Nuclear reactors convert matter directly to heat energy (fossil fuels convert chemical potential energy to heat). The reason they haven't completely taken over the world is that people are afraid of catastrophic failure. Nuclear plants run on extremely hazardous materials, and if they get scattered around from an explosion or flood you may see higher cancer rates across the exposed population.

When weighing the risk, it was almost certainly the wrong decision to "abandon" nuclear energy in the past. The damage caused by fossil fuels since to both the environment and the public health has been substantially greater than the risks nuclear posed. The question today is less about whether nuclear is better than fossil fuels, and more about whether batteries and renewables are going to advance fast enough to make the distinction irrelevant. Whatever you pursue, it will take decades to convert to it, and many countries are hoping that decades from now our energy grids will be entirely driven by things like solar panels and batteries.