Is Japan Doomed by Radioactive Hot Spots?

TOKYO—Japanese researchers discovered high levels of radioactive material in concentrated areas in Tokyo and Yokohama, more than 150 miles away from the Fukushima Daiichi nuclear plant, as increasingly thorough tests provide a clearer picture of how far contamination has spread and accumulated after the March disaster.

In Tokyo, a sidewalk in Setagaya ward, in the western part of the city, recorded airborne radiation levels, about 50 times higher than another location in Setagaya where the ward regularly monitors radiation levels.

"What's puzzling is that the levels detected on other parts of the same sidewalk were very low," said Ken Hatanaka, head of the ward's section in charge of radiation monitoring.

Setagaya ward is consulting experts to figure out what to do with the highly contaminated spot, Mr. Hatanaka said, noting that rainwater and sediments containing radioactive fallout may have been more likely to accumulate in such spots.

Radioactive contamination spreads unevenly depending on wind, weather and topography, and "radioactive elements later accumulate in specific spots that tend to gather dust and rain water, such as ditches," said Akira Hanawa, head of the Isotope Research Institute, a local private research institute in Yokohama.

Officials said the discoveries of such isolated, tiny 'hot spots' didn't indicate any immediate danger for residents of Japan's largest cities. Overall levels of radiation in those areas remain very low, even near the sites where the new contamination was found.

In Yokohama, the local government said last month that it detected 40,200 becquerels of radioactive cesium per kilogram of sediment collected from one part of a roadside ditch. Becquerels are a measure emitted radiation. By comparison, the Japanese government has banned the cultivation of rice in fields that are found to have more than 5,000 becquerels of cesium per kilogram of soil.

The discoveries come as Japanese researchers, local governments, and residents are stepping up their monitoring.

Yokohama is investigating another spot on an apartment rooftop where tests conducted by the Isotope Research Institute detected more than 60,000 becquerels of radioactive cesium per kilogram of sediment.

Both Setagaya ward and Yokohama discovered the concentrated spots after residents carrying radiation-measuring devices noticed the spots and reported them to local officials.

Still, those discoveries are unlikely to pose any major risks to Yokohama's more than three million residents, local officials say.

Although the cesium levels at those specific tiny spots appear high, airborne radiation levels near the spots aren't particularly high.

In the same roadside ditch where 40,200 becquerels were detected, another spot recorded only 3,030 becquerels.

"We've always known that there are 'hot spots' where contamination levels are higher than other areas, but these tiny spots are like 'micro hot spots,' " said John Kuramochi, who heads the section of the Yokohama city government in charge of monitoring contamination.

Write to Juro Osawa at juro.osawa@dowjones.com