This publication provides guidance for assessing the sustainability of a nuclear energy system (NES) in the area of nuclear fuel cycle facility (NFCF) safety. It deals with NFCFs that may be potentially involved in the NES such as, mining, milling, refining, conversion, enrichment, fuel fabrication, spent fuel storage, and spent fuel reprocessing facilities. It augments the information presented in the earlier INPRO publications on the methodology for sustainability assessments. The publication is intended for use by organizations involved in the development and deployment of a NES, including planning, design, modification and technical support for NFCFs.