Uranium
In nature, uranium is found as uranium-238 (99.2739–99.2752%), uranium-235 (0.7198–0.7202%), and a very small amount of uranium-234 (0.0050–0.0059%). Uranium decays slowly by emitting an alpha particle. The half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Spent nuclear fuel
If using a thorium fuel to produce fissile U-233, the SNF (Spent Nuclear Fuel) will have U-233, with a half-life of 159,200 years (unless this uranium is removed from the spent fuel by a chemical process). The presence of U-233 will affect the long-term radioactive decay of the spent fuel. If compared with MOX fuel, the activity around one million years in the cycles with thorium will be higher due to the presence of the not fully decayed U-233.