Answer:
The biggest risk is the future of the by-product, which includes highly-radioactive fuel, containers, tools, decomissioned plants, and anything that comes into contact with fuel. Since this material can be dangerous for hundreds of thousands of years after use, and since no human artifacts have been in continuous use for that span of time, it's doubtful humans will find a way to permanently store these materials. There are also acute dangers associated with atomic energy, such as the risk of reactor melt-down, where control is lost of the fissioning process, the containment and safety systems are breached, and massive amounts of radioactive material escape into the environment, contaminating a large region for hundreds or thousands of years (this is what happened at Chernobyl in Ukraine). Modern advocates of nuclear power claim that the risk of meltdown is low or nonexistent with modern designs, but it is hard for a designer to account for all scenarios (for example: terrorist attacks, asteroid impacts, extreme earthquakes, etc.)