A. N. Chebeskov

As noted in Chapter 1, the lack of availability of special nuclear mate­rial (SNM) that can be used to build a nuclear weapon is widely agreed to be a major barrier to nuclear proliferation (see Chapter 1). Thus, an essential part of understanding the proliferation risk associated with a research reactor involves understanding how straightforward it would be for a host state or terrorist organization to successfully misuse the reactor’s fuel material.

The attractiveness of a nuclear material from a proliferator’s point of view is determined in large part by a material’s ability to sustain a nuclear chain reaction. Material attractiveness is also influenced by whether it is necessary to process the material to make it usable in a nuclear weapon. To categorize fissile materials qualitatively, four categories (classes) might be used: very attractive, attractive, low attractive, and unattractive.

Several variables are relevant to the attractiveness of SNM. For ex­ample, for a given quantity of uranium, its attractiveness is proportional to both its enrichment and its mass. Higher-enriched materials are more attractive than lower-enriched materials; for example, HEU enriched to 90 percent uranium-235 is far more attractive than LEU, which is re­garded to be unattractive. Similarly, higher masses are more attractive than lower masses for a given level of enrichment. In general, the higher the enrichment, the less mass is required to obtain an equivalent amount of uranium-235.

Of course, nuclear weapons can be constructed using plutonium as well, but it is difficult to compare the attractiveness of different materials. Different grades of plutonium can be rated relative to one another as reac­tor grade (less attractive) and weapons grade (more attractive). However, very highly enriched uranium is the most desirable material for a potential proliferator, because of the relative simplicity of constructing a nuclear explosive device using HEU as opposed to plutonium.

As an example, at the MEPhI reactor, the small size and mass of very highly enriched fuel assemblies represent a higher theft risk than heavier power reactor fuel assemblies, especially for fresh fuel assemblies. The uranium contained in the MEPhI fuel assemblies would not need further enrichment to be usable in a nuclear explosive device. For irradiated fuel assemblies this risk is smaller because of the presence of strong radiation.