NEW : There is no immediate radiation threat , experts tell local officials . Local police cordons off an area in the Setagaya ward . Radiation there is higher than around Hiroshima . The damaged Fukushima nuclear plant could not be the source , experts say .
NEW : There is no immediate health hazard , experts tell local officials . Local government cordons off an area in the Setagaya ward . Radiation there is higher than around Fukushima . The damaged Fukushima Daiichi plant may not be the source , officials say .