Disaster 04 · Engineering disasters
The sea wall built for a smaller wave
A tsunami far taller than the plant was designed for flooded the backup power at Fukushima Daiichi, causing three reactor meltdowns, after warnings about larger waves had not been acted on.
- Tokyo Electric Power Company (TEPCO)
- Fukushima Prefecture, Japan
- 2011
- 3 min read
On this page
Before you readWhat would you do?
Your small shop keeps its sales records on a laptop, and the backup drive sits in the same drawer. Your area has started flooding more often in the rainy season. What is the most useful change?
Pick an answer now. The file tells you whether you were right.
01The file in one minute
1A wave above the design
The tsunami reached roughly 13 to 15 metres at a plant designed for about 5.7 metres.
2Backup in the basement
Flooding knocked out generators and electrical equipment, so the plant lost almost all power.
3Warnings delayed
Studies of possible larger tsunamis existed. A parliamentary commission called the accident man-made.
4The lesson
Plan for the risk the evidence shows today, and keep backups where the same event cannot reach them.
02Timeline
11 March 2011, 2:46 pm
A magnitude 9.0 earthquake strikes. The reactors shut down automatically.
About 50 minutes later
The tsunami floods the plant and cuts almost all power.
12 to 15 March 2011
Meltdowns and hydrogen explosions damage several reactor buildings.
July 2012
Japan's parliamentary commission concludes the accident was man-made.
03What happened
On 11 March 2011 a magnitude 9.0 earthquake struck off the north-east coast of Japan. The reactors running at the Fukushima Daiichi nuclear plant shut down automatically, and diesel generators started up to keep the cooling pumps running.
About 50 minutes later, the tsunami arrived. At the plant the waves reached roughly 13 to 15 metres. The site had been designed for a tsunami of about 5.7 metres. Water flooded the generators and electrical equipment, many of them in basements, and the plant lost almost all power.
Without cooling, the cores of reactors 1, 2 and 3 melted. Hydrogen explosions damaged several reactor buildings. More than 100,000 people were evacuated. The radiation itself caused few if any immediate deaths, but the evacuation was linked to many deaths among elderly and sick people.
In 2012 an independent commission set up by Japan's parliament concluded that the accident was man-made. It found that TEPCO and the regulators had known of studies suggesting larger tsunamis were possible and had delayed acting on them.
04Why it happened
The plant's defences were built around the biggest waves people had planned for, and new evidence of bigger ones was treated as something to study later. Critical equipment sat where a single flood could knock it all out at once.
The commission also blamed close ties between the operator and its regulators, and a reluctance to question earlier decisions. Admitting that the design basis was too low would have meant costly changes, so the old number stayed.
The idea behind this caseLesson: How machines learn →
05Lessons for decisions
- Plan for the worst case the evidence now allows, not the worst case you planned for years ago.
- Look for single points of failure, where one event could knock out both the main system and its backup.
- When new evidence suggests a bigger risk, set a date by which you will decide what to do about it.
- Keep your backups somewhere the same disaster cannot reach, such as a copy of important files off site.
07 — Sources
- The Fukushima Daiichi Accident: Report by the Director General · International Atomic Energy Agency · 2015
- Fukushima nuclear accident · Wikipedia · 2026