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Below this answer to Are there any safe-to-launch alternatives to RTG's for outer solar system exploration? I saw the comment:

...the Voyager RTG cores are still putting out plenty of heat. The problem is that the thermoelectric converters that turn that heat into electricity have worn out.

The isotope's decay rate is fixed by physics, 238Pu has a half-life of 87.7 years, which means a 1/e life of about 119.1 years or a year-on-year decay of about 0.8%. (envelope-back checking my math; (1-0.008)87.7 is indeed roughly 0.5)

Do the thermocouples on the older RTGs (circa Voyagers) actually lose efficiency at a faster rate than the 238Pu loses itself? Is it the heat or the radiation that gets them? Are modern RTG thermocouples longer-lived?

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  • $\begingroup$ related - I wonder why thermocouples rather than some other Peltier-based device were chosen. Probably size, weight, efficiency. $\endgroup$
    – Carl Witthoft
    Commented May 28, 2020 at 13:53
  • $\begingroup$ Qualitative description, "As thermocouples age in a process, their conductors can lose homogeneity due to chemical and metallurgical changes caused by extreme or prolonged exposure to high temperatures." I'm sure a few stray radioactive particles don't help the situation. $\endgroup$
    – Carl Witthoft
    Commented May 28, 2020 at 13:56
  • $\begingroup$ en.wikipedia.org/wiki/… $\endgroup$
    – Russell Borogove
    Commented May 28, 2020 at 14:32
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    $\begingroup$ @CarlWitthoft the Peltier devices I've dealt with haven't been very robust. While improvements could be made, the materials are both brittle and hard to bond. SiGe is actually pretty good anyway $\endgroup$
    – Chris H
    Commented May 28, 2020 at 18:56
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    $\begingroup$ I sat in on an RTG project review at JPL some years ago. The discussion involved any number of failure modes over the lifetime, and both radiation and temperature were discussed. Which limits any particular design depends on a large number of factors, and are engineering trade offs to be discussed for various mission profiles. For example, Pluto missions are very different from Jupiter missions, since the external rad environments are quite different. $\endgroup$
    – Jon Custer
    Commented May 28, 2020 at 19:08

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Surfing around,

This blogpost discusses the Multi-Hundred Watt RTG (MHW-RTG) which was developed for Voyager:

SiGe thermocouples were doped with Boron and Phosphorous. The main mode of failure in them was the germanium migrating out of solution over time, but the extent to which this occurred over the lifetime of the missions is unclear. To prevent sublimation and degradation, the thermocouples were coated with silicon nitride, which eliminated the need for the xenon cover gas used in earlier SiGe-based thermocouples.

The power conversion efficiency of the thermocouples was 6.5% at beginning of life, decreasing to 5.9 % at the end of design life (14 years, which the Voyager spacecraft have more than doubled). I am unable to find information about the current conversion efficiency of these systems.

The NASA Radioisotope Power System Program Office 2015 booklet Radioisotope Power Systems Reference Book for Mission Designers and Planners says (one of several links in previous reference)

Thermocouple performance may degrade over time due to precipitation of dopants in the material, sublimation of the thermocouple material, or changes in thermal conductivity of unicouple alloys. The output power degradation due to thermocouple degradation is ~0.8% per year, depending on the material and the operating conditions. Radioactive decay of the Pu-238 causes additional degradation at ~0.8% per year.

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  • $\begingroup$ Is it possible address and answer any of these directly as well? 2) s it the heat or the radiation that gets them? 3) Are modern RTG thermocouples longer-lived? Thanks! $\endgroup$
    – uhoh
    Commented May 29, 2020 at 2:04
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So with the first question already answered in that degradation of the thermocouples being about on par with that of the fuel (at least for RTGs run on 238Pu), from what I just read on Wikipedia on that topic I'm quite sure to be able to provide some insight on what's not contributing to that degradation and that's the radioisotope's radiation (which I guess the question mainly referred to) itself; however I have no idea about what cosmic-radiation's effect on the degradation process might be.

238Pu produces basically exclusively α-decay, which is really easily shielded. Thus no special shielding measures are required on top of whatever container you'd put the radioisotope in and you'd probably want to use a very tight and rigid container anyway as launch-safety will be rather high on your list of priorities.

With regard to modern thermocouples being less prone to degradation, I would be shocked, if they weren't, at least in theaory, after 40+ years of technical advancement. However I assume that real interplanetary-space experience with those new designs is about non-existing, so any predictions how they'll perform after decades of rad-hard bombardment out there would be quite a bit guesswork.

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