Nuclear Waste
Nuclear waste is the biggest problem facing nuclear energy. It should be at the top of the list of researchers, as the right solution to the waste disposal problem could cause a real resurgence in nuclear power usage.
Where does nuclear energy come from?
A teeny bit of physics now, if you don't mind. The most common fuel for nuclear plants is a specific isotope of Uranium, U-235. Uranium 235 is radioactive. What this means is that it is unstable, and any given U-235 nucleus could decay at any given time, in a random process known as radioactive decay.
The common image of nuclear waste barrels. Photo courtesy of StefrogZ
When a radioactive atom decays, it forms two smaller atoms, and releases a number of other particles in the process, as well as energy. For example, U-235 has 92 protons. When it decays, it emits a helium nucleus (2 protons and 2 neutrons), so the decay products are Helium and Thorium, which itself is radioactive. This is true of all nuclear fuels, and the waste is a lovely mix of many different elements.
The important part comes down to Einstein's ever famous equation, E = mc2. In this case, the U-235 atom has a greater mass than the Thorium + Helium. This is called a mass deficit, and if you plug this mass deficit into the equation, you see that the mass has been converted into energy - and a massive amount of energy too.
What is nuclear waste?
It's is a combination of many things, often depending on the fuel used in the reactor. It is often solid or liquid, and has a very long half life. (Half life is the amount of time for an amount of radioactive material to decay to half its original amount). It can in fact take tens of thousands of years for this waste to return to safe levels of radiation.
Nuclear waste disposal techniques
Honestly...bury it. The easiest method for dealing with the waste is to store it. This actually isn't as stupid as it may sound. The volume of nuclear waste isn't actually that great - it can be stored out of the way. This obviously raises major issues with environmental groups like Greenpeace but, physically at least, providing sufficient safeguards are in place to prevent leaks, this option is perfectly fine. Thanks to programmes like The Simpsons, the general public have a very bad view of this idea!
Proliferation
Another major problem facing certain types of nuclear power plant is the possibility of nuclear weapons proliferation. Breeder reactors create plutonium as a byproduct, which can be used in nuclear weapons if it gets into the wrong hands.