Nuclear reactor physics
Modern deposits of uranium contain only up to ~0.7% U (and ~99.3% U), which is not enough to sustain a chain reaction moderated by ordinary water. But U has a much shorter half-life (700 million years) than U (4.5 billion years), so in the distant past the percentage of U was much higher. About two billion years ago, a water-saturated uranium deposit (in what is now the Oklo mine in Gabon, West Africa) underwent a naturally occurring chain reaction that was moderated by groundwater and, presumably, controlled by the negative void coefficient as the water boiled from the heat of the reaction. Uranium from the Oklo mine is about 50% depleted compared to other locations: it is only about 0.3% to 0.7% U; and the ore contains traces of stable daughters of long-decayed fission products.Fermi age theory
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.