Nine nuclear plants along the northeastern coast are under states of emergency [Reuters]

Fears that one of Japan's nuclear reactors could go into meltdown have escalated after the operator of a quake-hit facility said water levels, which cool the plant, were almost empty.

Earlier officials said fuel rods that the Fukushima Daiichi complex's No.2 reactor had been temporarily exposed to the air, which can cause a meltdown.

Al Jazeera looks at Japan's nuclear situation and the possible outcomes.

How did this crisis begin?

On Friday afternoon an 8.9 magnitude earthquake struck off the northeast coast of Japan, triggering a devastating tsunami.

Nuclear reactors at the Fukushima Daiichi complex shut down when the tremors were felt, but the subsequent tsunami managed to crash over the sea walls surrounding the plant, some 270km northeast of Tokyo. It is understood that the plant lost electrical power to the reactor cooling systems, and diesel generators were found to be inoperable.

Emergency batteries also quickly ran out of power.

The water levels, which keep fuel rods in the reactor cool, began to fall, and by Sunday some of those rods were exposed to the air, a situation that can trigger a meltdown.

In addition, the nuclear fuel rods have reacted with steam vented from the plant let off in an attempt to reduce the pressure inside. That reaction has produced hydrogen, a combustible gas, leading to two explosions, one at unit 1 on Saturday and another on Monday.

One of the problems facing engineers working at the plant is that water level gauges at some of the reactors may be faulty, meaning no one is sure how much water is inside.

What area is affected?

Nine nuclear units, all boiling water reactors along Japan's northeastern coast, are under states of emergency. The three at Fukushima Daiichi, three at the nearby Fukushima Daini complex and another three at Onagawa.

Units 1 and 3 at the Fukushima Daiichi plant have been hit by explosions, while unit 2 has seen cooling water levels drop dramatically. At Daini, units 1,2 and 4 have experienced increased pressure inside their containment vessels, the area which holds the nuclear material.

In Onagawa, officials have said higher than usual radiation levels are being detected.

What's the current situtation?

The main tasks facing engineers and authorities right now are to keep the reactors cool and to protect the containment vessels surrounding the reactors.

Sea water and boron, an element that can absorb atomic particles, are being poured into the cooling systems in an attempt to keep the temperatures down.

Engineers are using firefighting equipment to force sea water into the chambers, a task described by an American official in the New York Times as "trying to pour water into an inflated balloon".

Justin Dargin, a nuclear energy analyst from Harvard University, told Al Jazeera that the procedures Japanese authorities have been using so far are untested, a sign of desperation.

"That just shows how extreme the situation is, and the governmentt is attempting to do whatever it can to try and mitigate what is looking like a partial meltdown," he said.

Some levels of radiation have already been released into the atmosphere from a method known as "feed and bleed", where engineers vent hydrogen from the plant to prevent pressure build ups.

Some 180,000 people have been evacuated from around the two Fukushima complexes, but it is feared 160 of those may have been exposed to levels of radiation.

What are the possible outcomes?

If the temperature inside the Fukushima reactor vessel continues to rise to 2,200 Celsius, then the uranium fuel pellets would start to melt.

Dargin said if the nuclear rods "are exposed for a certain duration of time then we could have a full meltdown, as opposed to a partial meltdown.

"When that happens we can have quite a disaster in the sense of radioactivity that would be leaked into the surrounding areas."

If fuel rods do begin to melt, the fuel could eat through the bottom of the reactor vessel, a thick steel container, then through the containment building and into the environment. There is a possibility that if this happened, another explosion could occur, sending particles even further into the air.

Dargin said the reactor at unit 1 is about 40 years old, and has not been reinforced to the level which would be able to withstand this recent earthquake.

"The situation does not look good. On the one hand while the government is trying to get this situation under control ... they have staff there but they are being exposed to radiation."

He said the current "feed and bleed" method, which sees the reactor flooded with seawater and the steam released, could go on for several months, if not a year.

"So what we're going to see is that people are going to have to evacuate for a long time. The worst case scenario is that we'll see a full-scale meltdown, but hopefully that won't happen, because Japanese authorities are working around the clock."

Source: Al Jazeera and agencies