The presence of U-232, U-234, and U-236 in recycled uranium has significant practical consequences for the fuel cycle:
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U-232 (68.9 year half-life) is present at only ~1 ppb in recycled uranium, but its decay chain includes Tl-208, which emits an intense 2.6 MeV gamma ray. The gamma dose rate from recycled uranium therefore increases with storage time as the Tl-208 daughter builds up, reaching a maximum after approximately 10 years. This means that recycled uranium fuel must ideally be fabricated and loaded into the reactor promptly after re-enrichment. Delays of more than a few years may require re-purification to remove the accumulated daughter products.
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U-236 (2.34 × 10^7 year half-life) is a neutron absorber (thermal neutron capture cross-section ~5.3 barns) that acts as a reactivity poison in the reactor. Its presence in recycled uranium means that fuel must be enriched to a higher U-235 content to achieve the same reactivity as fresh fuel — typically 0.2—0.4 percentage points higher. U-236 also transmutes under irradiation to produce Np-237, a long-lived alpha emitter that complicates waste management.
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U-234, already present in natural uranium, is concentrated during enrichment because it is lighter than U-235 and U-238. In recycled uranium, the U-234 content is higher than in fresh uranium of the same enrichment, further increasing the specific activity.
These complications mean that recycled uranium commands a lower market value than equivalent fresh uranium and must be processed through dedicated (segregated) conversion and enrichment facilities to avoid contaminating fresh uranium streams.