domestic nuclear power plants up to the time of the
accident and is also used for assessing tsunamis at nuclear power plants
all around Japan to report to the government including the ones located
along the Pacific coastline.Although TEPCO believed that the nuclear power plant safeguards put in place were sufficient per this standard, we deeply regret the accident that occurred on March 11th.
3. Incorporating the Latest Knowledge and Theories
Although TEPCO believed that nuclear power plant safeguards had been secured per the standard set down by the Society of Civil Engineers, much consideration was given to applying the latest knowledge and research to the power plant design and operations. In addition, close attention was paid to the latest research trends dealing with earthquakes and tsunamis for the purpose of conducting in-house investigations.
Although TEPCO believed that nuclear power plant safeguards had been secured per the standard set down by the Society of Civil Engineers, much consideration was given to applying the latest knowledge and research to the power plant design and operations. In addition, close attention was paid to the latest research trends dealing with earthquakes and tsunamis for the purpose of conducting in-house investigations.
For example, in July 2002, the HERP published a statement that "there is a possibility that an M8 earthquake could occur anywhere along the trench off the coast from Sanriku to Bousou"
.
The JSCE decided to respond to this statement by conducting a
probabilistic analysis method given that there was a large area along
the trench off the coast of Fukushima where no massive earthquake had
ever occurred in the past. The study and development of this method is
planned to take place from 2003. In 2006, TEPCO conducted a hypothetical
analysis utilizing this JSCE Probabilistic Tsunami Hazard Analysis
methodology and it showed that the occurrence of a tsunami exceeding 10 m
could only occur once every 100 thousand to a million years. However,
this analysis was conducted for the purpose of confirming adaptability
and improving methods that at the time were still in development. Thus,
TEPCO did not interpret this figure to literally mean the actual
frequency of tsunamis that could strike the nuclear power plant in
Fukushima.
Additionally, while reviewing the seismic safety evaluation, in 2008, TEPCO conducted another trial calculation utilizing the wave source model of the Meiji Sanriku-oki Earthquake along the trench off-shore Fukushima (the result: 15.7 m). However, the validity of this wave source model had been in question. TEPCO has requested that JSCE review the suitability of this hypothesis, including whether or not it would be feasible to assume the existence of a wave source in areas where no earthquake had previously occurred. The statement of the HERP assuming that an earthquake might occur anywhere along the trench off-shore Fukushima was not covered in the Central Disaster Prevention Council tsunami evaluation or in the tsunami evaluation for disaster prevention done at each municipality.
.
The JSCE decided to respond to this statement by conducting a
probabilistic analysis method given that there was a large area along
the trench off the coast of Fukushima where no massive earthquake had
ever occurred in the past. The study and development of this method is
planned to take place from 2003. In 2006, TEPCO conducted a hypothetical
analysis utilizing this JSCE Probabilistic Tsunami Hazard Analysis
methodology and it showed that the occurrence of a tsunami exceeding 10 m
could only occur once every 100 thousand to a million years. However,
this analysis was conducted for the purpose of confirming adaptability
and improving methods that at the time were still in development. Thus,
TEPCO did not interpret this figure to literally mean the actual
frequency of tsunamis that could strike the nuclear power plant in
Fukushima.Additionally, while reviewing the seismic safety evaluation, in 2008, TEPCO conducted another trial calculation utilizing the wave source model of the Meiji Sanriku-oki Earthquake along the trench off-shore Fukushima (the result: 15.7 m). However, the validity of this wave source model had been in question. TEPCO has requested that JSCE review the suitability of this hypothesis, including whether or not it would be feasible to assume the existence of a wave source in areas where no earthquake had previously occurred. The statement of the HERP assuming that an earthquake might occur anywhere along the trench off-shore Fukushima was not covered in the Central Disaster Prevention Council tsunami evaluation or in the tsunami evaluation for disaster prevention done at each municipality.
