KORIYAMA, Japan—Nearly eight months after the Fukushima Daiichi nuclear accident scattered radioactive material over surrounding communities, Japan still is struggling to figure out how to clean up the mess, exacerbating fears about health risks and fanning mistrust of the government.

Government guidelines provide scant detail about the $14-billion-plus effort. A new cleanup law doesn't take effect until January. Cities across Fukushima prefecture are scraping contaminated topsoil off school grounds and parks, but Tokyo hasn't...