A comprehensive study is performed on an energy conversion system which combines a gas turbine modular helium reactor (GT-MHR) and two transcritical CO2 cycles (tCO(2)). The aim of this study is to assess the energy, exergy and economic behavior of the proposed system, considering five indicators: the energy efficiency, the exergy efficiency, the total exergy destruction cost rate, the overall exergoeconomic factor and the total cost rate. A parametric study is also conducted to evaluate the influence of key decision variables on the GT-MHR/tCO(2) performance. Finally, the combined cycle is optimized to minimize the total cost rate. The results show that the energy efficiency of GT-MHR/tCO(2) cycle is 7.92% higher than that of the simple GT-MHR cycle at 850 degrees C. The largest exergy destruction rate takes place in the reactor, and after that in the helium turbine and the recuperator. The components in tCO(2) cycles have less exergy destruction. When the optimization is conducted based on the exergoeconomics, the overall exergoeconomic factor, the total cost rate and the total exergy destruction cost rate are 55.2%, 20,752 VI and 9292 $/h, respectively. (C) 2016 Elsevier Ltd. All rights reserved.