I'm looking to use water as a radiation shielding device, but do not want to waste space on my ship with waste and other water byproducts. Rather than just storing waste, I plan to use it as part of my radiation shielding, particularly the urine.

A bit of background on my ship. The ship is not large, and is a 30 meter diameter sphere on the front of an atomic engine (not radiation shielded) with different levels; 25 meters for a spinning artificial gravity, 4 for a water filled layer of insulation, and the last meter filled with....waste. The layers of (semi) liquid are completely heated by the excess heat of the engine, and are separated by an inch think metal.

This is not about how to get this craft into space, but rather to travel in space with it. Assuming there is a small waste reclamation plant on the ship at some location, I plan to give this ship a long travel time, (to Pluto and back) with a crew of 4 people.

This is a bit of an awkward question to ask, but how well does urine (and possibly other waste) act as an additional radiation shield?

I expect they may not like the smell however.....

Bonus: How well does flesh work as a radiation shield?

Edit: For all those curious about recycling the water for recirculation, I plan to, but the journey of the ship will be 34 years give or take. This means that not only will the water from urine be used, but also from fecal matter and other bodily fluids. Even then, there's diminishing returns on the amount of water reclaimed; although that's not a problem for the ISS, if my travellers are 3.5 billion miles out (~15 years at top speed) they will have a problem with replenishing their supplies along the way. I don't plan for them to encounter further water, so they'll need every drop. The process for fecal water reclamation is a bit longer and their water reclaimer is not suited to that amount of waste.

However, I don't feel that information is relevant to the question, but rather provides a bit of background as to why the question is in the form it's in and why I don't "just reclaim the water then store it".

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    $\begingroup$ Is this ship called the "Vomit Comet?" $\endgroup$
    – Braydon
    Sep 5, 2017 at 21:26
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    $\begingroup$ I'd assume no different from water. Are you aware what happens with urine on contemporary space stations? $\endgroup$
    – Raditz_35
    Sep 5, 2017 at 21:29
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    $\begingroup$ iflscience.com/space/… There you go (edited for a better link maybe). Yes, somehow real space travel is even more "disgusting" than your space ship. You should check out the toilets they have there. You have to be able to be super rational to be one of those astronauts, they deserve a lot of respect $\endgroup$
    – Raditz_35
    Sep 5, 2017 at 21:44
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    $\begingroup$ @Raditz_35: Filtering/purifying urine seems a lot less disgusting than having it all around the ship in its unfiltered form. Water you drink on Earth has been in and out of animals and plants many times since life evolved. $\endgroup$ Sep 6, 2017 at 0:49
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    $\begingroup$ Why don't you purify the urine back to water and use the water as shielding, like a sane person would...? $\endgroup$ Sep 6, 2017 at 1:28

8 Answers 8


Urine would work fine.

However, urine has a lot of dissolved salts and other nasty stuff in it that can, over time, create buildups that would block pipes and cause other problems.

Also, if it springs a leak, would you rather be breathing in floating globs of urine or floating globs of water? (assuming a micro gravity vehicle).

It doesn't take much to purify the urine into water (especially if you don't take it all the way to drinkable). Unless the vehicle has very low power generation/collection capabilities, there should be plenty of power to bring it back to drinkable. That would mean that you can either bring less water or you can have a reserve tank that you can drink in a pinch (say, your water tank gets punctured). Also, there will be fewer problems if those two tanks get mixed somehow.

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    $\begingroup$ It should be noted that urine is called that because it contains urea. Over time that can break down into ammonia, which is toxic at that point. So that might not be just urine leaking into the air... $\endgroup$
    – Machavity
    Sep 6, 2017 at 17:59
  • $\begingroup$ @Machavity, yeah, i knew some of that but I didn't feel I knew enough detail to specify it in my answer. $\endgroup$
    – ShadoCat
    Sep 6, 2017 at 18:31
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    $\begingroup$ Also, all that other (non-water) stuff in it is going to be lost to your enclosed system during the trip. If its a long trip, you might be far better off using it to replenish the soils your human's foods are growing in (or feeding it back into the food synthesizer or whatever). $\endgroup$
    – T.E.D.
    Sep 6, 2017 at 21:26
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    $\begingroup$ But OP after all just wants to be sounded by urine. So stop patronizing OP with all your sane thought, his space ship has to be Urine propelled! $\endgroup$
    – Zaibis
    Sep 7, 2017 at 10:58
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    $\begingroup$ @Machavity I think it's the other way around... The urea chemical compound is probably called that because it's found in urine $\endgroup$
    – BgrWorker
    Sep 7, 2017 at 14:33

I'm fond of a phrase I've coined: "technology dichotomy." You can't, for example, expect to have time travel without first inventing the wheel. How does this apply to your question?

Recycling technology preceeded space travel by a long, long way. Aristotle understood the basics of desalination. In other words, it is a technology dichotomy to have an interplanetary space ship without reclamation.

Next, add inefficiency. The cost of tanks to hold all the water you need without reclamation, plus the tanks for waste, the fuel to move it all ... all because you don't want to recycle.

Finally, add the benefits of recycling. In an emergency, you can ration and use reclamation to survive for a very long time. Without it, once you've drunk the last drop in the tank, you're basically dead.

Techniques could be used to make sure there is always adequate radition shielding, such as inner and outer tanks, one to hold source water, and one to hold waste such that there is always a minimum amount of water between you and outer space... but we're layering solution-upon-solution just to avoid reclamation, which is a millenial-old science and an obviously-available technology.

