Why isn't nuclear energy that popular? From what I know about energy sources, it seems to be the best option.

People are afraid and people also complain that Nuclear also has a waste product.  I think that maybe it is a bit expensive to start up as well, but I don't know all the details.

True, true. 

I'll stick to the problems in using nuclear energy in a car.

The biggest one is cooling. Water is great for cooling. A constant supply of cold water is needed to keep things under control in a nuclear reactor. This is why nuclear plants are placed near rivers. A submarine have access to cold water as well. However, in a car this isn't possible. I'm not sure if there could be an alternate way to keep the fuel at the right temperature.

Then there's the problem of size. I'm not sure if it's posssible to make a reactor that fits in a car. Some security measurements cannot be implemented in a car due to size as well. Even if it's doable, with the decrease in size efficiency will decrease as well and there's no way around it.

Size is a problem, and weight probably as well.

Fuel must be handled with extreme care. Not only it is radioactive, it needs to be submerged in water permanently to keep it at a reasonable temperature. Otherwise the temperature will increase and it can start a fire. Used fuel is even hotter and more dangerous.

If the vessel where the fuel is stored is damaged (in a traffic accident, for example), the water surrounding the fuel may escape. If this happens the temperature will increase, the fuel will start a fire, melt the vessel and escape (and the area surrounding the car will be affected with radioactivity).

At the time this was a pretty interesting report, however since then Haven't really herd much.

Yeah.. as long as you can dump the waste into some indian territory, and as with oil, supply of material is actually not unlimited. The US has enough desert area for solar energy to solve any problem. It does have a very efficient oil industry that won't allow that ever.

Political pressure. The legacy of nuclear accidents in the past and the very high cost of designing and building new ones has stopped it.

But compared to the reactors running now, modern nuclear designs are MUCH safer and more efficient. I believe they are a great stopgap between fossil fuels and an eventual 100% renewable future but not a permanent solution (even uranium is finite).

I'm a big fan of nuclear fusion as an upcoming technology, seeing as it's completely safe, no long-term waste, much more reliable than wind/solar. Only problem is it isn't quite ready.

Much more reliable? You mean the sun isn't shining in the desert tomorrow?

Of course when people think of fusion, they think of DD or DT fusion. These reactions create a mess of neutrons that destabalizes and "poisons" the equipment over time. If you dismantle a nuclear power plant, pretty much everything is radioactive waste, same is likely true for a fusion plant.

Fusion sounds interesting but the equipment will be way too complex/expensive to build/handle. Mirrors, water in a steam engine is much simpler technology available now. As the engineering rule number one goes "Fusion is the technology of the future. Always was and always will be".

Interestingly the inventor of that ended up dead less than two years after making his discovery due to pnemonia. Not too into conspiracy theories, but that's quite convienant.

Fear mostly.  Back when the cold war started, the US actual allowed and helped cultivate a bit of an overreaction about how dangerous nuclear weapons are kind of as an added deterant in the Cold War.

The effects of the worst nuclear meltdowns are actually a lot tamer then you would expect.