For outer solar system exploration, virtually the only feasible power subsystem are Radioisotope thermoelectric generators (RTGs). These include plutonium, which may carry considerable risks (see also this question). According to Space Mission Analysis and Design (Larson and Wertz, Third Edition, Eight Printing, 2006), page 335:
We must also consider safety issues, but RTG sources are probably safer than most propellants.
How true is this statement? In the case of catastrophic launch failure of a spacecraft carrying several kg of plutonium, how does the risk posed by the plutonium compare to the risk posed by propellants? By risk, I mean the danger posed to human health and to the local and global environment.
Edit: I'm looking for quantitative calculations. The question I linked above links to environmental impact statements that NASA did for the launch of Cassini, New Horizons, and Mars Science Laboratory. These contain risk calculations: for certain catastrophic scenarios, they estimate the consequences to human health (latent cancer risks) and cleaning costs. I'd like to see similar calculations for the consequences of large amounts of propellants, or their partial burning, being released locally or through a larger region.