The main isotopes within unirradiated fuel produced from mined uranium are listed in Table 3.1. These values are for natural uranium; for enriched fuel, the proportions of U-234 and U-235 are increased relative to U-238 (see worked example below).
Table 3.1: Isotopes within Fresh UO Fuel (Natural Uranium Basis)
| Isotope | Weight Fraction (%) | Decay Mode | Half-life | Specific Activity (Bq/g of isotope) |
|---|---|---|---|---|
| U-234 | 0.0055 | Alpha () | 2.46 10 years | 2.31 10 |
| U-235 | 0.72 | Alpha () | 7.04 10 years | 8.00 10 |
| U-238 | 99.27 | Alpha () | 4.47 10 years | 1.24 10 |
Key points:
- All three isotopes are alpha emitters, which also emit low-energy gamma rays with low abundances
- Combined with the self-shielding properties of the dense uranium material, the external radiation hazard from fresh fuel is negligibly small
- Typical surface dose rate: approximately ~20 Sv/h
- The dominant contribution to the total activity comes from U-234, despite it being present in the smallest proportion. This is because U-234 has a much shorter half-life and therefore a much higher specific activity.
Tip for students: It may seem counter-intuitive that U-234 (only 0.0055% of natural uranium) contributes more to the total activity than U-238 (99.27%). Remember: shorter half-life = higher activity. A small mass of a short-lived isotope can be far more active than a large mass of a long-lived one.