Why are uranium and plutonium used as nuclear fuels?

Answer 1

Because they are the most fissile materials available. Specifically, Uranium-235 (0.7% of natural Uranium), Plutonium-239 (produced in reactors from Uranium-238), and Uranium-233 (produced in reactors from Thorium-232) are the easiest to fission (split) with neutrons, producing enough new neutrons to continue the chain reaction.

Depleted Uranium (Uranium-238) is pretty easy to obtain, you can buy a chunk of it online (although it often sells out fast), whilst Plutonium is not available outside Military and Government controlled facilities.

Most other transuranic elements fission very effectively too, making them good reactor fuels also but they are very radioactive compared to Uranium and Plutonium, making them more dangerous to handle and store.

Answer 2

Plutonium and uranium are commonly used as nuclear fuels because they are fissile. Specifically, uranium-235 and plutonium-239 is fissile.

Fissile means that, for each fission event, one or more neutrons is produced that can go on to cause more fission events. This is a chain reaction. In a power plant, the moderator (usually water) is used to adjust the reactivity so that, on balance, one fission generates one neutron, which goes on to generate the next fission. In a weapon, there is no moderator, and the geometry is such that more than one neutron is generated per fission, with each generating a new fission, causing an exponential growth in activity and a devastating loss of control.