Problems continue to arise at the crippled Fukushima Daiichi nuclear power plant in Japan, despite progress in restoring the power needed to cool down its overheating reactors.

Japan's Prime Minister Naoto Kan bows in front of the Japanese flag with a black mourning cloth during a news conference at his official residence in Tokyo March 25, 2011. Kan said on Friday that the situation at a quake-damaged nuclear plant in northeast Japan remained very precarious. [Xinhua]

In recent days, injuries to workers, black smoke rising from No. 3 reactor and abnormal radiation have come one after another.

Some experts say the crisis might not end soon, while others insist its effects will be limited.

The crisis arose from the shutdown of the plant's cooling systems, which are critical to bringing down temperatures in the reactors' cores and stabilizing its nuclear fuel. The systems shut down after the March 11 earthquake and ensuing tsunami wiped out the power supply.

Workers are making efforts to bring the six-reactor facility's cooling systems back online and are spraying stricken reactors with seawater to cool damaged reactors and fuel rods

The Fukushima Daiichi nuclear power plant uses boiling water reactors, which went into emergency shutdown when the earthquake hit. The backup power started automatically to circulate cooling water to carry away the residual heat.

However, the earthquake destroyed the external power supply of the nuclear reactor. The emergency diesel power generators also failed when the tsunami arrived.

With the cooling system shut down, the residual heat built up, bringing down the water level of the fuel pool and threatening eventual core meltdown.

In 1979, a partial core meltdown occurred at Three Mile Island in the United States due to a cooling system failure. It remains the most severe nuclear leak accident in the country and forced the evacuation of at least 150,000 local residents.