Matthew Sharp, "Applications and Limitations of Nuclear Archaeology in Uranium Enrichment Plants," Science & Global Security, 21, no. 1, (2013): 70-92.
The uranium-235 content of a uranium enrichment plant's product is related to the uranium-234 content of its waste, allowing one to check with tails measurements consistency with a plant's declared past production. Verification works best with known feed material, but with unknown feed isotopics the production of low and high enriched uranium may still be distinguished based on tails measurements. Estimating product masses is harder, and concealment scenarios are discussed. With traditional nuclear accounting, relationships between product and waste isotopics, or nuclear archaeology, can increase confidence in the accuracy of declarations of past fissile material production.