Radioisotope thermoelectric generator behavior on reentry

Question

The RTG has become a major source of power in places where solar power just wont cut it. but there has always been an issue with sending them into space. if the launch fails, radiation comes raining down.

My question is, at what speed (if any) would the RTG just burn up and not cause any problems?

Answer 1

The only thing "raining down" will be a mostly intact RTG or intact GPHSes with no release of PuO2 in either case. They are designed to survive launch accidents due to the obvious concerns you express. The government doesn't let you just launch nuclear material without extensive engineering to deal with possible accidents.

From the Mars 2020 EIS, 4.1.4.3. MMRTG Response to Accident Environments:

• Explosion Overpressure and Fragments: Liquid propellant explosions and resulting fragments are expected to damage the MMRTG, but not result in any release of plutonium dioxide.

...

• Reentry: Impacts resulting from reentry of the MMRTG are dependent upon when and from where reentry occurs.
  • Most suborbital reentries are predicted to result in intact impact of the SV due to the presence of the SV aeroshell for Mars entry. Releases in these cases are similar in nature to those from SV impact near the launch pad.
  • Reentry from circular orbital decay or long-term reentry is predicted to cause breakup of the SV and the MMRTG with subsequent release of the GPHS modules. (This breakup of the MMRTG and release of the GPHS modules is intentional and designed to limit the release of PuO2 in this type of accident.) This will result in some heating and ablation of the surface of the GPHS modules, but no containment failure or release in the air. When these separated components impact land, there is a potential for release from the GPHS module if the impact is on rock or a similar hard surface. No release is expected from a water impact or soil impact.

A very high-speed hyperbolic reentry could release PuO2 from a GPHS and disperse it in the atmosphere. That is not considered to be a desirable outcome, as implied by your question. There would be the possibility of excess cancer fatalities in the human population of Earth from inhalation of the resulting particles.

This had to be dealt with for Cassini, which had a 19 km/s Earth flyby on the way to Saturn. A great deal of work was done to assure that the probability of an inadvertent reentry due to any cause, including spacecraft failures, was less than $10^{-6}$. Furthermore, the project had to convince expert, non-advocate review boards (i.e. people who didn't care if your project went forward or not) that that had all been engineered and calculated correctly.

Answer 2

Probably never.

According to wikipedia, PuO2 has a melting point of 3017 K, comparable to other materials which aren't expected to burn up on reentry like titanium alloy (1900 K) and silicon carbide (3103 K). Given that a failed interplanetary launch is likely to come down on a very steep trajectory, it would have even less time for heating than reentering from orbit, and so be less likely to burn up, just hitting the ground extremely fast.

Remember also that a launch can fail at any stage, including seconds after launch, when it wouldn't be going very fast at all.

As pointed out by Uwe in a comment, leaving a trail of plutonium vapour in the atmosphere would probably not be considered to be "not causing any problems", so in that sense the answer is definitely never.

Answer 3

Each of the Apollo missions carried an RTG in the lunar module, to be left behind on the moon to power scientific instruments. However, Apollo 13's lunar module returned back to Earth as a "lifeboat" for the crew, and was destined to re-enter Earth's atmosphere with the RTG. Even though the Apollo-era RTG was designed to be "indestructible", the Atomic Energy Commission insisted that the LM be targeted to the most remote place on Earth:

So we did move the landing point a little bit… to put the RTG in the deepest part of the Pacific we could find,” says Bostick. Whatever was left of the Aquarius, after its descent through the Earth’s atmosphere, would find its resting place about 10 kilometers beneath the waves in the Tonga Trench. Ultimately, no released radioactivity was ever detected, despite a helicopter survey of the area.

https://spectrum.ieee.org/tech-history/space-age/apollo-13-we-have-a-solution-part-3

RTG design has improved over the past 50 years, so the chance of a disaster is even smaller. Nonetheless, we do try to avoid such a possibility as much as possible.

Answer 4

Plutonium-238, the typical material used in RTGs, is an alpha emitter, with relatively low gamma and beta emissions. Alpha radiation is blocked by a couple inches of air, dead skin cells, clothing, and other thin materials. You can hold a chunk of plutonium-238 in your hand and with no real risk from radiation (as @MarkAdler pointed out you'd want an oven glove to protect you from the heat). The big danger from plutonium is from particles as they can be inhaled or ingested, coming into direct contact with tissues that have no protection. Plutonium can be absorbed by the body where it spends a very long time in your liver and bones. It's also a poisonous heavy metal.

So burning up is the absolute worst case scenario, the one thing you don't want to ever happen with that material. Raining big chunks is fine in comparison, you just pick them up, melt them down into a new RTG.