The RTG on Curiosity uses the contrast between heat of plutonium's radioactive decay and the chilly temperatures on Mars to generate electricity.
The form of plutonium used, plutonium-238, is unsuitable for nuclear explosive. The primary risk, according to physicists, comes if finely ground plutonium is inhaled or ingested along with with food.
The units housing the plutonium-dioxide pellets have undergone rigorous tests under conditions one might expect to see if a rocket carrying an RTG-bearing spacecraft has to be destroyed before it reaches orbit, NASA and US Department of Energy officials have said.
By some accounts, six missions since the 1960s, including the aborted Apollo 13 mission and Russia's Mars 96 mission, have ended with RTGs burning up high in the atmosphere or plunging into the ocean after surviving reentry. While early models released radioactive material as craft burned up on reentry, later models appear to have survived reentry and impact intact without releasing radioactive material.
NASA's environmental-impact statement for this launch puts the risk of a release from a launch-area accident at 1 in 420. But the agency's calculations put the risk of adverse health effects to any single individual from an accident near the launch site at less that 1 in 1 million.