Thoria is irradiated in the Hanford reactors for the production of U-233. The product quality is acutely sensitive to the control of the unseparable contaminant isotopes U-232 and U-238. Recent discussions between personnel of ARCHO and DUN indicate that product quality of a forthcoming separations campaign can be significantly enhanced by analysis of data for which we solicit your support. This letter describes the necessary background and the required analysis. Virgin thoria lots are processed to required transient impurity levels by the National Lead Company of Ohio. They are subsequently shipped to DUN identified by this virgin lot number and are accompanied by the transient impurity levels and U-238 contaminant levels based on statistical samples and analytic chemistry results. Virgin NOL thoria lots consist of various tonnages or fractions thereof, and two or three such lots are typically combined into a single DUN thoria element lot. By weighted averages, the U-238 contaminant level of DUN thoria lots is thus accurately known. The range of U-238 contaminant in any given DUN lot was further controlled by establishing three categories, A, B, and C, with U-238 ranges of 0-1.99, 2.0-3.3, and 3.4-5.0, respectively, on a thoria basis. Thus when a DUN lot …

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