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.
Actinides in the environment
Like plutonium, neptunium has a high affinity for soil. However, it is relatively mobile over the long term, and diffusion of neptunium-237 in groundwater is a major issue in designing a deep geological repository for permanent storage of spent nuclear fuel. Np has a half-life of 2.144 million years and is therefore a long-term problem; but its half-life is still much shorter than those of uranium-238, uranium-235, or uranium-236, and Np therefore has higher specific activity than those nuclides.