This publication provides guidance on assessing of sustainability of a nuclear energy system (NES) in the area of environmental impact of stressors. The INPRO methodology is a comprehensive tool for the assessment of sustainability of an NES. Basic principles, user requirements and criteria have been defined in different areas of the INPRO methodology. These include economics, infrastructure, waste management, proliferation resistance, environmental impact of stressors, environmental impact from depletion of resources, and safety of nuclear reactors and fuel cycle facilities. The ultimate goal of the application of the INPRO methodology is to check whether the assessed NES fulfils all the criteria, and hence the user requirements and basic principles, and therefore presents a system for a Member State that is sustainable in the long term.