They used graphite core in the reactor building. None of the old federal reactors were constructed with modern containment buildings. This was similar to the soviet era reactors. The difference was Soviet reactor scientist were allowed to experiment with their reactors by pushing them to new limits which is what happened at Chernobyl.
American reactors were designed to operate to produce nuclear weapons grade materials and there was no need to experiment or play around with them as if they were a giant physics lab.