The KRUSTY reactor (the first actual test under the Kilopower project) is shown in this image.

enter image description here

As can be seen, about half of the mass is shielding. Suppose we were using the reactor on an umanned vehicle (ion drive tour of the Jupiter or Saturn system, for instance) could that shielding be omitted? The reactor could be on a boom (or even a tether) at some distance from the spacecraft electronics, and obviously no human is going to go anywhere near it after it achieves criticality. Since power/mass basically gives you the acceleration you can achieve with your ion drive, this makes a significant difference.

  • $\begingroup$ Changing the question after 24 hours and two answers have been posted sometimes makes people grumbly, but when I saw your numbers I realized I'd left off a few zeros on a few numbers, though the conclusions are the same. Still, you really shouldn't rewrite questions after people post answers. Instead, post a new question, or post a supplemental answer with the new material. $\endgroup$ – uhoh May 5 '18 at 10:00
  • $\begingroup$ Actually I checked and you completely changed the question. I've rolled it back. If you want to demonstrate that you know how to answer your own question, then post an answer! You can even accept it. That's 100% okay. But dramatic changes to a question after this much time has passed and after two people have taken the time to post answers is just not the right thing to do. $\endgroup$ – uhoh May 5 '18 at 10:15
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    $\begingroup$ Ok I started partying it as an answer and the prompt suggested editing the question. Since it was only half an answer anyway, that's what I did. Didn't matter anyway, several answers and comments are now including similar numbers $\endgroup$ – Steve Linton May 5 '18 at 11:53
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    $\begingroup$ Partying should be typing $\endgroup$ – Steve Linton May 5 '18 at 12:23
  • $\begingroup$ Ha! apparently you might have been "partying" as well! If you take a break from it, you can reconsider posting anything interesting as a supplemental answer. It might be interesting and potentially helpful to someone reading here someday in the future. I think the prompt was just there because sometimes people (including me) start writing comments in the answer box by mistake. $\endgroup$ – uhoh May 5 '18 at 15:10

The gamma and neutron fluxes from a reactor are huge, and both are difficult to shield against. Even on robotic spacecraft there are components that are sensitive to radiation damage and interference from the neutrons and gamma rays you'd get from a reactor: avionics, star trackers, imaging instruments, etc. You could probably get away with less shielding than for humans, but not a lot less.

  • $\begingroup$ Perhaps an avenue of weight savings -- Without atmosphere and a living ecosystem around you, you only really need to place shielding between the radiation source and the sensitive gear, right? So instead of surrounding it with shields, it's just a shade. $\endgroup$ – Saiboogu May 4 '18 at 17:55
  • $\begingroup$ @Saiboogu The shielding doesn’t absorb neutrons; it just bounces them around and eventually causes them to fly free away from the craft. That means you can’t just put a shield/shade in front of specific components: neutrons can hit unshielded parts and bounce around until they arrive at the sensitive component. A spacecraft designer might be able to group sensitive components inside small/light $4\pi$ shields, but a power system designer will prefer shielding the entire craft. $\endgroup$ – Bob Jacobsen May 4 '18 at 18:54
  • $\begingroup$ Obviously some amount of shielding becomes enough to block or deflect enough radiation to keep the ship safe. I'm referring to placing that much in a shield between the reactor and the rest of the ship, and letting radiation going in other directions just fly free. With nothing else within millions of kilometers, nothing will deflect that radiation back towards the payload. $\endgroup$ – Saiboogu May 4 '18 at 18:58
  • $\begingroup$ Shadow shields - that's what I think this question is after, and I feel like you passed by - ieeexplore.ieee.org/document/4315932 $\endgroup$ – Saiboogu May 4 '18 at 19:07
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    $\begingroup$ @uhoh , After seeing your edit I saw what you meant by your earlier comment, and indeed my original answer was ambiguous. I've edited it to remove that ambiguity. $\endgroup$ – Tom Spilker May 5 '18 at 5:12

Much of the the already radiation-hardened electronic components of the Juno Spacecraft needed to be enclosed within a thick and heavy (~200 kg) titanium vault (see also Juno Radiation Vault) in order to protect them from one or two dozen passes through the radiation from particles trapped in Jupiter's magnetic field, lasting (roughly) a few days each, and radiation damage of some components less effectively shielded is still what is likely to degrade and perhaps destroy several of Juno's instruments by the end of the mission.

This total dose is nothing compared to the radiation effects being a few meters away from a nuclear fission reactor for months or years!

To address @Hobbes's comment with an envelope-back-class answer, according to Wikipedia 235U fission releases about 200 MeV per fission event. A one-kilowatt source would then have something like 3E+13 decays per second.

The article says that for each event, about 7 MeV of that is released as gamma rays. Assuming 0.5 MeV per photon, that works out to 33,000,000,000 gamma rays per second per square centimeter at 1 meter, or almost 3 Watts of gamma rays per square meter, 0.2% as bright as the Sun's 1360 W/m^2 of radiation, but 100% ionizing!

That's also about 2,000,000,000 neutrons per square centimeter per second, which are can also be damaging and problematic.

I'll update this and compare with proximity to Jupiter as well as deep space background if I can find/locate all of the sources and conversion factors. While this answer makes it look easy by quoting a post-nuclear survivalist's blog about bomb shelters, this it's actually not a simple calculation to do correctly unless you do it for a living.

However, I can estimate that a person standing 1 meter from this reactor absorbing only 10% of the 3 Watts/cm^2 of gamma rays would receive about 500 Sv per day, or about 100x higher rate than the rate for "colonists" on Jupiter's moon Europa.

For more about Juno's orbit through Jupiter's radiation field, see for example:

below: Image from Installing Juno's Radiation Vault Cropped and complete (click for full size view). Credit: NASA

enter image description here

enter image description here

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    $\begingroup$ "This total dose is nothing compared to the radiation effects being a few meters away from a nuclear fission reactor for months or years" - that statement could use some numbers... $\endgroup$ – Hobbes May 5 '18 at 8:23
  • $\begingroup$ @Hobbes indeed. Neutron and gamma flux from a fission reactor should be available somewhere, if not I'll estimate it somehow. If I don't add an estimate in the next day, ping me again. But with even just one neutron and a few gamma rays produced for every event (ignoring radiation from all the fission products), it's going to be extremely hot! Consider that some materials can't even be used near a reactor due to intense radiation damage; some types of pipe even becomes brittle from metallurgical deterioration, etc. I'm pretty confident that the numbers are going to be strikingly different. $\endgroup$ – uhoh May 5 '18 at 8:48
  • $\begingroup$ @Hobbes for even a small, 1 kW reactor at 1 meter, I guesstimated the radiation to be 100x stronger than standing on Europa, not counting neutrons or fission products. See edit. It's not an easy calculation to do quantitatively, I'll see what else I can round up. $\endgroup$ – uhoh May 5 '18 at 9:43

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