The nation's fist nuclear power plant with a prestressed concrete reactor pressure vessel began producing electricity on February 21, 1974 at Fort St. Vrain, a 330 megawatt station about 30 miles northeast of Denver. At least three similar but larger nuclear plants are scheduled for construction starts within the next two years. The new generation of power plant, called high temperature gas cooled reactors, seems to be rapidly gaining acceptance even among some antinuclear factions for several reasons: they cause no thermal pollution of waterways; they produce almost no gaseous or liquid radiation emissions; the use of prestressed concrete reactor vessels and helium as a coolant offers significant safety advantages; they have the highest thermal efficiency of all nuclear power plants; they function as converter reactors, "producing" slightly more than 60 percent of their fuel by neutron bombardment of readily available thorium-232, which converts by capture into uranium-232 and undergoes fission within the reactor core, thereby saving scarce uranium supplies. Safety appears to be an overriding attraction. The entire primary system of nuclear components plus steam generation is contained within the PCRV. Test to failure of models and scale mockups have established that a PCRV under extreme internal pressures would crack, release excess pressure and then reseal itself. Since the helium coolant is basically inert, little radiation would be released and most of the contents of the vessel would remain intact, even in the event of a catastrophic failure from runaway temperatures or pressures.