The sort of plutonium-powered radioisotope thermoelectric generators (RTG) Cassini carries have been used many dozens of times on Nasa space missions without any release of radioactive materials or loss of life. Two missions with RTGs on board have had serious accidents - one was Apollo 13 - but the RTGs worked exactly as designed.
The plutonium is in the form of a ceramic - plutonium dioxide - which makes the RTG extremely robust and the chances of a significant release of radioactive material under extreme conditions, such as the failure of the launch system or accidental re-entry of the vehicle into the Earth's atmosphere, very much less than one in a million, and then both slight and confined to the immediate vicinity of such an improbable event.
If Mr Brierley wants something to worry about, perhaps he would like to consider what would happen in the distinctly more likely event of a one-mile wide asteroid hitting the Earth, with the probable loss of up to one billion human lives. He might consider how such a catastrophe might be prevented without sending nuclear material, in the shape of nuclear weapons, into space.