Development reports on closed-cycle gas turbines (CCGT) as proposed by Ackeret and Keller (AK system) and promoted mainly by Escher Wyss Ltd., Zurich, Switzerland, and Gutenhoffnungshütte (GHH), Germany, have been presented since 1945 at ASME meetings about every five years (1). This, the sixth paper, reports on the operating experience with some newer fossil-fuel fired plants made by different manufacturers and gives the study results of European designers for nuclear gas turbines which can be built already with today’s technology for the 600 to 1000-Mw range. The special physical properties of air and helium and their influence on plant design are discussed. The combination of a CCGT and a high-temperature reactor offers many possibilities for simplifications of nuclear plants and lowering capital costs.