Alpha and beta decay are both natural processes, which transform the nucleus. However, the nucleus of an atom can also be transformed by man made processes. This occurs when we shoot an alpha, or some other particle, at high speed into an atom. The nucleus of the atom absorbs the particle, and then almost always immediately decays, releasing a different particle. There are some atoms, though, that when bombarded transmute into a stable isotope. Uranium-238 will change into uranium-239 when bombarded with a high speed neutron. This isotope will beta decay immediately to neptunium-239, which will in turn quickly beta decay into plutonium-239. This isotope of plutonium never naturally occurs, but is stable for almost twenty-five thousand years. The uranium atom has been artificially transmuted, and transmuted upwards; a totally new occurrence.

As said before, most nuclei that absorb a particle will almost immediately emit another particle. We've also seen that some atoms don't do this, instead remaining stable for a long period of time There is one more thing that can happen: The atom, upon absorbing the particle, can, instead of emitting a new particle, split almost in half! For example, uranium-235 will split into two different elements, barium-141 and krypton-92, plus three neutrons when bombarded by one neutron. This is fission. Many large nuclei can undergo the fission reaction.

Note that in the reaction we described with uranium-235 one neutron went in while three came out. Now, consider many uranium-235 atoms together in one place. If one of these atoms undergoes fission, it releases three neutrons. These neutrons can cause three more atoms to undergo fission, and since each of these releases three more neutrons, nine more atoms will undergo fission, etc. etc. etc. A chain reaction occurs. To produce a chain reaction, elements must be used that produce more neutrons than they absorb when they undergo fission. Only three elements/isotopes do this: uranium-233, uranium-235, and plutonium-239. Only uranium-235 is found naturally, and it is always mixed in with a much greater quantity of uranium-238, which makes it very difficult to separate and use. Plutonium-239 is actually much cheaper to produce, and is used most often in modern bombs and reactors. Nuclear bombs and reactors produce a lot of energy. Where does this energy come from? Well, a lot of the energy comes from the breaking of nuclear bonds. It takes energy to hold the nucleus of an atom together, and when the nucleus is broken apart, that energy is released. Most of the energy, though, comes from a loss of mass. When the exact masses of the particles going through a fission reaction are added up and compared with the mass of the particles coming out of the reaction, it is found that there is more going in than coming out. Now, since the mass can't just disappear, it must have been transformed into energy by Einstein's e=mc² equation. This means an awful lot of energy.

Nuclear reactors, unlike bombs, produce controlled fission. There are several different kinds of reactors. Breeder reactors expose uranium-238 to very high energy neutrons, transforming it into lutonium-239 to use in fission. The most common nuclear power plants in the United States are boiling water reactors. They use uranium-235 as fuel. The uranium is formed into rods, which are immersed in water. The rods contain only about three percent uranium-235, the rest is uranium-238. When neutrons hit the first rod, they cause fission to occur, producing more neutrons. Most of these neutrons "escape" into the water, slowing them down before they reach the next rod. Since uranium-238 only absorbs very high speed neutrons, all of the slowed down neutrons go to produce fission in the uranium-235. To control the speed of the reaction, control rods made of a boron-carbon alloy are inserted into the reactor. These rods absorb neutrons. Any neutrons that were not slowed down by the water are absorbed by the uranium-238, which, in a few days, transmutes it to plutonium-239. This plutonium also undergoes fission. However, relatively little energy comes from plutonium fission in boiling water reactors. The energy produced by the fission heats up the water, causing it to boil. The boiling water produces steam, which flows through turbines to produce electric energy. Nuclear waste is the term given to used up uranium rods. These rods are highly radioactive, and are very dangerous to humans.

Breaking up large nuclei into smaller nuclei produces large amounts of energy. Fusing together small nuclei into a larger nucleus produces an even greater amount of energy. This process is called Fusion. Fusion is the process that powers the sun, and all other stars. Fusion in stars occurs when two protons combine to produce hydrogen-2 (plus the emission of a positron: a positive electron.). This hydrogen isotope combines with another proton to produce hydrogen-3. This combines with yet another proton and then beta decays to produce helium. At each step, the mass decreases a little bit more, releasing huge amounts of energy as specified by Einstein's e=mc² formula. Fusion is the energy of the future, as the fuel needed is in endless supply and no harmful waste products are produced. However, due to the fact that fusion is a thermonuclear reaction, that is, it only occurs in temperatures of millions of degrees, there is still a long way to go before fusion is a workable energy source. There is still plenty of work left for the physicists of the future.