Spain faces the challenge of 80-95% greenhouse gas emissions reduction by 2050 (European Energy Roadmap). As a possible first step to fulfill this objective, this paper presents a two-level analysis. First, we estimate the carbon footprint of a hypothetical nuclear facility in Spain. Using a hybrid multiregional input-output model, to avoid truncation while diminishing sector aggregation problems and to improve environmental leakages estimations, we calculate the CO2 equivalent emissions associated with the different phases of the nuclear life-cycle--construction, fuel processing and operation and maintenance--taking into account the countries or regions where the emissions have been generated. Our results estimate a nuclear carbon footprint of 21.30 gCO2e/kWh, of which 89% comes from regions outside Spain. In some regions, the highest impacts are mostly direct (92%, 95%, and 92% of total carbon emissions in the U.S., France, and UK, respectively), meaning that these emissions are linked to the inputs directly required for nuclear energy production; in other regions, indirect emissions are higher (83% in China), which becomes relevant for policy measures. Second, through the analyses of different scenarios, we unravel and quantify how different assumptions that are often taken in the literature result in different carbon emissions.