This publication presents the results of the INPRO collaborative project: Proliferation Resistance and Safeguardability Assessment (PROSA) Tools. The PROSA assessment process has been developed to specifically address the needs of national self-assessors performing an INPRO Nuclear Energy System Assessment of sustainability. The main objective of this project is to explore approaches to make the INPRO sustainability assessment of proliferation resistance simpler and easier to understand. In addition to lessons learned, the publication includes the results after applying the PROSA assessment to an illustrative case, demonstrating use of the refined methodology. The outcome of this project may also serve as a basis for a future revision of the INPRO methodology manual in the area of proliferation resistance.