Waste
High level radioactive waste is very dangerous. It lasts for tens of thousands of years before decaying to safe levels. It is highly radioactive and is probably the biggest hurdle we face if nuclear power is going to be taken seriously.
If there is to be a "nuclear renaissance", a sophisticated method of dealing with this waste must be refined. This point itself has sparked a surprising number of debates.
For example, how do you write "danger" on a concrete box, when in 5,000 years the word "danger" may no longer exist? What symbols could we use to let people know to leave it well alone?
As you can see from the graph below, the radioactivity of nuclear waste takes about 10 000 years to return to that of the original ore. The black line, indicating the total waste products, is the important one.
Disadvantages of nuclear power: waste. (Source)
Accidents
Consider the words "Chernobyl" or "Three Mile Island". Do these provoke an emotional response in you? I wouldd imagine they get some kind of response at least - they are usually used by anti nuclear activists to put people off nuclear power.
Without going too deeply into what happened at these incidents, it's true that they were an example of nuclear power going somewhat awry. It's also true that a "meltdown" style accident simply cannot occur in modern reactors.
Modern reactors are designed to switch off in the event of a loss of coolant, not heat up until the whole reactor melts. This is not the problem, though. The problem is fear.
People were so terrified of the threat of radiation and various other scary things from previous accidents that even the idea of such things happening again makes people completely averse to nuclear power as a whole.
In terms of disadvantages of nuclear power, this is also a big one. The sense of danger, or at least misgiving, currently goes hand in hand with it. Lots of work will need to go into easing the collective public mind.
Proliferation
Some forms of nuclear reactor, known as "breeder" reactors produce plutonium, which can, conceivably, be used to make nuclear weapons, most likely in the form of a "dirty bomb". This is a conventional explosive mixed with radioactive material with the intention of spreading the material across a wide area to do even more damage.
In all fairness to nuclear power, the probability of terrorists getting hold of this material is small.
There are other reactors which do not have this problem, but it is another issue which must be addressed before the possibility of a nuclear future can be taken seriously. In fact, there are reactor designs, and fuels for them, which would also significantly reduce the amount of waste produced.
Fuel
Nuclear fuels are, physically, even rarer than fossil fuels. Fossil fuels at least are made on Earth, albeit over millions of years. Heavy elements like Uranium are only made as stars die, in supernovas. Our solar system actually formed from the remains of another star, at which point heavier elements were made.
Essentially, once they're gone, they are well and truly gone. Only in particle accelerators can heavier elements be made.
That said, there is plenty of Uranium, and if we switch to other fuel cycles, then there's enough to last for a very long time.
Return to nuclear fission from disadvantages of nuclear power