Following are main developments after a massive earthquake and tsunami devastated northeast Japan and crippled a nuclear power station, raising the risk of an uncontrolled radiation leak.

• Engineers work through the night to pump nitrogen gas into a reactor at the Fukushima Daiichi nuclear power plant crippled by the March 11 quake and tsunami to prevent an explosive buildup of hydrogen gas. Earlier efforts stopped highly radioactive water leaking into the sea at another of the facility's six reactors.

• Plant operator Tokyo Electric Company (TEPCO) says a buildup of hydrogen after attempts to cool the reactors with water could produce an explosion, as occurred early in the crisis in reactors No. 1 and 3. But the likelihood of this was "extremely low."

• After using water to cool fuel rods, engineers must still pump 11.5 million liters (11,500 tons) of contaminated water back into the ocean as they have run out of storage space.

• The head of a U.N. Scientific body says the situation at the plant is not expected to have any serious impact on people's health. Data shows much lower levels of iodine than in the 1986 nuclear accident at Chernobyl in Ukraine.

• Radioactive iodine detected in the sea now stands at about 600 times the legal limit after being recorded at 4,800 times the limit previously.

• A top official for the U.S. Nuclear Regulatory Commission says the Commission does not believe the core of reactor no. 2 melted down.

• The coast guard says the quake shifted the seabed near the epicenter in northern Japan by a record 24 meters (79 feet).

• TEPCO this week began paying "condolence money" to nearby local governments to aid people evacuated because of the crisis.

• Japan has asked Russia to send a floating radiation treatment plant, used to decommission nuclear submarines, to solidify contaminated liquid waste from the plant, Russian media reported.

• Japan has warned it could take months to stop radiation leaking from the plant.

• Authorities do not plan to expand the evacuation zone around the plant, a senior nuclear official said. The government created a 20-km (12-mile) evacuation zone around the site after the earthquake and tsunami.

• TEPCO has said it will scrap at least four reactors once they are under control, but this could take years or even decades. The Fukushima Daiichi and nearby Fukushima Daini plants produce 4%  of Japan's power and local politicians say reopening them will be politically difficult.

• A total of 12,554 people were confirmed dead by Japan's National Police Agency, while 15,077 are missing as of Wednesday. A total of 162,481 households were without electricity and at least 170,000 without running water.

• Estimated cost of damage to top $300 billion, making it the world's costliest natural disaster. The 1995 Kobe quake cost $100 billion while Hurricane Katrina in 2005 caused $81 billion in damage.