Engineering360: "Pu-238: Fuel for Deep Space Journeys"

"..“it’s little helium atoms without nuclei can be stopped with a piece of paper...'

Hmmm. Nope. Little helium atoms without electrons, sure. Electrons are not nuclei.

Pu-238 conversion to energy only 5% thermal efficiency? Not good.

Why not do something exotic smart, and use a thermal radioactive source to heat silicon (or other essentially inert material) to 1414 °C, (or even higher), several container materials already proven OK, including SiC, C, W, Ta, etc. (you physics geniuses would have to work out the likelihood of neutron or alpha activation of the thermal storage media, and the container) Next the light emitted at that temperature is well suited to TPV devices that have already reached above 20% TE (in their sleep mode). Some say we will have TPV conversion efficiency near 80% in the not distant future. Band selection is key.

Do not bother with attempting thermopile in addition to, as it will just siphon off more energy than it produces.

I think it is the mass and volume constraints of going to space that lead to a low efficiency. My guess is that the heat sink is the weak link....being able to keep components to a temperature that affords some reliability without having the radiator be gargantuan must be tricky.

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By the way, what is this alpha activation of which you speak?

Neutron production by pu238 is not very strong.

Probably that was a questionable statement, but suppose you accelerated an alpha to near light speed. I makes a good cue ball for some other nucleus at that point, and could result in all manner of havoc. Do not ask me how it would be accelerated to near c.

I had second thoughts about including that statement. I agree the neutrons are not too strong from it.

Why have a heat sink at all when you want the thing to be hot? The Thermo-Photovoltaic elements absorb energy at their active spectral band, and reflect back the rest, by design. In space, it should in fact be easier to make thermal silicon tech work better. If you are talking about radiative cooling of CB components, in space, that is a horse of a totally different color. One wants to use high impedance circuitry in space to minimize power consumption in the first place. I expected that all board cooling is done radiatively.

"...Why have a heat sink at all when you want the thing to be hot? The Thermo-Photovoltaic elements absorb energy at their active spectral band, and reflect back the rest, by design. In space, it should in fact be easier to make thermal silicon tech work better..."

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Why have a heat sink at all? - b/c keeping everything organized and doing its intended job becomes much more difficult when sporting solid phase for key components becomes exceedingly passé and subject to thermodiscrimination.

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Assuming the absorber/converter is semi-decent at absorbing the frequencies it can use and reflecting most of the rest back, the emitter is going to continue to increase in temperature. This will lead to the absorber/converter receiving more and more radiation. Normal real world inefficiencies in the converter would yield more and more added heat. Some form of catastrophic self disassembly would be a likely outcome.

I am not wishing to start an argument with you, but here I go anyway.

(1) nuclear fuel source is regulatable to X heat output.

(2) X raises temperature of the silicon (in the case of zero G) to just sub-melting, since liquid containment at zero G becomes only slightly more complicated.

(3) Y heat output is couple out of the silicon (or other suitable high temperature "black body")

(4) Z portion of Y is converted to electricity in TPV panels (that include the necessary band selection optics). (one still has to apply the thermal efficiency of the TPV to Z).

(5) Y-Z heat output of the silicon is immediately returned in the band selection optics to the source.

(6) Heat (actual selected light) not converted to electricity heats up the TPV (actually increasing net efficiency (to some limit)). This quantity is τ (tau), which once radiative equilibrium is surpassed between the silicon (or other) and the TPV leads to the following: Y- Z is reflected heat into the storage medium (true), but there is still the quantity τ to be absorbed and converted or radiated into space. If there is a coaxial material to the TPV that can absorb and utilize a portion of τ, then the residual heat is further reduced to long wave thermal radiation peak (to somewhere in the infrared). Call this quantity ξ that will be coupled out the top (and bottom if you can decide up and down in space) of the secondary converter. Supposedly, in this way the system is regulated as to temperature (especially considering any turn-down on the reactor pile), and will operate at a stable set of temperatures indefinitely (until the fuel runs out).

There is no significant negative (or positive) temperature reactivity associated with decay heat. These are not reactors in the sense there is no neutron population controlling the power.

Decay heat is what is beingutilized. Basically things heat up until the energy output matches the energy input. In insulating vacuum of space, for objects of that aren't very big, a significant continual heat input can lead to very high equilibrium temps.

Okay, so we are talking decay heat, and there is no apparent way to moderate the rate of decay, so that leaves us back in the swamp? I expect the only way to further control that might be to expend some of that energy (heat) as radiation out the back of the craft, and to thus provide a minuscule thrust boost integration over time.

Hey, I thought you might find this interesting. Not sure which post would be most appropriate...,but this one was at least close.

I looked the first few pages, and I find that sort of thing fascinating, especially when it matches iron content of the real object.

When I first saw it, I though O2 + Si → SiO2, then I realized you are talking about billions of degrees and millions of grams/cc. I bit more drastic than any conditions I hope to be doing any actual testing on any time soon, (except with a very good telescope). Awesome sauce! Thank you, and happy morning!

So the 'explosive' part in the title, had be thinking the 'burning' was of the combustion type. And then I started reading and realized they were talking about stars, yet it was still very interesting. I'm glad you had a similar experience.

Plutonium melts at 639.5° C. That would pose some interesting packaging and reliability problems, particularly if it forms alloys with the containment material, goes through it, or begins sloshing around inside. The chemical and mechanical stability refers to the solid form.

Plutonium in metal form is unlikely to be used.

Voyager missions used ²³⁸PuO₂ (which has a much higher melting point and lower chemical reactivity in general) spheres clad in platinum alloy packed in graphite and sealed in an irridium alloy container. The design was to minimize risk of spreading plutonium over the Earth should the container experience reentry.