The 1970s era Fukushima No. 1 plant was rocked by the 9.0-magnitude quake on March 11 and then hit by the 14-metre (46-foot) tsunami it triggered, cutting it off from the national electric grid and knocking out its backup power supply.

This shut down the cooling systems needed to keep the fuel rods inside reactors, and spent rods in containment pools, from overheating and boiling off the water around them, which can lead to their melting down and releasing radiation.

To stop a catastrophe, crews have pumped thousands of tonnes of water into the reactors and pools, urgent work now creating a massive amount of radioactive water and sparking a fresh fear of contamination in the Pacific Ocean.

Emergency workers have found turbine buildings and trench tunnels outside submerged in highly radioactive water, also discovering water leaking from a cracked pit into the ocean.

They have successfully stopped the water leakage and are now checking for any more water runoffs from the plant.

In order to find safe storage space for the most radioactive water -- so dangerous that it is halting crucial repair work -- operator TEPCO has been forced to empty containers with lower-level radioactive water into the ocean.

The Tokyo Electric Power Company has released 11,500 tonnes, or more than four Olympic pools-worth, into the sea, despite protests from local fishermen.

Here is the current state of each of the six reactors at the plant.

-- REACTOR ONE --

Overheating has caused a partial meltdown of the reactor core. TEPCO believes some 70 percent of its 400 fuel rods have been damaged.

Workers have injected pure water, switching from sea water used last month, into the pressure vessel via a pump, but the cooling system has not been restored yet.

With hydrogen and oxygen likely to have accumulated inside the reactor vessel, workers began pumping inert nitrogen gas early Thursday to prevent a possible hydrogen blast.

Workers had begun pumping out radioactive water from the basement of the adjacent turbine building, but found more in a trench outside the turbine building, 56 metres (yards) from the ocean.

-- REACTOR TWO --

Also thought to have suffered a partial meltdown, with about 30 percent of 548 fuel rods likely damaged.

The torus -- the reactor's suppression pool which controls the pressure inside the reactor container -- has likely been damaged.

Spent fuel rods in the pool were fully exposed at one stage, but TEPCO has said they are now submerged in water and in a stable condition.

A puddle of highly contaminated water was found in the basement of the turbine building and outside in a trench, where a radiation reading of over 1,000 millisieverts per hour was measured.

Workers have injected pure water containing boric acid into the pressure vessel, after dumping sea water as an emergency means.

They found a crack in a seaside concrete pit near this reactor, which was leaking highly radioactive water.

After several failed attempts to seal the crack, using cement, and even newspapers and sawdust, workers stopped the leak Wednesday morning after injecting sodium silicate, a chemical agent known as "water glass", to solidify soil near the pit.

-- REACTOR THREE --

A hydrogen explosion badly damaged the outer building, and a partial meltdown is also suspected. TEPCO said about 25 percent of the 548 fuel rods may be damaged.

Three workers were exposed to high levels of radiation last month when they stepped in contaminated water at the basement of the turbine building. They were found to have suffered no major injury.

Workers had used sea water to cool both the reactor and spent fuel pool but have now changed to fresh water.

-- REACTOR FOUR --

The reactor was undergoing maintenance when the quake struck and there are no rods in the core.

Fires broke out in the building several days after the quake. These were tackled with water, which made its way into the spent fuel pool.

Firefighters doused the spent fuel pool using a concrete pumping vehicle, usually used in the construction industry. TEPCO has said its spent fuel pool is now submerged in water.

Contaminated water was found in the basement of the turbine building but workers have yet to remove it.

-- REACTORS FIVE AND SIX --

Undergoing maintenance when the quake hit, but fuel rods were already placed in the cores as they were prepared for operation.

Workers have created three holes in each of the reactor buildings, aiming to vent hydrogen out and prevent an explosion.

They have restarted the cooling systems both for the reactors and the spent fuel pools, which have remained stable.

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