As for the Cassini mission, the sort of plutonium-powered radioisotope thermoelectric generators (RTG) it carries have been used many dozens of times on Nasa space missions over the last 30 years without any release of radioactive materials. Two of these had serious accidents - one was Apollo 13 - but the RTGs worked exactly as years of extensive and rigorous testing had predicted. There was no release of radioactive material.
The plutonium in RTGs is in the form of a ceramic - plutonium dioxide - which as well as being entirely unsuitable for weapon use has been chosen exactly because it makes the RTG extremely robust and the chances of a life-endangering release of radioactive material under extreme conditions, such as the failure of the launch system, about one in a million.
Ms Roddick could instead worry about the more likely event of a one- mile wide asteroid hitting Earth and causing the loss of up to 1 billion human lives. And then she might consider how such a catastrophe might be prevented without sending nuclear weapons into space.