Possible fuels for nuclear fusion reactors optimal reaction to produce energy by fusion is deuterium and tritium due to its high cross section. It is also therefore the most commonly used in experimental trials. The reaction is: D T 4He n Get deuterium is not difficult as it is a stable and abundant element that was formed in large quantities in the primordial soup of particles (see Big Bang). In water one part per 5000 is deuterium. This means that there are 30 grams of material in each cubic meter of water. In a self-sustaining reactor deuterium-tritium reaction would generate energy and neutrons. Neutrons are the negative part of the reaction and must be controlled because the neutron capture reactions on the reactor walls or at any atom of the reagent can induce radioactivity. In fact, neutrons, with sufficient time may come to weaken the structure of the container itself with the consequent risk of occurrence of dangerous cracks.This are the moderators and neutron shielding such as heavy water, beryllium, sodium or carbon as moderators widely used in fission plants, or boron and cadmium, used as products that completely stop absorbing neutrons. If you want to build a reactor that will really clean look for other formulas. It has been proposed a twofold solution to the problem of neutrons and tritium abundance. Tritium is not found in nature as it is unstable so you have to make it. To get it you can use fission power, which can be generated by activation of the hydrogen contained in water, or the bombardment of lithium, abundant material in the crust, with neutrons. 6Li n 4He 7Li n T T 4He n There are two stable isotopes of lithium and lithium-6 lithium-7 the latter being much more abundant. Unfortunately, the reaction that absorbs neutrons is given with lithium-6, less abundant.This also prevents many neutrons it impacted with the wall of the reactor itself with the subsequent production of radioactive atoms. However one of the proposals for ITER is to coat the walls with lithium-6 which would stop much of the neutrons to produce more tritium. Because of all these problems are being investigated reactions of high cross section but cleaner. One of the most promising is the more deuterium helium-3. D 3He 4He p The problem in this reaction lies in the smaller cross section with respect to the deuterium-tritium in itself obtaining helium-3 which is the rarest isotope of the element. The protons do not pose as much danger as these neutrons are not easily captured by atoms due to the Coulomb barrier which must pass through the charge-neutral particles like neutrons does not occur. Additionally, a proton can be manipulated by electromagnetic fields.A solution for artificial helium-3 would be to incorporate into the reactor itself, the deuterium-deuterium reaction. D D 3He n The problem is that, again, we obtain a residual neutron, which again returns us to the problem of neutrons. Maybe the key was getting natural helium-3, but this is extremely rare on Earth. Keep in mind that the little helium-3 produced by natural radioactivity tends to escape our dense atmosphere. The funny thing is that this isotope is abundant on the moon. It is spread over its surface and comes from the solar wind over billions of years has bathed the bare lunar surface with ionized particles. This lunar helium could, in future, the key to fusion reactors. While much is being investigated in materials but are activated only lead to short-lived isotopes, which let stand a short period such material could be considered as conventional waste (not radioactive).The main problem in any case, would still be the difficulty of maintaining the structure of the core conditions without which this would deteriorate and would have to be changed every so often.