Effects of Separation Processes on Minor Uranium Isotopes in Enrichment Cascades

Abstract

In natural uranium, there are trace amounts of the “minor isotope” ²³⁴U in addition to the more abundant ²³⁵U and ²³⁸U. Uranium irradiated in a reactor and separated from the spent reactor fuel by reprocessing will contain additional minor isotopes. In uranium enrichment, cascades of separating units are tuned to produce the desired level of the isotope ²³⁵U, but the levels of other isotopes will also change. Measurement of the assays of the ²³⁵U in the feed, product and withdrawal streams of each stage allow the calculation of the stage separation factor for the ²³⁵U. The separation factors for the other isotopes may be calculated. In this aritcle, two methods of modeling the separation factors in a way that represents the physics of two enrichment processes, gas centrifuge and gaseous diffusion, are explored. This technique could be a valuable tool in nuclear forensics. For example, it could be important to know the origin of a sample of highly enriched uranium, found either in intercepted material or in explosive debris. Analysis that could point to the enrichment plant where the uranium was produced could immediately identify weaknesses in safeguards and physical security systems.

Acknowledgments

The author acknowledges the support of the Program on Science and Global Security at Princeton University where he spent half a year on sabbatical. During this time, the idea of using separation factors based on the physics of the process was suggested by Alexander Glaser and R. Scott Kemp. Their encouragement for this effort is greatly appreciated. The author also thanks Ed Von Halle, who graciously modified his M* code and provided the code to perform the calculations presented in this article.