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.
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.
Uranium and plutonium are both actinides that are used in nuclear reactors.
in reactors: yellowcake, a uranium oxide; after that probably metallic uraniumin stars: ordinary hydrogen; after that helium
The most common fuels used in nuclear chain reactions are 235U (uranium) and 239Pu (plutonium).
Uranium is the main fuel used, but in some countries a mixture of uranium and plutonium is also used (MOX fuel)
Plutonium I think.
Uranium and plutonium can be used as nuclear fuels for nuclear reactors.
Plutonium obtained in nuclear reactors with uranium fuels after recycling of the burned fuels can be used also as a nuclear fuel.
Uranium and plutonium are both actinides that are used in nuclear reactors.
in reactors: yellowcake, a uranium oxide; after that probably metallic uraniumin stars: ordinary hydrogen; after that helium
Plutonium is an artificial element; traces (extremely low) of plutonium isotopes of natural origin exist in uranium ores. Plutonium is used in nuclear weapons and nuclear fuels.
The most common fuels used in nuclear chain reactions are 235U (uranium) and 239Pu (plutonium).
Uranium and most transuranic elements. Plutonium and Americium are particularly good reactor fuels.
Usually uranium with various U-235 percentages.
Many things, but the fuels required are Uranium-233, Uranium-235, Plutonium-239, Deuterium, Tritium, and Lithium, depending on the design.
Nuclear energy appears as heat in a nuclear reactor. It comes from the fission of uranium or plutonium
Plutonium is used for nuclear fuels not for control rods.
The most useful isotope is uranium 235 which occurs naturally as 0.7 percent uranium as found. Plutonium 239 is also useful but has to be made in a reactor from uranium 238