U235 part one

There are nine naturally occurring radioactive elements including uranium and thorium. A radioactive element is unstable and breaks apart into new elements. It is a natural, random and uncontrollable process. Some elements are fissile meaning they can sustain the breakdown to the point of critical, leading to a nuclear explosion. Uranium, like most elements, is composed of different types called isotopes. The two isotopes vary by the number of neutrons in the nucleus. U238 has three more neutrons than U235 and it is only the U235 that can be used in a bomb. Uranium in nature is 99.3% U238 and only .7% U235, so it must be concentrated or enriched before it can be useful. The standard reactor needs U235 at 3.5 to 5%. These are formed as small pellets and put into 12-foot-long tubes and then placed in the reactor core and they are replaced about every five years. For the newer small nuclear reactors called modular reactors, the fuel must be enriched to 15% and is in the form of pellets about the size of a jelly bean. These are fed into the reactor on a continuous basis as needed and if the supply is interrupted, the reactor shuts down. This is known as walk away safety, since the unit shuts down if the power goes off and the fuel supply stops. U235 must be enriched to 20% to be used in weapons but most is enriched to 90%. One of the advantages of thorium is that it cannot be used for bombs but can be used in pellet form for the safety feature. Also, thorium is more abundant and does not have to be enriched. The first reactor made, used thorium but the government changed to uranium so they could make bombs.