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The 2011 disaster at the Fukushima Daiichi Nuclear Power Plant was the worst nuclear event since the meltdown at Chernobyl in the former Soviet Union 25 years prior.

It started with an earthquake. It resulted in 465,000 evacuations, $360 billion in economic losses and increased radiation levels in Tokyo, 140 miles away.

As with most disasters, several things had to go wrong to produce such a catastrophic outcome. Below is a detailed account of how the devastation unfolded. 

Fukushima Timeline: How an Earthquake Triggered Japan’s Nuclear Disaster

A broken picture frame lies in the mud within the exclusion zone, about 12 miles (20 kilometers) away from Fukushima Nuclear Power Planton April 7, 2011 in Minamisoma, Fukushima Prefecture, Japan. 

March 11, 2011: An Earthquake Precipitates Crisis

2:46 pm: The westward moving Pacific Plate, an oceanic tectonic plate, lurches downwards beneath the North American plate, causing an earthquake 43 miles off the northeastern coast of Honshu, Japan’s most populous island. The earthquake has a magnitude of 9.1, making it the largest earthquake in Japan’s history—and one of the five most powerful earthquakes globally recorded since modern record-keeping began.

3:27 p.m.: The earthquake sets off a tsunami. The first wave arrives at the Fukushima Daiichi Nuclear Power Plant in the form of a 13-foot-high wave, which is deflected by a sea wall built to withstand waves up to 33 feet high.

3:35 p.m.: A second wave, this one over 50 feet high, breaches the wall. It destroys seawater pumps, drowns power panels that distribute energy to water pumps, and surges into basements where backup generators are housed. In five of the six reactors, AC power is lost; without the power, water pumps can’t provide the steady flow of cool water to the reactors’ intensely hot cores. Without the regular flow of cooling water, a meltdown will inevitably follow.

READ MORE: Chernobyl Timeline: How a Nuclear Accident Escalated to a Historic Disaster

3:37 p.m.: With flooding having destroyed the generator’s backup batteries, Unit 1 loses DC power as well. The control room for Units 1 and 2 goes dark, depriving power plant operators any capacity for monitoring the two reactors.

Just before 6 p.m.: A work crew goes to the 4th floor of the Unit 1 reactor building without protective clothing. Their dosimeters read off-the-scale levels of radiation, indicating that the core of Unit 1 is exposed and its fuel rods ruptured.

7:03 p.m.: Prime Minister Naoto Kan declares a nuclear emergency.

9:00 p.m.: The Japanese government issues evacuation orders for the several thousand residents living within a 1.9-mile (3 kilometer) radius of the power plant. 

Fukushima Daiichi Nucler Power Plant

Fukushima Daiichi Nucler Power Plant seen the day after an 9.0 magnitude strong earthquake struck on March 11, 2011 off the coast of north-eastern Japan. 

March 12: Evacuation Area Expands, the Roof Blows

Shortly before 6 a.m.: Prime Minister Kan decides to go to Fukushima. He orders authorities to widen the evacuation zone to 6.2 miles (10 kilometers). With the loss of coolant, temperature and pressure builds inside the reactors.

10:09 a.m.: The Tokyo Electric Power Company (TEPCO) announces they have vented some steam from Unit 1 in an attempt to lower the temperature and pressure. The venting means that some radioactive material has been released into the air.

10:58 a.m.: Unit 2, it is announced, has likewise been vented.

3:36 p.m.: A hydrogen explosion blows the roof off Unit 1, collapsing concrete walls and leaving behind only the steel framework. Four workers are injured in the explosion. In addition to the harm to the workers, the explosion damages electric cable that workers had been laying for the purposes of restoring power to Units 1 and 2. The explosion also damages fire hoses that workers had arranged, hampering the plant’s ability to deliver coolant to the reactor core.

Just before 6:30 p.m.: The evacuation area is expanded to a 12.4- mile (20 kilometer) radius.

8:20 p.m.: TEPCO begins injecting seawater into Unit 1, as a substitute coolant. The decision to use seawater is the death knell to Reactor 1: Unlike fresh water, it irreparably corrodes pumps and pipelines. Around the same time, Japan’s Nuclear and Industrial Safety Agency (NISA) detects harmful radiation levels of cesium 137 and iodine 131 near the plant.

