Despite taking an oath of secrecy regarding their jobs, employees at a nuclear weapons plant in Iowa will be allowed to talk to doctors and scientists about hazardous chemicals to which they may have been exposed, the Pentagon determined in a report issued yesterday. The oaths have posed problems for thousands of current or former employees of the Iowa Army Ammunition Plant seeking medical care or federal benefits, or attempting to take part in health studies. From the late 1940s to the mid-1970s, the plant assembled and test-fired nuclear weapons components; it currently manufactures conventional weapons. Workers there may have been exposed to silica, beryllium, solvents, explosives, epoxies, and heavy metals. With the easing of the rules, workers will be able to name the substances with which they worked, but not discuss how each substance was used. Sen. Tom Harkin (D-Iowa) welcomed the change but called the Pentagon report “woefully short on information about possible radioactive and toxic exposures at the plant.”