All 6 reactor units of the Daiichi Nuclear Power Station have been shut down.

The plant is 40 years old. The tsunami overpowered the facility’s diesel generators, leading to emergency shutdowns of the reactors. Key to the situation: cooling the reactor cores. Workers are using seawater and boron in cooling efforts -- but if they don’t do it fast enough there’s risk of a full-blown meltdown. Experts say if more than about 35 per cent of the uranium cores are exposed, meltdown to some degree is inevitable.

There are six reactors in all at Tokyo Electric Power Company's Fukushima Daiichi plant, located in northeastern Japan about 65 km south of Sendai.

These images by DigitalGlobe show units 1 through 4, with the first three units experiencing the most problems.

Partial meltdown likely underway. Sustained substantial damage with upper structure largely destroyed with the roof and side panels blown off in a Sunday, March 13 explosion. Four people were injured.

Workers are injecting sea water and boric acid, which absorbs neutron into the reactor pressure vessel.

This is the oldest of the six units at Daiichi, according to the World Nuclear Association. It began operation in March 71.

Partial meltdown likely underway. Three blasts in four days. Pressure readings indicate damage to the reactor’s suppression pool -- a donut-shaped reservoir at the base of the containment vessel.

Under Japanese government orders, measures underway to lower pressure of the reactor containment vessel. Japan's Nuclear and Industrial Safety Agency said Tuesday that up to 2.7 meters (8.8 feet) of the No. 2 reactor's control rods – which amount to about half -- have been uncovered. Workers are injecting sea water and boric acid to cool the reactor.

No. 2 began commercial operation in 1974.M

A hydrogen explosion rocked this reactor on Monday, March 14. A big plume of white smoke was seen rising from the building and drifting east approximately 2 km.

The reactor containment vessel remains intact for now, according to TEPCO. Eleven workers were injured in that blast. Workers have been pumping seawater and boron into the reactor since Monday.

This reactor began commercial operation in 1976.

A U.S. official said all the water from one of the spent fuel rod pools was gone on Wednesday, March 16. Meaning there’s nothing to stop the rods from getting hotter and melting. But TEPCO denied that claim. The official also said radiation levels are extremely high at No. 4.

Earlier in the week the rooftop sustained damage in an explosion, attributed to disaster responders having failed to fully extinguish a fire that struck the same reactor on March 15.

This reactor was shut down for regular maintenance before the March 11 earthquake crippled the power plant.

This is a November 21, 2004 satellite image of the Fukushima Daiichi Nuclear Power Plant in Okuma, Japan. Daiichi translates to number 1.

The plant’s six boiling water reactors have a combined power of 4.7 gigawatts (GW) -- making it one of the 15 largest nuclear power stations in the world. As a point of comparison, the Hoover Dam pumps out about 2 GW and the largest nuclear plant in the U.S. outputs about 4 GW.

Daiichi was the first nuclear plant to be constructed and run entirely by TEPCO - the Tokyo Electric Power Company.

Reactors for Units 1, 2, and 6 were supplied by General Electric; units 3 and 5 by Toshiba; and unit 4 by Hitachi.