Moreover, TEPCO has reviewed the paper that mentioned the Jogan tsunami occurrence in 869 (Satake, 2008 ),
issued in April, 2009. However, the wave source models in the paper
were based on the results of the tsunami deposit survey in the Sendai
Plain and Ishinomaki Plain. Therefore, the location and scale of the
tsunami remained unverified. The tsunami deposit survey in the coastal
area of Fukushima Prefecture and other adjustment areas were required
for their establishment. TEPCO conducted tsunami deposit surveys, and as
a result, an altitude of about 4m in the northern part of Fukushima
Prefecture was confirmed for the tsunami deposits carried by the Jogan
tsunami while no tsunami deposits were found in the southern part. As
there were inconsistencies between the results of the investigation and
the trial calculation via using the proposed wave source model (the
result: 9.2 m), TEPCO considered it necessary to conduct further
investigation and research in order to determine the wave source of the
Jogan tsunami.
),
issued in April, 2009. However, the wave source models in the paper
were based on the results of the tsunami deposit survey in the Sendai
Plain and Ishinomaki Plain. Therefore, the location and scale of the
tsunami remained unverified. The tsunami deposit survey in the coastal
area of Fukushima Prefecture and other adjustment areas were required
for their establishment. TEPCO conducted tsunami deposit surveys, and as
a result, an altitude of about 4m in the northern part of Fukushima
Prefecture was confirmed for the tsunami deposits carried by the Jogan
tsunami while no tsunami deposits were found in the southern part. As
there were inconsistencies between the results of the investigation and
the trial calculation via using the proposed wave source model (the
result: 9.2 m), TEPCO considered it necessary to conduct further
investigation and research in order to determine the wave source of the
Jogan tsunami.
(The Earthquake in March, 2011)
As shown in the figures below, the focal area of the earthquake in March, 2011, expanded widely from the region off-shore of Iwate to Ibaraki prefectures, an area stretching 500 km by 200 km. The scale completely differs from either the assumed earthquake along the trench off-shore Fukushima proposed by the HERP, or the Jogan earthquake. The scale was so massive that it could be described as an action caused by several simultaneous wave sources, which were required to be assessed individually in the JSCE "Tsunami Assessment Method for Nuclear Power Plants in Japan". Consequently, based on the accumulated knowledge at the time, the Tohoku-Chihou-Taiheiyo-Oki Earthquake could not have been foreseen and TEPCO also considers the height (scale) of the tsunami that followed this earthquake to have been unpredictable.
As shown in the figures below, the focal area of the earthquake in March, 2011, expanded widely from the region off-shore of Iwate to Ibaraki prefectures, an area stretching 500 km by 200 km. The scale completely differs from either the assumed earthquake along the trench off-shore Fukushima proposed by the HERP, or the Jogan earthquake. The scale was so massive that it could be described as an action caused by several simultaneous wave sources, which were required to be assessed individually in the JSCE "Tsunami Assessment Method for Nuclear Power Plants in Japan". Consequently, based on the accumulated knowledge at the time, the Tohoku-Chihou-Taiheiyo-Oki Earthquake could not have been foreseen and TEPCO also considers the height (scale) of the tsunami that followed this earthquake to have been unpredictable.
In spite of the conclusion reached that the occurrence
of a tsunami the size of the one that occurred on March 11th were beyond
the predictive powers of TEPCO and any other Japanese organization
engaged in seismic activity research, we deeply regret and apologize
that the safeguards and measures put in place consequently turned out to
be insufficient. From here on out, we will do our very best to apply
the lessons learned from the tragic events that unfolded on that fateful
day and make every effort to ensure the utmost in preparedness should
another large-scale natural disaster occur.
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Reactor buildings will not collapse even if earthquake with the same scale as that of Tohoku - Pacific Ocean Earthquake occurs again.
