Fort St. Vrain (FSV) is the only graphite-moderated, helium-cooled nuclear power plant in the United States. It was preceded by the 40-MW(electric) Peach Bottom high-temperature gas-cooled reactor (HTGR), which was permanently shut down in 1975. The Public Service Company of Colorado owns and operates FSV, and the core design and manufacture were performed by General Atomic Company (now GA Technologies, Inc.). Extensive physics testing of the 330-MW(electric) FSV HTGR was conducted to confirm the adequacy of the calculational models used in the core design. The physics testing performed during the first three cycles has confirmed that the calculational models used for the core design have been eminently successful in predicting the core nuclear performance from initial cold criticality through power operation and refueling.