New categories of nuclear materials for safeguards

The application of safeguards is very much dependant on the definition of material categories. If a category is too broadly defined, it may not be accepted as fungible in terms of safeguards. Fungible materials from different origins can be accounted for collectively as one category and mixed therein without keeping track of the identity of single batches, since it is basically the same substance one is dealing with. A similar problem would arise for the principle of equivalence, if highly enriched uranium would be declared “equivalent” to a larger quantity of depleted uranium on the sole basis of the contained uranium-235. Two different categories of materials, e.g. different uranium enrichments, can be declared equivalent, when they are \"exchangeable\" for accounting or safeguards purposes, and thus can be substituted for each other. The paper elaborates on the benefits of changing formally the definition of some categories of nuclear materials, e.g. of using with flexibility the “natural”, but yet “artificial”, limit of 0.711% for natural enrichment, of creating an additional category each for low and high enriched uranium and for plutonium.