The unique high temperature nuclear heat source the modular high temperature reactor (MHTGR) is should make it possible to associate the nuclear energy and an efficient heat conversion means, following the example of present, successful, fossil-fired combined cycle power plants. In order to reach this goal, a topping closed gas cycle is proposed, but only in combination with a bottoming steam cycle, for the investigated applications. The adopted reactor outlet temperature is 900°C. Results are given for four examples which have been thoroughly evaluated but are not yet final, optimal versions. Two of them are direct He cycle concepts which afford net plant efficiencies of 47.2 and 47.5 per cent. The aspects of arrangement simplification and primary circuit concerns are discussed. Two other cases feature an indirect topping gas cycle either a single expansion one or a reheat one. Owing to a greater cycle adaptation flexibility, net plant efficiencies of 48.3 and 50.4 per cent respectively are obtained.Temperature profiles, plant diagrams, main parameters values and primary circuit arrangements are presented. By comparison with most of the nuclear reactors currently in operation, for a same power output, 35 per cent less uranium fuel can be burnt, 35 per cent less fission products and radioactive wastes can be produced and 50 per cent less waste heat can be rejected.