Japan's nuclear crisis

Timeline: How Japan's nuclear crisis unfolded

     By Bryony Jones, for CNN

(CNN) -- Authorities in Japan are battling to resolve a series of crises at the nuclear power plant at Fukushima Daiichi, which was badly hit by last Friday's devastating earthquake and tsunami. Here's a look at efforts to contain the damage and avert a potential nuclear meltdown. (all times and dates are local).

Friday, March 11

2.46 p.m. (0.46 a.m. ET/5.46 a.m. GMT): A magnitude 9.0 earthquake strikes an area 370 kilometers (230 miles) northeast of Tokyo, Japan, at a depth of 24.5 kilometers.The offshore quake, the fifth largest worldwide since records began, sparks a major tsunami warning across the Pacific. Within an hour a wall of water up to 9 meters (30 feet) high hits the Japanese coast, sweeping away towns and villages in its path. The quake causes serious damage at Tokyo Electric Power Company's (TEPCO) Fukushima Daiichi nuclear power plant, about 65 kilometers south of Sendai. Three of the plant's six reactors, which came into service between 1970 and 1979, were already shut down for inspection at the time the disaster struck. Those still in operation are designed to also shut down in the event of a quake, with diesel generators pumping water around the reactors to keep them cool. But when the tsunami hits, flood water swamps the generators, causing them to fail. The reactors begin to heat up.

8.15 p.m.: The Japanese government declares an emergency at Fukushima Daiichi power plant.

10.30 p.m.: Authorities reveal the cooling system at the plant is not working, and admit they are "bracing for the worst."

Saturday, March 12

2.06 a.m.: Radiation levels in the No.1 reactor at Fukushima are reported to be rising.

3.24 a.m.: Japanese trade minister Banri Kaieda warns that a small radiation leak could occur at the plant.

6.45 a.m.: TEPCO says radioactive substances may have leaked at Fukushima. Japan's Nuclear and Industrial Safety Agency says radiation near the plant's main gate is more than eight times the normal level.

4.19 p.m.: Japan's Nuclear and Industrial Agency reveals a small amount of radioactive cesium has escaped from the power plant, possibly caused by a fuel rod melting.

EXPLOSION: 6.22 p.m.: A hydrogen explosion at Fukushima's reactor No.3 blows the roof off the containment structure around the No.1 reactor and injures four people.

8.18 p.m.: Residents living within 20 kilometers of the plant are told to evacuate the area. Some 200,000 people leave.

8.54 p.m.: Authorities insist no harmful gases were emitted as a result of the explosion at the Fukushima plant, blaming the blast on "water vapor that was part of the cooling process."

10.35 p.m.: Radiation levels around the plant fall as officials prepare to flood the containment structure around the reactor with sea water to cool it. Meanwhile, authorities prepare to distribute iodine tablets to residents near the damaged nuclear plant to prevent radiation poisoning.

Sunday, March 13

3.20 a.m.: Three people randomly selected from a group of 90 test positive for radiation exposure in Fukushima prefecture.

5.37 a.m.: Japanese authorities say Saturday's explosion at the Fukushima Daiichi plant occurred outside the primary containment vessel, adding that the vessel's integrity has not been compromised.

4.46 p.m.: Chief Cabinet Secretary Yukio Endo warns of the possibility that a second explosion could happen at the No.3 reactor.

Monday, March 14

EXPLOSION: 11 a.m.: Hydrogen explosion at the No.3 reactor damages the cooling system at the No.2 reactor and injures 11 people. A wall at the plant collapses as a result of the blast, but officials say the containment vessel surrounding the reactor remains intact. Authorities begin pumping a mixture of sea water and boron into the No.2 reactor to cool its nuclear fuel rods. Those residents living within 20 kilometers of the plant who have so far ignored evacuation orders are warned to stay indoors. Up to 2.7 meters of the No.2 reactor's control rods are left uncovered because the pump which keeps them cool has run low on fuel after being left unattended. It causes them to heat up generating radioactive steam.

Tuesday, March 15

EXPLOSION: 6 a.m.: An "explosive impact" rocks the No.2 reactor -- the third blast at the plant in four days -- and damages its suppression pool.

7 a.m.: The U.S. Navy begins repositioning ships and planes after detecting low-level "airborne radioactivity" in the region. Three people on the U.S.S. Ronald Regan earlier tested positive for low levels of radiation.

8.30 a.m.: Chief Cabinet Secretary Yukio Edano says he cannot rule out the possibility of a meltdown at all three of the plant's damaged reactors. He says radiation levels at the plant have increased to "levels that can impact human health", and warns anyone living within 20 and 30 kilometers of the plant to remain indoors. Almost all of the plant's staff, about 800 people, are evacuated from the site, with just 50 remaining to carry out emergency operations.

FIRE: 8.54 am: Fire breaks out in a cooling pond used for nuclear fuel at the No.4 reactor -- which had been shut down before Friday's quake. Japanese Prime Minister Naoto Kan warns that "there is still a very high risk of further radioactive material coming out," but urges the public to remain calm. The government imposes a no-fly zone within a 30-kilometer radius of the plant.

11 a.m.: The fire in No.4 reactor is reported to have been extinguished. The International Atomic Energy Agency reveals that radiation levels at the plant have been recorded at 167 times the average annual dose of radiation, but that level is expected to drop quickly.

11.10 p.m.: The IAEA reports that Monday's blast at reactor No.2 "may have affected the integrity of its primary containment vessel."

11.45 p.m.: TEPCO says it plans to use helicopters to pour water onto reactor No.4 in order to cool the nuclear fuel rods.

Wednesday, March 16

FIRE: 7 a.m.: The second fire in two days is discovered in the building of the No. 4 reactor at Fukushima Diiachi. This one is in the northeastern corner of the building, an official with Tokyo Electric and Power told reporters.

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