Safety of RTGs

[rpayne88]

What would have happened if NASA had to abort a probe carrying an RTG during its launch? I forget the website, but I think Cassini carried enough plutonium to deliver a lethal dose of radiation to every person on the planet. Is this the truth? If so, what would NASA have done to prevent a malfunctioning rocket from irradiating the Florida coast?

[K^2]

It's a matter of the rate at which the radiation is released. Yes, there is probably enough nuclear fuel in a large RTG to give everyone on the planet a lethal dose, but if you split the fuel up into small enough chunk, the rate at which the radiation is released by each one is going to be so slow that it's going to take longer than human life span to do so.

Because Plutonium-239 has fairly long half-life, in tens of thousands of years, to give off dangerous levels of radiation it needs to be reasonably concentrated. That means if you spread the RTG fuel over a large chunk of Florida's coast, it's not going to be a total disaster. Situation with nuclear meltdowns is actually very different and produces short-lived isotopes that are the real danger, and there isn't enough fuel in RTG to result in a meltdown.

But more generally, dispersal isn't going to be even that bad. The rocket will become a chemical fireball in the worst case scenario. That's probably enough to melt some of the fuel, but not to evaporate it. Most of the fuel will end up landing in a very small area. Some of it might turn into a molten spray and cover a slightly larger area. This can still be bad if this lands in populated area, but rocket landing in populated area isn't much better. Given typical trajectories, this will most likely result in some radiation of either area around the launch pad or over the ocean, but nothing too serious, so long as majority of the fuel is promptly recovered.

All in all, it is a hazard, but not as bad as it might sound. It's comparable to all of the standard hazards of lighting a controlled explosion under a giant tank of (frequently toxic) fuels.

[lajoswinkler]

There's little if any chance that a RTG module would be dispersed in mid air. It might break upon impact with the ground, though.

RTG is plutonium dioxide pellets in a strong casing. Very nonreactive ceramic material. If it hit a farmer's field, it would contaminate the field, that's it. You just scoop it up.

As usual, the press likes to scare the gullible laymen.

[TheSaint]

Actually, NASA already had an RTG crash back on Earth. The Apollo lunar modules each carried one to power one of the lunar surface experiments. All of them stayed on the Moon. Except for Apollo 13. That one reentered Earth's atmosphere and landed in the Pacific Ocean near the Tonga Trench. After reentering at 11,000 m/s, coming to rest under 10,000 m of water (which is almost 1,000 atm of pressure), and being submerged in seawater for over forty years, there is no indication that any radioactive material has been released.

I think they're pretty safe.

[Eric S]

Because Plutonium-239 has fairly long half-life

Slight correction, the Cassini RTG(s?) used -238, which has a much shorter half life (87.7 years). However, failed launches, reentry, etc. was all considered when the RTGs were designed.

[Nuke]

yea 239 is weapons grade. 238 is what is used in rtgs.

[Kerbin Dallas Multipass]

I think Cassini carried enough plutonium to deliver a lethal dose of radiation to every person on the planet.

Lethal dose: "few miligrams"

1mg per human would be 7 billion * 1/1000g = 7,000,000g or 7000kg. Cassini had 33kg on board so there isnt enough for everyone to die.

I'm assuming the stuff was packed away safely so it would have survived a launch malfunction. However, Cassini did a close flyby of earth in order to slingshot to Jupiter. WP states:

with Cassini passing by 500 km (310 mi) above the Earth on August 18, 1999.[19] Had there been any malfunction causing the Cassini space probe to collide with the Earth, NASA's complete environmental impact study estimated that, in the worst case (with an acute angle of entry in which Cassini would gradually burn up), a significant fraction of the 33 kg[19] of plutonium-238 inside the RTGs would have been dispersed into the Earth's atmosphere so that up to five billion people (i.e. almost the entire terrestrial population) could have been exposed, causing up to an estimated 5,000 additional cancer deaths

http://en.wikipedia.org/wiki/Cassini%E2%80%93Huygens#Plutonium_power_source