A system analysis has been performed to develop the concepts for a fusion reactor and to identify the design parameters by using the tokamak system analysis code at KAERI (Korea Atomic Energy Research Institute). The system code elucidates the device parameters which satisfy the plasma physics and engineering constraints by taking into account a wide range of plasma physics and technology effects, simultaneously. The calculation of 1-D neutronic system code was coupled with this tokamak system code to optimize the reactor parameters. The numerical simulation for blanket neutronics was performed with MCNP5 code to calculate the tritium breeding ratios and neutron multiplications, which were the input parameter of system code. With the coupled system analysis and one-dimensional neutronic calculation, we assessed various types of DEMO blanket concepts with the requirements for the DEMO selected as to demonstrate the tritium self-sufficiency, to generate a net electricity amount, and for a steady-state operation.