Emissions of radioactive materials from the crippled Fukushima nuclear plant have been cut by half over the past month, its operator said Monday.

Tokyo Electric Power (TEPCO) also said in its monthly review of work on the Fukushima Daiichi plant that it was on track to bring damaged reactors to a stable "cold shutdown" by the year end.

In a regular joint news conference with the government, TEPCO said that the temperature of the three damaged reactors was below 100 degrees Celsius (212 degrees Fahrenheit). Keeping them there is a key requirement for cold shutdown.

The company warned, however, that a large tsunami remained as a major risk for the work at the Fukushima plant, which has been hit by countless aftershocks since the March 11 earthquake and tsunami.

The company stressed that the availability of multiple water supply sources, including on-site fire trucks, minimised the risk of further damage to the plant's cooling system.

Another major earthquake could also pose danger to the plant, it added.

Tens of thousands of people remain evacuated from a 20 kilometre (12 mile) no-go zone around the plant and in pockets beyond, after it unleashed huge amounts of radiation in the worst nuclear accident since Chernobyl in 1986.

Hotspots of radioactive fall-out have been detected as far away as Tokyo, 220 kilometres from the crippled facility.

Variable winds, weather and topography result in an uneven spread of contamination, experts say, and radioactive elements tend to concentrate in places where dust and rain water accumulate such as drains and ditches.