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. It is used only to make plutonium-238 when bombarded with neutrons in a lab.