A selection of Rankine and Brayton power cycles for fusion and fission nuclear power plants (NPPs) has been modeled in a framework that integrates the engineering equation solver (EES) and the PROCESS systems code. The combined production of heat and electricity has been studied, and efficiency improvements have been obtained for all cycles. The primary energy savings (PES) indicator has been evaluated for the different layouts. The economic viability, which considers the production and distribution costs of the layouts, is discussed. The combined production decreases costs and greenhouse gas emissions and appears profitable for Rankine and supercritical CO2 cycles. Potential pricing of the externalities of emissions and an increase in fossil fuel prices would improve the competitiveness of all presented layouts.