If research reactors will continue to be needed in the foreseeable future it is important to understand as clearly as possible their risks. As noted previously, conversion of research reactors from HEU to LEU lowers risk. However, some reactors may not be able to be converted, so it is important to understand the risks associated with their continuing operation. This risk goes beyond the reactor itself to involve all facilities and associated infra­structures, including fuel manufacturing; transportation; fresh fuel storage; irradiated fuel storage; and reprocessing or final repository placement.

Robert Bari described two different types of risk associated with re­search reactor facilities and infrastructures (systems) as follows (Bari, 2011):

• Proliferation risk of an HEU-fueled research reactor’s fuel cycle is associated with the diversion or undeclared production of nuclear material or misuse of technology by a host state seeking to acquire nuclear weapons or other nuclear explosive devices.

• Terrorism risk of an HEU-fueled research reactor’s fuel cycle is associated with the theft of materials suitable for nuclear explosives or radiation dispersal devices and the sabotage of facilities and transportation by sub-national entities and/or non-host states.

The following sections describe two methodologies to structure and improve the understanding of proliferation and terrorism risk: First, as­sessing the relative attractiveness of various nuclear materials; and second, proliferation risk assessment methods.