Some of the smallest children in Koriyama, a short drive from the ruined Fukushima No. 1 nuclear power plant, barely know what it's like to play outside — fear of radiation has kept them indoors for much of their short lives.

Though the strict safety limits for outdoor activity set after the multiple meltdowns at the Fukushima plant in 2011 have now been eased, parental worries and ingrained habit mean many children still stay inside.

And the impact is now starting to show, with children experiencing falling strength, lack of coordination, some cannot even ride a bicycle, and emotional issues like shorter tempers, officials and educators say.