NASA took exhaustive efforts to make the RTGs safe
even if the Titan IV blew up. You can read more about what they did, including the full text of their Environmental Impact Statement, at this site.
One problem with dealing with issues like nuclear material in spacecraft is that no matter what precautions you take, some self-proclaimed expert will loudly proclaim that it isn't enough. I've seen the same kind of pronouncements about things like aviation safety since 911. Of course, the Press loves controversy so these types often appear on the tube and in print.