For seven generations, Yoshitoshi Sewa and his ancestors have tilled this farm in a gently curving valley filled with green rice paddies. But now he will not let his young grandchildren play outside their tile-roofed home for fear of an invisible and potentially long-lasting threat, radiation. Yoshitoshi Sewa, a farmer, in his home in Tenei, 40 miles from Japan's stricken nuclear plant. He will not let his grandchildren play outdoors.