The exergetic analysis, a thermodynamics methodology that quantifies Exergy losses associated with irreversibility, allows to optimize each stage of a transformation process and thus its overall efficiency. In this contribution, a novel approach to exergetic analysis is applied to energy transformations taking place within the core of a nuclear reactor. To perform such analysis, reference was made to a pressurized lightwater reactor (PWR), modeling heat exchanges between fuel assemblies and coolant fluid in the core by balancing incoming and outgoing mass and energy flows, incoming and outgoing in the reactor. Analysis results are validated through a comparison with actual reactor operating parameters. Main goals of the work -part of a wider ongoing research effort-are to develop the thermo-economic analysis of a PWR nuclear power plant (NPP) to assess the actual cost of the products obtainable downstream of the NPP (electric energy and thermal energy for industrial and civil users, very different products in terms of exergy contents), and to compare such costs those of similar products obtained from conventional thermal power plants.