April 5, 2012
At 11:04 pm on April 1, a 5.9-magnitude earthquake
centered in the coast of Fukushima Prefecture occurred. Hama-dori of
Fukusihma Prefecture registerd intensity 5 lower on the Japanese seismic
(intensity) scale of 7. No abnormalities were detected at facilities
for water injection into the reactors, nitrogen gas injection, cooling
of spent fuel pool, and the treatment of highly contaminated water at
Fukushima Daiichi Nuclear Power Station. They all operate normally after
the quake. As for the degree of the shake of the reactor buildings,
Unit 6's reactor building's foundation registered 40.7 gal in horizontal
direction and 19.4 gal in vertical direction.
We, TEPCO, evaluates earthquake-proof safety by developing Design Basis Earthquake Ground Motion Ss as large-scale quake which would possibly occur in future. For example, the degree of shake of Unit 6's reactor building's foundation against the Design Basis Earthquake Ground Motion is 448 gal in horizontal direction and 415 gal in vertical direction (, which is around 10 times large in horizontal way and around 20 times large in vertical way compared with the quake occurred on April 1, 2012). We assess that the level of this Design Basis Earthquake Ground Motion is almost same as the one recorded for the Tohoku - Pacific Ocean Earthquake. Based on the Motion, we simulated the damaged situation of the current reactor buildings of Unit 1 to 4, having implemented quake response analysis for the reactor buildings as well as equipments and pipes which are important in terms of safety. As a result, we confirmed that there are no negative signal, such as shear/ twist of quake-proof walls of buildings, the fact that the stress of facilities/ piping lowers the standard value, and the fact that buildings collapse and facilities/ piping lose their functions.
We, TEPCO, evaluates earthquake-proof safety by developing Design Basis Earthquake Ground Motion Ss as large-scale quake which would possibly occur in future. For example, the degree of shake of Unit 6's reactor building's foundation against the Design Basis Earthquake Ground Motion is 448 gal in horizontal direction and 415 gal in vertical direction (, which is around 10 times large in horizontal way and around 20 times large in vertical way compared with the quake occurred on April 1, 2012). We assess that the level of this Design Basis Earthquake Ground Motion is almost same as the one recorded for the Tohoku - Pacific Ocean Earthquake. Based on the Motion, we simulated the damaged situation of the current reactor buildings of Unit 1 to 4, having implemented quake response analysis for the reactor buildings as well as equipments and pipes which are important in terms of safety. As a result, we confirmed that there are no negative signal, such as shear/ twist of quake-proof walls of buildings, the fact that the stress of facilities/ piping lowers the standard value, and the fact that buildings collapse and facilities/ piping lose their functions.
Related material and data
- Submission of Reports about the study regarding current seismic safety and reinforcement of reactor buildings at Fukushima Daiichi Nuclear Power Station (1)
- Submission of Reports about the study regarding current earthquake-proof safety and reinforcement of reactor buildings at Fukushima Daiichi Nuclear Power Station to Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry (2)
- Submission of Reports about the study regarding current earthquake-proof safety and reinforcement of reactor buildings at Fukushima Daiichi Nuclear Power Station to Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry (3)
We have confirmed that the water level of accumulated water inside of Primary Containment Vessel, Unit 2 is approx.60cm from bottom of the vessel, concluding that the inside of the vessel is cold enough.
April 5, 2012
On March 26, investigating inside of the PCV of Unit 2
by means of the industrial endoscope, it was confirmed that the water
level was 60 cm from the bottom of the PCV, the water temperature was in
the range of 48 to 50 ℃, and that the atmospheric temperature on the
water surface was in the range of approx. 42 to 45 ℃. Some media
reported, "the water level inside the PCV was only 60cm from the
vessel's bottom," in spite of water injection to the vessel at the rate
of 9m3/h. The level was lower than expected. But what is important here is twofold:
・It was visually confirmed using the industrial endoscope, not estimated from the outside of the vessel, that there was indeed accumulated water inside the Unit 2's PCV. In addition, the temperature of the water inside was measured using the thermometer calibrated outside.
