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I've recently been making an attempt to write a short story that leans very heavily towards hard sci-fi. My area of expertise is primarily in biology and neurology, and the backbone of the story is mostly based around these elements. However, I'm less well-versed in reactor design and rocket science, and these are all currently elements I'm struggling with.

For context, the story follows a group of three people stationed on a moon that have been stranded due to the loss of their shuttle and communications, and are slowly dying from radiation poisoning themselves.

In order to achieve this outcome, I was initially thinking about using an automated probe powered by a Kilopower nuclear reactor. A malfunction in its navigation system causes it to end up slamming into the surface of the moon, all too close to their base. The control rod would be ejected from the nuclear reactor in the probe or the reactor core would be deformed into a favourable geometry, and it would go supercritical. The resulting criticality accident would expose the entire crew to radiation and damage semiconductor components enough so as to knock out electronics in their base and their shuttle.

I thought this would be a fairly easy bit of worldbuilding, looking further into it has convinced me that I was wrong about that.

In order to estimate radiation exposure, I have looked at the Kiwi-TNT event, detailed here. Reactivity was inserted into a nuclear rocket engine prototype by turning all its control drums at a high speed, and its effects were studied. This is not exactly analogous because the Kiwi-TNT experiment was done on Earth, whereas the moon in question in my story has no atmosphere, but it's good enough.

As explained in page 34 of the linked report, all radiation exposures at a distance of 300 feet would be fatal, exposing anybody within that radius to over 1000 rads. The table on page 25 seems to indicate that at a distance of 100 feet, a person would be exposed to gamma radiation amounting to 3,000-5,000 rads, and at a distance of 200 feet, a person would be exposed to gamma radiation amounting to 800-2000 rads. This seems fine for my purposes, until you consider several things:

Unless the engineers of this base were extremely incompetent, with the lack of a magnetic field to shield from cosmic rays there is no way the base would not be radiation shielded to some extent. A shielding that blocks out something like say, 90% of gamma radiation would attenuate radiation exposure enough to not be fatal for the crew (hundreds of rads is enough to induce sickness, but would not necessarily be fatal). The only way to expose every single crew member to a definitively fatal dose of radiation would be to have them all be spacewalking outside the base at that point, and that seems like a ridiculously risky thing to do especially considering that automation exists in this world, I can't think of a scenario which would justify it. Furthermore, knocking out the electronics in their shuttle and communications system would be difficult with radiation alone considering that radiation hardening even today is capable of making things shockingly resilient, with space grade semiconductor chips being able to withstand 1000-3000 grays (note: 1 gray equals 100 rads). Radiation hardening is a consumable, but that's a lot of radiation to be able to withstand, and all of these things would likely remain inside a shell that itself provides radiation shielding.

Now, instead of a kilopower nuclear reactor, I've been looking over nuclear thermal propulsion rockets in order to see if I can generate a criticality accident severe enough in those to achieve everything I would personally like, but there's a lot of literature to push through on that and not necessarily a lot of data about possible radiation exposures from an accident.

Can anyone help with this?

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    $\begingroup$ RBMK has a enriched history of doing exactly what you want. $\endgroup$
    – ErikHall
    Commented Jun 10 at 20:37
  • $\begingroup$ A base would be shielded by mass not magnetic fields, ie it would likely be buried under a few meters of regolith. Less expensive long term and functional against all radiation, unlike magnetic fields.. $\endgroup$ Commented Jun 10 at 21:01
  • $\begingroup$ @GaultDrakkor Perhaps I should have made it clearer, I mean the lack of a magnetic field on the moon to protect from cosmic rays would entail their use of mass-based radiation shielding. $\endgroup$ Commented Jun 10 at 21:03
  • $\begingroup$ @ErikHall Agree that this would be a good solution. I just can't think of a reason as to why an RBMK-like reactor would need to be there in the first place. (Note, the issue of providing energy to the base has been solved using geothermal energy from the tidally heated interior, so this might not be a viable reason, and gets worse when you consider RBMK is a fairly old reactor design that won't necessarily be used in near future applications). $\endgroup$ Commented Jun 10 at 21:04
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    $\begingroup$ Am I allowed to suggest a badly maintained Russian one $\endgroup$ Commented Jun 10 at 21:51

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with the lack of a magnetic field to shield from cosmic rays there is no way the base would not be radiation shielded to some extent

Here's the thing about cosmic rays, and meteorites for that matter: you know exactly where they're coming from. Up! Or across, depending on the nature of the local terrain. Lets imagine that you build your base in a crater, though. The crater rim limits the directions in which hazardous stuff can come in. You then slap a nice big thick roof slab on it... a few of meters of regolith, maybe sintered a bit, and that'll do great. Have the roof overhang the walls, and then you can have things like non-armored airlocks or even windows, because there's simply no line of sight for the bad stuff to reach you. Nice big defensive mushroom.

Except, uh, actually if an intense man-made radiation source kinda crashed into the crater and bounced and rolled up under the shield layer to your lovely viewing deck then actually maybe quite a lot of radiation could get in. Seems kinda obvious, in retrospect. But how often do nuclear devices fail deadly by your front door?

If it is hot enough, there's no way anyone is going to be able to go out there and move it away from the window. If the robots available aren't radiation hardened, they might have issues moving it too. So, everyone soaked up rather more sieverts than they'd have liked, and the whole top level of the base is out of bounds because the wreckage is still pouring gamma rays and maybe neutrons through those lamentably non-multi-meter-thick vertical walls and those windows that didn't even have a good view in the first place. Hope there wasn't anything important up there.

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Not a reactor at all: propellant storage for a nuclear salt water rocket, their only means of getting back home in a reasonable amount of time, takes an unfortunate micrometeorite hit. The leaking propellant pools beneath the tanks, the water boiling off. It reaches criticality and ruptures the tanks, making the runaway reaction much bigger. The resulting nuclear bonfire spreads vaporized, highly radioactive fuel across the vicinity. Due to negligence or necessity, it was located too close to the main base for safety, and contaminates everything.

The crew wouldn't necessarily need to have immediately received a fatal dose...the entire exterior of the base is covered in highly radioactive material. They're slowly accumulating exposure while they stay inside the shielded base, but they'll rapidly receive a lethal dose if they step outside. Simple heat from the runaway reaction destroyed the cable runs to the communication terminals. This is simple enough to fix, but again...they're dead if they go outside to do it.

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Gas Core Nuclear Reactors

With a gas core nuclear reactor (especially if it's a open cycle one), when the automated probe crashed into the moon, theoretically the hot uranium gas, being thrown forwards by momentum, would have immediately wrecked it's way through anything that stands in front of it, first the probe then quite possibly alot of the regolith as well, by the virtue of being hot enough to vaporise itself in the first place. Even though it might not be hot enough to actually penetrate into the base(I have no idea what the regolith is composed of, so this is a guess) having uranium (and fission fragments) condensing inside the radiation shielding is still likely to boost the radiation dose several times over, not to mention that if the gas condensed into a lump then it might be able to continue a fission reaction on top of the base.

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The radiation shielding failed.

A reactor emitting enough radiation to fatally irradiate the crew of a buried base is pretty unlikely if it was designed properly. Thus, I suggest an alternative; The reactor is putting out the same amount of radiation that you mentioned, but the shielding failed. The crash ended up cracking the rock the base was buried under, exposing them to fatal radiation doses. This isn't that unreasonable, since while the base is likely designed to withstand small meteorite impacts, it probably isn't made to take a rocket falling on top of it.

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