Disposal of nuclear waste is very expensive. As it is radioactive it has to be disposed of in such a way as it will not pollute the environment.
Decommissioning of nuclear power stations is expensive and takes a long time. (In fact we have not yet decommissioned one!)
Nuclear accidents can spread 'radiation producing particles' over a wide area, This radiation harms the cells of the body which can make humans sick or even cause death. Illness can appear or strike people years after they were exposed to nuclear radiation and genetic problems can occur too. A possible type of reactor disaster is known as a meltdown. In a meltdown, the fission reaction of an atom goes out of control, which leads to a nuclear explosion releasing great amounts of radioactive particles into the environment. See Chernobyl.