.
Chance of once in 10,000 years: Three Mile Island [1979] – Chernobyl [1986] – Fukushima [2011]

    According to the Japanese Nuclear Safety Organization, a probabilistic safety assessment (PSA) was performed on all nuclear power facilities by 2002, with results showing the chance of a core damage accident was 1/100,000 or less per year per reactor. The chance of an accident leading to containment damage is 1/1,000,000 or less per year per reactor. Because of the many safety measures in place, “the occurrence of [a] severe accident is practically unlikely from an engineering viewpoint.”

One day ago: Japan nuclear plants nearest earthquake safely shut down

March 11, 2011 — Latest reports indicate that the four nuclear power plants in Japan closest to the epicenter of the 8.9 earthquake have been safely shut down, according to the International Atomic Energy Agency.

“The four Japanese nuclear power plants closest to the quake have been safely shut down,” the agency said in a statement.

The IAEA, based in Vienna, is monitoring the situation closely and will continue to seek information on any nuclear facilities that might be put at risk by the earthquake or the tsunami triggered by it.

Earlier reports from Japanese news wires said that the reactor cooling systems in two units of Fukushima No. 1 Nuclear Power Plant had failed. Other reports held that the plant’s operator, Tokyo Electric Power Co., was considering venting radioactive vapor into the air to relieve reactor pressure that had risen.

President Barack Obama held a news conference in which he said that the Japanese government had assured him there had been no reports of radiation leakage from the Japanese power plants.  

Today the Japanese government declared a state of emergency, its first ever at a nuclear plant …

Japan quake causes emergencies at 5 nuke reactors

TOKYO–Japan declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability in the aftermath of Friday’s powerful earthquake. Thousands of residents were evacuated as workers struggled to get the reactors under control to prevent meltdowns.

A single reactor in northeastern Japan had been the focus of much of the concern in the initial hours after the 8.9 magnitude quake, but the government declared new states of emergency at four other reactors in the area Saturday morning.

The earthquake knocked out power at the Fukushima Daiichi plant, and because a backup generator failed, the cooling system was unable to supply water to cool the 460-megawatt No. 1 reactor. Although a backup cooling system is being used, Japan’s nuclear safety agency said pressure inside the reactor had risen to 1.5 times the level considered normal.

Authorities said radiation levels had jumped 1,000 times normal inside Unit 1 and were measured at eight times normal outside the plant. They expanded an earlier evacuation zone more than threefold, from 3 to 10 kilometers (2 miles to 6.2 miles). Some 3,000 people had been urged to leave their homes in the first announcement.

UPDATE
Japan official: Meltdown at nuclear plant possible

SENDAI, Japan — A nuclear power plant affected by a massive earthquake is facing a possible meltdown, an official with Japan’s nuclear safety commission said Saturday.

Ryohei Shiomi said that officials were checking whether a meltdown had taken place at the Fukushima Daiichi power plant’s Unit 1, which had lost cooling ability in the aftermath of Friday’s powerful earthquake.

Operators at the Fukushima Daiichi plant’s Unit 1 scrambled to tamp down heat and pressure inside the reactor after the 8.9-magnitude quake and the tsunami that followed cut off electricity to the site and disabled emergency generators, knocking out the main cooling system.

Japan’s nuclear safety agency said the situation was most dire at Fukushima Daiichi’s Unit 1, where pressure had risen to twice what is consider the normal level. The International Atomic Energy Agency said in a statement that diesel generators that normally would have kept cooling systems running at Fukushima Daiichi had been disabled by tsunami flooding.

"But I will not let myself be reduced to silence."