Safe is measuring how many deaths per user various power generations sources cause. Coal etc is less safe then nuclear because it causes more deaths. Yes when a nuclear accident happens, it can be bad, but that doesn't happen that often at all. Compared to the numerous accidents and deaths that happen every day with say coal. Also coal is outputting pollutants into the atmosphere which causes more deaths (a slower death but dead is dead). Cleaning up that air is also just as difficult as cleaning up nuclear waste.