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Does Depleted Uranium (DU) have a role in spacecraft shielding?

Crewed spacecraft require shielding to protect crew from hazards of space, particularly:

  1. Micrometors. The chief defense is a Whipple Shield https://en.wikipedia.org/wiki/Whipple_shield consisting of “bumper” layers to break up the micro-meteor before it hits the main shielding layer. This strategy is similar to “air gap armor” used in tanks. DU is incorporated in Abrams tank armor plate due to its high resistance to penetration. DU could be used as the main, inner shielding layer for a Whipple Shield.

  2. Primary cosmic rays are high speed positively charged atomic nuclei including protons. Unfortunately, when primary cosmic particles hit a spaceship hull (or shielding), they produce a spray of secondary particles. Hydrogen (in the form of fuel, water or hydrogen-rich plastic) is the most mass-efficient shielding for cosmic rays.

  3. Gamma rays are high energy electromagnetic radiation. Heavy atomic nuclei (Tungsten, Gold, Lead, and Uranium) are the best shielding materials. Uranium is the most mass-efficient shielding for gamma rays.

Based on this information, I would expect mass-efficient integrated shielding (to protect against micro-meteors, cosmic rays and gamma rays) to consist of:

  1. Spacecraft design utilizing water and fuel storage as shielding, when practical.
  2. Multiple bumper layer Whipple shielding to protect from micro-meteors
  3. Plastic between the Whipple layers to absorb cosmic rays.
  4. An inner layer of depleted uranium to protect from gamma rays and micro-meteor fragments spallated by the Whipple shield.

Depleted uranium is the opposite of “enriched uranium”: it has a lower percentage of the fissile isotope U235, consisting of 99.7% U238 with a half life about the age of the Earth.

Due to its long half life, health hazards of DU are chiefly chemical rather than radiation. It has heavy metal toxicity (similar to lead) with renal, CNS and cardiac toxicity. It has a short elimination half life of 15 days, but can accumulate in internal organs.

Has depleted uranium been considered for radiation shielding in crewed spacecraft beyond LEO?

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    $\begingroup$ Then there is also the fact that water and/or hydrocarbons would be more useful on the far end… $\endgroup$
    – Jon Custer
    Apr 29, 2023 at 19:58
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    $\begingroup$ I wouldn't exactly be thrilled to hear that my spaceship's radiation shield was a toxic heavy metal known for spalling into extremely flammable dust in an impact. Water is a lot more benign should something go wrong in an unexpected manner. $\endgroup$
    – Cadence
    Apr 29, 2023 at 22:28
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    $\begingroup$ @Cadence ... good point. But DU is used in tank armor as a defense from penetrating munitions. The DU radiation/meteor shield would be inside the normal micro meteor shield. If a meteor gets through both layers, inform Houston you have a problem. $\endgroup$
    – Woody
    Apr 29, 2023 at 23:06
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    $\begingroup$ Vaguely related answer of mine to Why make X-ray shielding from titanium, when lead is 6 times lighter?. TL;DR: "best radiation shielding" is very dependent on the kind of radiation you're expecting to be receiving, and stuff that's good against one flavor (eg. x-rays) is likely to be poor against another (eg. neutrons). $\endgroup$ Apr 30, 2023 at 13:34
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    $\begingroup$ I also have a sneaking suspicion that bombarding U-238 with high energy protons is going to generate a larger quantity of radioactive byproducts than if you used something rather more stable for shielding, but I don't have an easy way to quantify that. $\endgroup$ Apr 30, 2023 at 13:43

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Is DU, in fact, the best shielding by mass?

The first thing to note is that the given quote from Wikipedia is not very relevant for the case of shielding of spacecraft: It refers to shielding against radiation from radioactive isotopes which is quite different to radiation encountered in space.

In LEO we do have a strong contribution of protons and neutrons in the energy range between 0.1 MeV and 10 MeV which are not generated by radioactive isotopes (outside of fission reactors). To compare the effect of materials against these we need to compare their "nuclear interaction length", e.g. from this table by the Particle Data Group. The relevant values are 50 g/cm² for hydrogen and 200 g/cm² for uranium - you need 4 times as much material if you want to use uranium to shield against neutron and proton radiation. This actually can be understood in a quite simplified view: To stop a proton it needs to hit an atomic nucleus head-on. The single protons of hydrogen are spread rather uniformly across the material. For uranium some nucleons "hide on the backside" of their nucleus and don't provide additional cross-section.

The second important contribution to radiation in space is gamma rays. Here uranium has an advantage because two of the dominant ways for their interaction are the photoelectric effect (which scales with the atomic number to the fourth power: $Z^4$) and pair production that scales with $Z^2$. Here the very strong field of the highly charged nuclei increases the shielding effect substantially. Comparing the "radiation length" value from the table above, uranium is 10 times more efficient than hydrogen in this respect.

The third component of radiation are electrons and positrons, but here the difference between materials is much less pronounced.

Last but not least, it also has to be considered that it's not sufficient to just stop incident particles, the amount and kind of secondary radiation needs to be taken into account as well. E.g. it doesn't help if protons are stopped but produce a nasty mess of radioactive isotopes in the process, e.g. by causing fission of heavy nuclei. In this respect light elements are a clear winner.

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    $\begingroup$ Surely the density of the material also plays into it. H2 (solid) is about 0.07 g/cm^3 whereas uranium is about 19 g/cm^3, so much much heavier to lift and less effective at shielding. $\endgroup$
    – bob1
    Apr 30, 2023 at 21:09
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    $\begingroup$ @bob1 I think that "4 times as much material" refers to 4 times the mass per unit area (as expressed in say grams per square centimeter), not 4 times the thickness in linear dimension. In radiation "shieldingology" we frequently (usually) refer to thicknesses in those mass/area units. $\endgroup$
    – uhoh
    Apr 30, 2023 at 21:45
  • $\begingroup$ @asdfex I totally forgot about protons, which I should not have since (a million years ago) I did a lot of reading on proton spallation as an excellent way to generate lots of interesting and very radioactive isotopes. Very nice answer! $\endgroup$
    – uhoh
    Apr 30, 2023 at 21:49
  • $\begingroup$ @uhoh I realized the 4x unit was about material, but the fact that U is significantly more massive per unit volume plays heavily into the good 'ol tyranny of the rocket equation, so it's not only that H is better, U is harder to get there in terms of payload. $\endgroup$
    – bob1
    May 1, 2023 at 0:31
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    $\begingroup$ @uhoh Neutrons are as good as protons when it comes to proton radiation. For gammas "useless neutrons" are outweighed by the powers of the atomic numbers. Stopping neutrons and protons; and high energy gammas by pair conversions needs the nuclei, not the electrons. $\endgroup$
    – asdfex
    May 1, 2023 at 8:54

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