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Comment by acidburnNSA | Hacker News Reader

Most SNAPs were RTGs but SNAP-10A was an honest to goodness fissioning nuclear reactor [1].

[1] https://en.wikipedia.org/wiki/SNAP-10A

Highly enriched uranium is still barely radioactive, very similar to natural uranium. The half-life of U-235 is 703 million years. As half-life approaches infinity, atoms approach stability. The dose rate of holding navy nuclear fuel is modest compared to the hazards of fission products.

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I don’t disagree that in comparison to many other nasty byproducts of the U235 chain, most Uranium isotopes are not that “hot” but the danger of HEU comes partly from the increased presence of the U234 isotope, not just U235. So that isotope solely is not the right half life to do math around. While U235 is about as stable as U238, most purification techniques result in selecting the lower weight isotopes, which selects the hotter U234 as well. And while there is less of it, if I recall correctly eventually U234 dominates in terms of radiation output.

Again this is a semantics game, SNAP 10A was still driving a thermo electric converter, thus "scaled up RTG", nothing resembling a traditional naval reactor.

The on-contact for a HEU billet is over 10mrem/hour. You and I have very different ideas about nuclear safety apparently and I presume you haven't worked professionally in the field with that attitude.

Frankly, it seems to me that you're the one trying to make it a semantics game, by trying to define what is and is not a nuclear reactor based on how you collect power from the device.

According to that Wikipedia article, SNAP 10A was centered on a device that created and maintained a controlled, sustained nuclear fission reaction. I would call that a "nuclear reactor" even if no attempt were made to harvest the power. I think that the rest of the world is probably with me on this. The Chicago Pile 1 is widely regarded as the first nuclear reactor, and nobody particularly cares how it generated electricity. What they care about is that it demonstrated a controlled, sustained nuclear fission reaction.

10 mrem/hour seems to confirm the "barely radioactive" argument; the CDC says (https://www.cdc.gov/nceh/radiation/air_travel.html) that a cross-country flight exposes you to 3.5 mrem. No one's saying you should hold said uranium in your hand for weeks at a time; they're saying the risk of chucking some around after a (very rare) spacecraft crash is pretty minor, especially as it'll be launched over water.

>"Again this is a semantics game, SNAP 10A was still driving a thermo electric converter, thus "scaled up RTG""

You are getting the absolute basics wrong, so you have no standing to question the OP's atittude.

Nuclear reactor runs a nuclear chain reaction, hence the name, RTG does not. There is no scope for debate here. The difference is night and day and is obvious if you look at fuel, power to weight ratio, or do physics 101.

RTGs run on decay heat and use plutonium 238, they cant be turned off, their power slowly drops off over decades. Reactors use U235, have active control and starting/stop procedure and 10-100x higher power to weight

The issue is these designs are ~3% efficient like RTGs and have few moving parts unlike ~35% efficient nuclear reactors on earth. While they sidestep most of the complications of traditional nuclear reactors like radiation shielding, they really aren’t useful designs having low energy output, terrible energy density, relatively short lifespans, and extreme cost.

According to https://www.etec.energy.gov/Operations/Major_Operations/Majo... it's even worse than that; 1.83%.

That said, it seems like, all by itself, conversion efficiency is a tricky measure of the usefulness of a design for spaceflight purposes. Wouldn't it be more useful to consider the total cost to deliver a given energy production capacity to space? In that case, rocket fuel itself, and the tyranny of the rocket equation, becomes a major consideration. If an efficiency gain comes at the cost of increasing the weight of the energy generation system in some way, then perhaps it doesn't end up being a net win over the less efficient design.

Efficiency is a big deal in part because you need to radiate out all that thermal energy. A radiator that’s dumping 100kw of thermal energy for 1.3kw of electricity is much heavier than a 1.3kw solar panel anywhere near earth. Add 50kg for fuel and and things look even worse.

By comparison the voyager probe RTG used ~1/10th the fuel for a little over 1/10th the power. https://en.wikipedia.org/wiki/MHW-RTG So the only advantage was cheaper fuel.

They have same radiation shielding and powet output as any other reactor, electricity generation is a separate concern that should not be confused with the reactor itself.

The same reactor couod be hooked up to a 20% efficient stirling engine to keep it low maintenance, to a >50% efficient convined cycle or have 0% electricity output and be used for heat or water desalination.

Nuclear reactors aren’t plug and play, and you can sell they skipped a lot of radiation shielding simply from the weight of these systems.

By comparison SL-1 a 400kw thermal design so 4x power used a 12,000 kg pressure vessel including shielding. https://en.wikipedia.org/wiki/SL-1

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