Depleted uranium (DU)—the U-235-lean byproduct of the enrichment process—and thorium occupy a distinct regulatory tier from enriched fissile materials, yet both remain subject to NRC licensing as source materials under the Atomic Energy Act. DU's exceptional density makes it commercially valuable for radiation shielding, counterweights in aircraft, and armor-penetrating applications, while thorium compounds are used in specialty alloys, high-temperature crucibles, and emerging thorium-cycle reactor research. Import of either material requires NRC source material license authorization, and thorium-bearing minerals may also trigger NORM (naturally occurring radioactive material) regulations at the state level. Classification accuracy matters because DU and thorium products are sometimes misidentified as conventional heavy metals.

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