Providing guidance to those assessing a planned nuclear energy system (or a nuclear reactor), this publication provides detail on how to apply the INPRO methodology for sustainability assessment in the area of safety of nuclear reactors. It augments the information presented in the earlier INPRO publications on the methodology for sustainability assessments. The INPRO assessment is expected either to confirm the fulfilment of all INPRO methodology criteria in the area of reactor safety, or to identify which criteria are not fulfilled and the corrective actions that would be necessary to fulfil them. The publication is intended for use by organizations involved in the development and deployment of a nuclear energy system, including planning, design, modification and technical support for nuclear power plants.