50 YEARS OF OPERATION 5 6 1.  Lowering reactor vessel in the ATR reactor building. 2.  ATR Canal under construction. 3.  View looking northeast through refueling port showing interior of reactor vessel. 4.  ATR predecessor, the Engineering Test Reactor (ETR), was the second reactor at the ATR Complex. A view of ETR core. 5. ATR Critical Facility. 6.  ATR safety rod drive flange tube being welded to vessel nozzle in the safety rod drive area. 7. Hydrostatically testing loop penetration plugs in reactor vessel top head closure plate. 7