A nuclear weapon uses a fissile material to cause a nuclear chain reaction. The most commonly used materials have been uranium 235 (U-235) and plutonium 239 (Pu-239). Both uranium 233 (U-233) and reactor-grade plutonium have also been used. The amount of uranium or plutonium needed depends on the sophistication of the design, with a simple design requiring approximately 15 kg of uranium or 6 kg of plutonium, and a sophisticated design requiring as little as 9 kg of uranium or 2 kg of plutonium. Plutonium is almost nonexistent in nature, and natural uranium is about 99.3% uranium 238 (U-238) and 0.7% U-235. Therefore, to make a weapon, either uranium must be enriched, or plutonium must be produced. Uranium enrichment is also frequently necessary for nuclear power. For this reason, uranium enrichment is a dual-use technology, a technology that "can be used both for civilian and for military purposes". Key strategies to prevent proliferation of nuclear arms include limiting the number of operating uranium enrichment plants and controlling the export of nuclear technology and fissile material.
