Why is plutonium reprocessing considered a greater proliferation risk than direct use of low-enriched uranium in reactors?

Plutonium reprocessing is considered a greater proliferation risk than the direct use of low-enriched uranium (LEU) in reactors because reprocessing separates plutonium from spent nuclear fuel, resulting in a concentrated form of plutonium that is directly usable in nuclear weapons. Low-enriched uranium, typically containing less than 5% uranium-235, is not directly suitable for weapons production and requires further enrichment to reach weapons-grade levels (typically 90% or more uranium-235). While LEU used in reactors is under IAEA safeguards, the spent fuel still contains plutonium generated during reactor operation. Reprocessing involves chemically separating this plutonium from the other components of the spent fuel. This separated plutonium can then be diverted for weapons purposes with relatively less processing compared to enriching natural uranium or LEU to weapons-grade levels. Although reprocessing facilities are also subject to IAEA safeguards, the risk of diversion is higher because the plutonium is already in a form that can be readily used in nuclear weapons. Therefore, the availability of separated plutonium, which is a direct weapons-usable material, makes reprocessing a greater proliferation concern than the use of LEU, which requires further enrichment.