1. Most of the uranium is composed of the isotope uranium-238 which does not undergo fission. The uranium-235 that does exist is spread too thin to create a chain reaction.

4. Since a neutron has no electrical charge it doesn't have to overcome the electircal repulsion that a proton would or electron would. It is also much more massive than an electron.

12. To prevent the radioactive contaminants produced in the reaction chamber from reaching the outside envornment.

13. The atomic nucleus is less massive than the separated nucleons that it is made of.

16. Fission would release energy from gold. Fusion would release energy from carbon. Neither would release energy from iron.

23. The fuel used for fusion is hydrogen, the most abundant element in the universe. Uranium and plutonium, which fuel fission reactions are relatively rare. Also, fusion produces no radioactive byproducts.