IWAKI, Japan — Every day, at 10 a.m. and 2 p.m., a part-time worker at one of Fukushima's most well-known beaches walks toward the shoreline and lowers a dosimeter to the water. The device measures radiation, and its readings this summer have delivered the best news that one can hope for 40 miles south of a still-leaking nuclear plant: Nearly 21 / 2 years after a series of meltdowns at the coastal