A working group was created in 2002 by the Generation IV International Forum for the purpose of developing an internationally accepted methodology for assessing the proliferation resistance of a nuclear energy system (NES) and its individual elements. A case study was performed by the working group using this methodology to assess the proliferation resistance of a hypothetical NES called the Example Sodium Fast Reactor (ESFR). This work demonstrates how the proliferation resistance assessment methodology can be used to provide important information at various levels of details to NES designers, safeguard administrators, and decision makers. The study analyzed the response of the complete ESFR NES to different proliferation and theft strategies. The challenges considered include concealed diversion, concealed misuse, and "break-out" strategies. This paper describes the work done in performing a qualitative assessment of concealed diversion scenarios from the ESFR.