Radiation levels in areas within 80 km of the crippled Fukushima No. 1 power plant fell by nearly half over the 20-month period between April 2011 and last November, a government-affiliated body said.

The level is declining more quickly than anticipated thanks to rain and the two-year half-life of cesium-134, the Japan Atomic Energy Agency said Sunday. The rain apparently washed the fallout produced by the three reactor core meltdowns triggered by the March 2011 quake and tsunami to other areas via the regional drainage system. The decline was particularly rapid in areas with many buildings and asphalt roads, the agency said.

The agency analyzed radiation readings taken 1 meter aboveground, using data collected by the Education, Culture, Sports, Science and Technology Ministry. According to the estimates, radiation levels fell 30 percent from April 2011 to November 2011, and had dropped to half by November 2012.

The agency believes most of the drop is due to the elements, not government decontamination efforts. Where the tainted rainwater ends up, however, is another matter.

In November, the fall slowed after the radiation from cesium-134, which has a half-life of two years, fell to half, the agency said. But the energy from the more prevalent cesium-137, whose half-life is 30 years, hasn't changed.