U.S. Atomic Tests in 50's Exposed Millions to Risk

National Cancer Institure reports that atmospheric nuclear bomb tests in Nevada from 1951 to 1962 exposed millions of American children to large amounts of radioactive iodine, component of fallout that can affect thyroid gland; says doses were large enough to produce 25,000 to 50,000 cases of thyroid cancer nationwide, of which 2,500 can be expected to be fatal; map; photo (M)