Uranium is a dense heavy metal that decays–meaning it’s radioactive and gives off particles and energy and transforms into a slightly lighter metal, thorium, which is also radioactive. It emits an alpha particle, the Mac truck of subatomic particles, which is also a helium nucleus. This is where earthly helium comes from! All forms of uranium are radioactive..but not the helium it emits. Don’t worry, your party balloons are safe. Uranium is unstable and thus radioactive. The word radioactive was coined by the Curies in 1898, with radio being related to ray as in a ray of light Many radioactive elements and nuclear reactions cause their surrounding to glow due to their energy. It’s slowly radioactive with all isotopes having long half-lives. It can be found in deposits across the globe.
Uranium can be made into a source of power when it undergoes fission. During fission, the core of the atom (the nucleus) is hit with a neutron and split into smaller pieces and new lighter elements are made. The lighter elements are more stable and the energy needed to hold the large unstable uranium together is released. The process is shown in the figure below. You can see that more neutrons fly out and if enough atoms of the right isotope of uranium are nearby, they split other uranium atoms. A chain reaction ensues and this keeps the energy release going. If the reaction is fast enough, a bomb is created.
Here’s the catch, not all forms of uranium undergo fission. Only the isotope with 92 protons and 143 neutrons in the nucleus, uranium 235 or U-235, is unstable enough to be broken in this fashion. And it’s not very plentiful. Only 0.7% of naturally occurring uranium is this isotope. And to allow for the chain reaction to occur, you need to concentrate this form of the metal. This is needed for both weapons grade and power reactor uranium but weapons grade uranium needs more concentration aka enrichment. This is not easy. Why does it take so much work? Chemical reactions occur with the outside of the atom–the electron cloud. This is an easy way to separate chemicals–by their different reactivities due to different electron clouds surrounding them.
All isotopes of uranium have the same cloud of 92 electrons. This means the isotopes have to be separated by mass. The uranium is reacted with fluoride and forms a gas, then is passed through a porous membrane which only lets the smaller 235 isotope through. Alternately, it might be centrifuged. There are a few other less efficient methods of enrichment. This process demands lots of energy. Monitoring the energy use of enrichment facilities is one way to watch to see if a country is working on producing weapons grade U-235.
What’s going on in Iran? They have used centrifuge technology to enrich uranium to a concentration of 4.5%. The allowed limit with the Nuclear Deal was 3.67%. However, it takes 90% enrichment to make a bomb because a bomb reaction must go faster with more U-235 atoms close to each other. Getting to this level is a huge challenge needing a high tech centrifuge. Yes, Iran can get there if the nuclear deal remains sour for years.
Right now, the world has a surplus of enriched uranium because of the many enrichment plants world wide. What country has a surplus of weapons grade uranium? The United States. Germany, the Netherlands, and Japan also have plenty of the stuff. Scientists worked hard to create the bomb. Some did it unknowingly and other suffered remorse at how it was used. Scientists approved the Nuclear Deal, they supported it, and scientific collaboration is suffering at its end.