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This case has been updated. Compatible choices are kept; revisit the evidence before drawing your conclusion.
01 · The briefing
Begin with the question.
On 11 March 2011 the Great East Japan Earthquake, of magnitude 9.0, struck off the north-east coast of Japan at 14:46 local time. At Fukushima Daiichi, three of the six reactors were operating at full power, and the three operating units shut down automatically when the ground motion was detected. The earthquake also cut the plant's connection to the grid, so the plant depended on emergency diesel generators to keep cooling water moving through the reactor cores.
The question is why the tsunami that followed defeated those defences. Several explanations are possible. The wave may have been larger than anyone could reasonably have expected. The seawall may have been too low for the site. The backup power may have been placed where a flood could reach it. Or the operator's own earlier estimates may have been ignored. Each explanation predicts different evidence: the height of the wave, the design height of the wall, the location of the backup equipment and the record of tsunami estimates before 2011.
This case uses the IAEA's 2015 report to the Director General and the World Nuclear Association's account to separate what the wave did from what the plant was designed to withstand, and it notes where the two sources disagree.
The timeline
A consensus-based method for tsunami flood levels is developed in Japan, and the operator's reassessments using it give values above the original design basis.
TEPCO calculates a potential tsunami height of 15.7 m at the Daiichi site, according to WNA. The IAEA reports a trial calculation of about 15 m for 2007 to 2009.
At 14:46 Japan Standard Time a magnitude 9.0 earthquake occurs. The earthquake also cuts off the plant's off-site power, and reactor protection shuts down Units 1 to 3.
The wave that floods the site arrives at 15:36, according to the IAEA. Its run-up of 14 to 15 m overtops the seawall, disables the seawater pumps and emergency diesel generators, and leaves Units 1 to 5 without AC power.
At 15:36 an explosion occurs on the service floor of the Unit 1 reactor building, according to the IAEA.
The IAEA publishes its report to the Director General on the accident (GC(59)/14).
A researched reconstruction for learning. These reasoning steps simplify a complex record; no score can capture the full history.
02 · The evidence
What does the record show?
Read each exhibit before settling on an explanation. The source links below every exhibit lead to the record behind it.
Exhibits reviewed 0 of 4
The seawall's design maximum and the second wave
The IAEA's 2015 report describes tsunami barrier seawalls designed to protect against a maximum tsunami height of 5.5 m. It says a second and larger wave, with a run-up of 14 to 15 m, arrived about ten minutes after the first and overwhelmed the seawalls. This establishes the mismatch between the wall's design maximum and the run-up the report describes. The passage does not explain how the 5.5 m figure was derived.
Source record s1www-pub.iaea.org
A 2007 to 2009 trial calculation
The IAEA report describes a TEPCO trial calculation for 2007 to 2009 that postulated an earthquake of magnitude 8.3 off the Fukushima coast. It gives a tsunami run-up of around 15 m at the Daiichi site, similar to the actual height on 11 March 2011, which would inundate the main buildings. The report also says TEPCO did not take interim compensatory measures in response to these increased estimates. This is a scenario calculation, not a forecast.
Source record s1www-pub.iaea.org
A design basis revised upward in steps
WNA gives the original design basis as 3.1 m, set from the 1960 Chile tsunami, with the plant about 10 m above sea level and the seawater pumps 4 m above it. It says the basis was revised to 5.7 m in 2002 and to 6.1 m in 2009, and that TEPCO calculated 15.7 m in 2008 without major action following. This is a secondary summary, and its design figures differ from the IAEA's 5.5 m.
Source record s2world-nuclear.org
How the flood caused the blackout
The IAEA's account says the wave flooded and damaged the unhoused seawater pumps and the water-cooled emergency diesel generators, and that water flooded the reactor and turbine buildings and damaged the emergency generators or their power connections. Units 1 to 5 therefore lost all AC power, a station blackout. Only the air-cooled emergency diesel generator at Unit 6 was unaffected and kept Unit 6's safety systems cooled. This establishes the mechanism of the blackout.
Source record s1www-pub.iaea.org
03 · Your reasoning
Follow the strongest explanation.
Choose an interpretation, then compare it with the evidence. You can revise your choice. The goal is understanding, not a perfect score.
04 · The findings
The chain behind the outcome.
Your notebook
Case report
Next in engineering & technology: The pressure test read as a seal
The sources agree on the core sequence. The earthquake of 11 March 2011 shut down the three operating reactors and cut off off-site power. A tsunami then came over the seawalls, damaged the seawater pumps and the emergency diesel generators, and left Units 1 to 5 without AC power, a station blackout. Only the air-cooled generator at Unit 6 survived. The IAEA gives the seawall's design maximum as 5.5 m and the run-up of the second wave as 14 to 15 m. WNA gives the design basis as 3.1 m in the 1960s, revised to 5.7 m in 2002 and 6.1 m in 2009, and reports that TEPCO's 2008 calculation gave 15.7 m. The IAEA reports a 2007 to 2009 trial calculation of about 15 m, and says TEPCO took no interim compensatory measures. The defences were therefore built to values below the operator's own later estimates. The blackout's mechanism is the best supported part of the record. Why the calculations were not acted on depends on institutional decisions that the physical evidence cannot settle.
Contributing causes and conditions
- A magnitude 9.0 earthquake shut down the operating reactors and cut off off-site power, so the plant relied on its emergency generators.
- The seawall was designed for a maximum tsunami of 5.5 m, well below the later calculations of about 15 m.
- The second wave, with a run-up of 14 to 15 m, overtopped the seawalls about ten minutes after the first.
- The flood damaged the seawater pumps and the emergency diesel generators and their power connections.
- With the AC power gone, Units 1 to 5 lost the cooling that the backup systems would have provided, a station blackout.
OutcomeFukushima Daiichi accident (2011)
Take the method with you
- Compare a safety margin with the most recent hazard estimate, not only with the figure used when the design was approved.
- Check whether a defence was designed to the same scenario that a later analysis produced.
- When sources give different figures, report the range and say which figure each source uses.
You can read findings at any time. Exploring every reasoning step marks this case as complete in your notebook.
The open record
Sources & further reading
Read the original record and its scope before relying on a conclusion. Source references support the exhibit summaries; Faultory’s reasoning prompts are editorial reconstructions.
- s1
- s2World Nuclear Association — Fukushima Daiichi accident
world-nuclear.org
Written and curated for Faultory by Yunus Emre Vurgun, with AI-assisted research and drafting. Sources and interpretations can need correction. Read our editorial standards or report an issue.
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