I've heard of a kind of energy called plutonium-238, is it nuclear? I also want to know, what kind of nuclear is most dangerous? I've heard of a few, like the neptunium-237, what is the name of the most dangerous one. Also, if they ever uses nuclear for space travel and want it to last long, what would they use?

Why can't we use nuclear power to power the world?

Plutonium-238 (Pu-238), Neptunium-237 (Np-237), etc are called "Isotopes". The number following the chemical name or symbol is the sum of the neutrons and protons in the isotope. (The atom, itself, is determined by the number of protons in the atom.) U-238, and U-235, for example, are two isotopes of uranium. U-238 is not fissionable while U-235 is fissionable (can be used in an atomic bomb or a nuclear reactor to generate power).

Some isotopes of a given element are stable, others are unstable (radioactive). If it's unstable, there are three ways it can decompose. It can emit an alpha particle (two neutrons + two protons), a beta particle (actually an electron emitted from the nucleus) and often a gamma ray will be emitted. How dangerous an element is depends on how fast it decomposes and whether it produces an alpha particle, a beta particle, or a gamma ray. Alpha particles are not very dangerous outside of the body. They can easily be stopped by skin. Beta particles are a bit more dangerous. Gamma rays are high-energy electromagnetic particles, more energetic than x-rays. They are in the class that is usually called "ionizing radiation". They can induce changes in DNA in the body as well as other changes.

The rate of decomposition is determined by the half life of the isotope. These are found in tables. The shorter the half life, the quicker the isotope decomposes and therefore, generally, the higher the flux of radiation.

If they ever decide to use nuclear power to run space ships, I can't say what they would use.

Why can't we use nuclear power to power the world? There are nuclear reactors in use throughout the world. Why can't we use it to power the whole world? We probably could. The reasons are largely political. Many people worry that the waste fuel rods will pollute the world or that there will be a cataclysmic event (like the Chernoble nuclear reactor meltdown) that will endanger the world. Whether someone will be able to overcome the technical problems with powering the world is an open question at the current time.

Good answer! Thank you, that was very informational