The Fukushima Dai-ichi nuclear power complex was Japan's oldest nuclear power plant, with six reactors, all built in the 1970s.
They are boiling-water reactors -- an older nuclear technology that differs from the heavy water CANDU reactors used at Canada's seven nuclear power plants. The reactors were housed in six square buildings in a complex on the edge of the Pacific coast.
Here's what is believed to have happened to set off the nuclear crisis underway at Fukushima:
• On March 11, 2011, the facility was rocked by a 9.0-magnitude earthquake just before 3 p.m. local time.
• At the time, three of the reactors were operating. Reactors 4, 5, and 6 had been shut down ahead of time, for planned maintenance.
• The quake tripped the facility's scram system, which automatically shut off reactors 1, 2 and 3.
• The reactors had been installed with neutron-absorbing boron control rods between fuel rods to halt the usual nuclear activity inside the reactor. The control rods did exactly as they were designed to do, absorbing enough neutrons to stop the nuclear chain reaction within a matter of seconds.
• However, even once the chain reaction stopped, radioactive materials in the reactor core continue to produce heat, called decay heat. That heat falls to a quarter of its original level in the first hour after a shutdown, and then disappears more slowly. But that's only if the core is kept cool after the shutdown.
• Once a nuclear reactor is shut down, the fuel needs to be kept cool or it will begin to overheat. In fact, even once nuclear fuel is used up, the fuel rods need to be kept cool for a number of years until the natural decay processes fizzle out.
• Normally, heat inside a reactor is removed by water that circulates around the core. That water is circulated with electric pumps.
• When the quake hit Japan, it knocked out power to the Fukushima area. The facility had 13 diesel-powered generators on site that were intended to kick in if the plant ever lost power. But the generators were housed in secure, underground rooms right on the water's edge. The quake's resulting 7-metre tsunami flooded the generator building, knocking all but one of the generators out of commission.
• There were batteries on site designed to act as backups to the generators. But the power within them was consumed within a few hours.
• The lack of electricity caused the cooling system to shut down, allowing the reactors to begin to overheat. Once the fuel inside reaches roughly 4,000 degrees Fahrenheit (2,200 Celsius), the uranium fuel pellets start to melt – something that Japanese authorities conceded did happen at all three reactors, at least partially.
• Workers were forced to manually pump in seawater into the thick steel container called the reactor vessel. Since salt water is highly corrosive, the move likely permanently destroyed the reactors. But they had trouble maintaining enough water in the reactor vessels and water levels soon began to drop.
• By 7 p.m. local time on Friday, March 11, the crisis inside the facility was becoming clear, prompting Prime Minister Naoto Kan to declare a nuclear emergency, prompting the first evacuations.
• By Saturday, March 12, the pumped-in seawater had caused a buildup of hydrogen in the reactor vessels. Engineers were forced to vent the hydrogen into the atmosphere, which released gas containing small amounts of radioactive particles. The venting is what set off two explosions at the facility.