READ MORE: The Chernobyl Coverup: How Officials Botched Evacuating an Irradiated City

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March 13

6:23 a.m.: A NISA official announces that the emergency cooling system in the Unit 3 reactor has failed.

10:05 p.m.: TEPCO begins injecting seawater into Unit 3.

10:09 p.m.: TEPCO announces a plan to inject seawater into Unit 2, the first acknowledgement of emergency at that reactor.

In this satellite view, the Fukushima Dai-ichi Nuclear Power plant after a massive earthquake and subsequent tsunami on March 14, 2011. Japanese officials report that a fire at the stricken Fukushima nuclear power plant has released radioactive material into the air in the latest development in the chaos wrought by the recent earthquake and tsunami that have left at least 10,000 people dead in northeastern Japan. 

In this satellite view, the Fukushima Dai-ichi Nuclear Power plant after a massive earthquake and subsequent tsunami on March 14, 2011. Japanese officials report that a fire at the stricken Fukushima nuclear power plant has released radioactive material into the air in the latest development in the chaos wrought by the recent earthquake and tsunami that have left at least 10,000 people dead in northeastern Japan. 

March 14: Explosions Continue

11:01 a.m.: There’s a hydrogen explosion at the Unit 3 reactor. 11 workers are injured, and the building’s structure is severely damaged.

March 15

6:14 a.m.: A hydrogen explosion occurs at the Unit 2 reactor.

Throughout the day: Seawater pumping continues at Units 1, 2 and 3. Near the plant, radiation levels are measured at 400 millisieverts per hour. By comparison, the average person is exposed to about 2.4 millisieverts of radiation per year, meaning that radiation at Fukushima is 1.46 million times stronger than it would be in an average environment.

March 17

The military begins using helicopters to dump seawater onto Unit 3, where radiation levels are at 17 millisieverts per hour.

March 19

Replacement diesel generators are successfully implemented at Units 5 and 6, pumping water back into those reactor cores. Elsewhere, the extent of damage becomes clearer: Milk and water in the greater Fukushima Prefecture show excessively high levels of radioactive iodine.

READ MORE: Chernobyl: 7 People Who Played a Crucial Role in the World's Worst Nuclear Disaster

Radioactive soil has been decontaminated from the land around Fukushima and packed in black bags that are stacked on piles while waiting to be transported to the intermediate storage.

Radioactive soil has been decontaminated from the land around Fukushima and packed in black bags that are stacked on piles while waiting to be transported to the intermediate storage.

March 20: Things Start to Stabilize

Temperatures stabilize at Units 5 and 6, bringing about the safe harbor of “cold shutdown” conditions. Electrical power is restored to Unit 2.

March 22

Eleven days after the initial disaster, electrical power is restored to the control rooms of Units 1 and 2. In the wastewater just south of the plant, radioactive iodine is measured at 126.7 times higher than the legal limit.

March 25

The Unit 1 Reactor temperature is brought down to 204.5 degrees Celsius, safely inside its design limits. The Japanese government advises those residents who are between 20 and 30 kilometers away from the the plant to voluntarily evacuate the area.

READ MORE: How the Three Mile Island Accident Was Made Even Worse by a Chaotic Response

March 26

Seawater that’s tested near the plant has 1,250 times the legal limit of iodine 131.

April 11

A new earthquake, of magnitude 7.0, rocks eastern Japan. For 50 minutes, Fukushima loses power, preventing cooling water from reaching Units 1, 2, and 3.

April 12: Atomic Disaster Declaration

The International Atomic Energy Agency rates the Fukushima Crisis a disaster magnitude of 7, the highest of their scale.

May 11

Evacuees who have abandoned homes within 20 kilometers of Fukushima are given two hours to return for important documents or belongings left behind in the initial haste of their evacuation.

February 2, 2012

Nearly a year after the disaster, the village of Kawauchi—one of nine evacuated municipalities less than 20 kilometers from the plant—announces plans to reopen in the spring.

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