・If the fuel which was melted down to the bottom of the vessel was not fully immersed in the water, the temperature inside the vessel should have been higher than measured this time. The measured water temperature as well as the atmospheric temperature show that the inside of the vessel has been cold enough.
While it is true that the water which was injected at the rate of 9m3/h was leaked from inside the vessel to the reactor building and to the turbine building, there has been no leakage of the water from each building to groundwater. We, TEPCO, will continue to stably operate the circulating injection cooling system which retrieves the leaked water from the turbine building, removes radioactive materials and salt out of the water, and injects it into the reactor again. By doing this, we make it sure to keep the cold shutdown status. We plan to suppress the leakage by various measures, including to check the degree of damage at torus room located on the first basement floor of the reactor building employing robots and to develop construction methods to block water among buildings.
・It was visually confirmed using the industrial endoscope, not estimated from the outside of the vessel, that there was indeed accumulated water inside the Unit 2's PCV. In addition, the temperature of the water inside was measured using the thermometer calibrated outside.
・If the fuel which was melted down to the bottom of the vessel was not fully immersed in the water, the temperature inside the vessel should have been higher than measured this time. The measured water temperature as well as the atmospheric temperature show that the inside of the vessel has been cold enough.
While it is true that the water which was injected at the rate of 9m3/h was leaked from inside the vessel to the reactor building and to the turbine building, there has been no leakage of the water from each building to groundwater. We, TEPCO, will continue to stably operate the circulating injection cooling system which retrieves the leaked water from the turbine building, removes radioactive materials and salt out of the water, and injects it into the reactor again. By doing this, we make it sure to keep the cold shutdown status. We plan to suppress the leakage by various measures, including to check the degree of damage at torus room located on the first basement floor of the reactor building employing robots and to develop construction methods to block water among buildings.
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Related material and data
TEPCO never proposed a full evacuation.
March 1, 2012
The Media has widely alleged that TEPCO attempted a
full-scale evacuation of all staff from Fukushima Daiichi Nuclear Power
Station. However, this is a misrepresentation of the facts. On March 15,
2011, at approximately 6:30am, the former President Shimizu issued an
order "to evacuate all staff except for the minimum number of staff
needed". In response, the plant general manager ordered the "team
leaders to appoint those who would be remaining". The staff of partner
companies and TEPCO employees (approx. 650 people) who were not directly
involved in the restoration work were temporarily moved to safer
locations and the remaining staff (approx. 70 people) continued to
implement the work.
The message TEPCO communicated to the Prime Minister's Office is that "TEPCO may begin considering the temporary evacuation of staff not directly related to the restoration work and who would eventually have to be evacuated anyway in light of the severe plant conditions". At around 4:30am on March 15th, former President Shimizu was summoned to the Prime Minister's Office and was questioned by former Prime Minister Kan concerning TEPCO's intentions regarding a possible site evacuation. Former President Shimizu replied that a full-scale evacuation was the last thing on his mind. Former Prime Minister Kan also subsequently related the contents of this conversation to the Budget Committee of the House of Councillors on April 18th, 25th, and May 2nd. Former President Shimizu also provided the same response at the press conference on April 13th that a "that a full-scale evacuation was the last thing on his mind".
The message TEPCO communicated to the Prime Minister's Office is that "TEPCO may begin considering the temporary evacuation of staff not directly related to the restoration work and who would eventually have to be evacuated anyway in light of the severe plant conditions". At around 4:30am on March 15th, former President Shimizu was summoned to the Prime Minister's Office and was questioned by former Prime Minister Kan concerning TEPCO's intentions regarding a possible site evacuation. Former President Shimizu replied that a full-scale evacuation was the last thing on his mind. Former Prime Minister Kan also subsequently related the contents of this conversation to the Budget Committee of the House of Councillors on April 18th, 25th, and May 2nd. Former President Shimizu also provided the same response at the press conference on April 13th that a "that a full-scale evacuation was the last thing on his mind".
