re: MERs and New Horizons
It all depends on believing NASA and DOE's numbers. The interesting point is that the anti-Cassini crowd believed all the numbers of how much PU-238 was used and was going into space on other missions, but not any of the other numbers from the government.
No matter how often a person pointed out that most of the PU-238 numbers they quoted were over optimistic and for unfunded missions. They still would quote these silly nonsense numbers and pretend this was proof of the militarization of space.
The two MER carried about 1.5 ounces each.
The RHU have the same kind of protections as RTG have.
The types of launch accidents that would cause vaporization of the RHU and spread of PU-238 are improbable.