A pattern of illness doctors cannot explain has been emerging on and around East Tennessee's Oak Ridge reservation, where nuclear weapons fuel and components have been manufactured for five decades. More than 200 workers and residents report neurological, respiratory and immune system disorders. Meanwhile, scientists are discovering that environmental contamination on and around the reservations is far worse than previously believed. Many of ill suspect - but cannot prove - their health problems are linked to this contamination. So, too, do some parents and educators in one county adjoining the reservation where special education enrollment more than doubled from 1990 to 1996.