Related material and data
- Topics/Details
February 9,2012 Asahi Newspaper's "Trap of Prometheus" Article
January 13,2012 Asahi Newspaper's "Trap of Prometheus" Article
January 13,2012 Asahi Newspaper's "Trap of Prometheus" Article
Damaged thermometer caused increase in temperature in the lower part of Reactor Pressure Vessel of Unit 2. Found out it remains cold shut down status.
February 16, 2012
At the beginning of February, it was found that one of
the thermometers in the lower part of Reactor Pressure Vessel (RPV)
indicated the temperature increase. TEPCO investigated both the
possibilities; whether the temperature increased locally or the
thermometer was damaged. We continuously monitor the situation while we
increased the water injection up to double on step-by-step basis in
order to prioritize the cooling of the Reactor.
We concluded from following two reasons that the increase in temperature was due to the damage of the thermometer and the lower part of the RPV is actually cooled adequately; all other thermometers in RPV and Primary Containment Vessel (PCV) indicated the temperature drop in line with increased the water injection, and higher resistance value than normal was measured when investigated the electric circuit.
It was also confirmed that Xenon concentration was below detection limit when gas sampling of PCV, which means there no re-criticality was happened. There is no difference of the concentration of the radioactive Cesium, compared with that measured before the temperature increase.
We concluded from following two reasons that the increase in temperature was due to the damage of the thermometer and the lower part of the RPV is actually cooled adequately; all other thermometers in RPV and Primary Containment Vessel (PCV) indicated the temperature drop in line with increased the water injection, and higher resistance value than normal was measured when investigated the electric circuit.
It was also confirmed that Xenon concentration was below detection limit when gas sampling of PCV, which means there no re-criticality was happened. There is no difference of the concentration of the radioactive Cesium, compared with that measured before the temperature increase.
Related material and data
- Press release (February 16, 2012)
- Sampling result of the Gas Control Sysem of Primary Containment Vessel of Unit 2
→Xenon 135 was below detection limit mesured before and after the temperature increase.
Sampling date and time: Feb. 6, 12:34
Sampling date and time: Feb. 12, 3:22
Sampling date and time: Feb. 15, 11:06
Sampling date and time: Feb. 6, 12:34
Sampling date and time: Feb. 12, 3:22
Sampling date and time: Feb. 15, 11:06
- Nuclide Analysis Results of Radioactive Materials in the Air at the upper part of Unit 2 Reactor Building
→Both Cesium 134 and 137 were no change before and after the temperature increase.
Sampling date: Feb. 13
Sampling date: Feb. 6
Sampling date: Jan. 13
Sampling date: Feb. 13
Sampling date: Feb. 6
Sampling date: Jan. 13
Cooling of Unit 4 spent fuel pool properly accomplished
February 14, 2012
The temperature of the spent fuel pool of Fukushima
Daiichi NPS Unit 4 is maintained between 25 - 30 degrees Celsius, and
the top of the spent fuels are 7 meters below the water surface of the
pool. Due to the earthquake that occurred on January 1, of which the
epicenter was near Karasujima, the water level of the skimmer surge tank
became temporarily low, but there was no change in the water
temperature and the water level of the pool. We put a camera in the pool
on February 9 in order to measure the clarity, and we confirmed that,
although there is a certain amount of debris due to the explosion of the
building, as we have confirmed on May 7 last year, the spent fuels
themselves are secured inside the rack.
Furthermore, on January 22, we have inspected the condition of the fifth floor of the reactor building, and we have confirmed that the floor was parallel to the water surface of the reactor well, which indicates that the building is not leaning.
Furthermore, on January 22, we have inspected the condition of the fifth floor of the reactor building, and we have confirmed that the floor was parallel to the water surface of the reactor well, which indicates that the building is not leaning.
Related material and data
- Latest image of inside the pool
(2) A picture taken inside the spent fuel pool of Unit4 (2:07)
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