...And this all assumes you can drink water that has been used for radiation shielding. Various sources (1, 2) suggest irradiated water is unlikely to be a problem. But I'm not sure I want to drink the water and wait for the warm, fuzzy feeling. IfyouknowwhatImean... They're not testing water that has been used for interplanetary radiation shielding.

You're probably looking for shock value, but in reality, keeping the shielding and hydration sources separate is far more efficient, meaning a more cost-effective operation. If you don't do it, your competitor will, just to save fuel costs if nothing else.

I'm not a nuclear physicist, but I suspect a layer of heavy water (deuterium oxide or 2H2O) would be a better radiation shield than regular water or urine, and that your ship would only carry enough water to compensate for loss due to imperfect reclamation and emergencies.

As for flesh. I'm going to go out on a limb and assume you're thinking of a biologically-grown shielding and that you're not putting people or animals at risk. Either way, flesh would be an unlikely candidate for raditation shielding due to its tendency to suffer from Melanoma, better known as "skin cancer," which is caused by radiation.

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    $\begingroup$ For shielding against high-energy beta radiation, water would be (slightly) preferable to heavy water as it has lower potential to generate bremsstrahlung. You don't really need any shielding against alphas or low-energy betas. For shielding against gamma radiation and X-rays it's really just a question of mass, where heavy water has an edge by virtue of its greater density -- you have to lug just as much of it around, but at least it's slightly more compact. Finally shielding against neutrons is probably too much to ask of water of either kind. $\endgroup$
    – Charles
    Sep 6, 2017 at 7:38
  • $\begingroup$ I'm not reinventing it, but one of the problems with long space travel is liquid and solid waste management. With urine acting as a radiation shield and the feces water being reclaimed due to the heat of the engine and other distillation processes, the availability for multiple members to constantly recycle waste matter in ship much smaller than the ISS is to say the least a bigger problem than simple recycling can handle in short periods of time. $\endgroup$
    – Anoplexian
    Sep 6, 2017 at 16:50
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    $\begingroup$ Water doesn't really become irradiated. OK, some very small fraction might absorb enough neutrons to become tritium but that would be very rare (and has a short half life). Also, unless you are using a fusion reactor, deuterium is not a good thing to carry since it is not safe to drink in large quantities. The heavier molecules chemically react slower. I've heard that problems can arise if more than 20% of your body's water is heavy. $\endgroup$
    – ShadoCat
    Sep 6, 2017 at 17:10
  • $\begingroup$ @ShadoCat, I'm not suggesting deuterium be used for drinking water at all. Quite the opposite. I believe drinking water and shield water should be two completely separate systems. $\endgroup$
    – JBH
    Sep 6, 2017 at 18:37
  • $\begingroup$ Unless you have a use for the deuterium, that's a lot of extra weight to carry. You can get all the benefits of deuterium by just adding 10% or so more ordinary water and save 80-90% of the weight (plus plumbing). If you use a deuterium based fusion reaction, I'd still add that extra 10-20% normal water for when you get low on the deuterium. $\endgroup$
    – ShadoCat
    Sep 6, 2017 at 18:45

No no, that's a terrible idea.

Radiation shielding tends to become radioactive.

When an atom is bombarded by radiation, it is often altered - neutrons flipped to protons, lose a neutron, gain one, etc. When it's just water, you're dealing with only 2 atoms - hydrogen and oxygen - you know what they do. You know how to deal with it. And water is pretty well-behaved, doesn't take much to clean up water in a light-water reactor.

 From "Hogan's Heroes": 
 Mueller: This water is for use in nuclear experiments. It is known as "heavy" water.
 Klink: I drank some of that water. (gasps) Will I die from it?
 Mueller: Only if Berlin finds out.

Urine is made of myriad elements: these are, after all, things the human body is rejecting. So it could be almost anything, now you have to contend with what radioactivity could induce onto all those atoms and isotopes.

Cites for shielding: Cosmic-ray activation of some elements... Activation generally (Wikipedia) Rutgers on activation and shieldng

  • $\begingroup$ Could you provide an example element mapping of what elements change into some other elements? Of particular interest would be a mapping showing a chain of how non-radioactive elements are changed into radioactive elements. $\endgroup$ Sep 6, 2017 at 15:17
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    $\begingroup$ Is "Hogan's Heroes" really a reliable source for material safety data? $\endgroup$
    – Philipp
    Sep 6, 2017 at 19:54
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    $\begingroup$ @NoctisSkytower, List of nuclides would be a good starting point. Pick a stable isotope of an element, add a neutron, and see where the decay chain leads you. For example, stable Potassium-41 activates to 42K, which decays by beta emission with a half-life of 12 hours to 42Ca, which is stable. Stable Oxygen-16 activates to 17O, which is stable, which activates to 18O, which is stable. Ordinary hydrogen activates to 2H, which is stable, which is why we use water for shielding: it takes at least two neutron captures to make it radioactive. $\endgroup$
    – Mark
    Sep 6, 2017 at 22:44
  • $\begingroup$ @Philipp In sufficient quantity heavy water will kill you. I'm not having any luck with a LD50 and it appears that the lethal level is way above the lethal dose for water itself--the only way to die of it would be to drink it for an extended period so the deuterium builds up in your body. (But I did find it suggested for a stealthy murder plot.) $\endgroup$ Sep 7, 2017 at 1:40
  • $\begingroup$ @LorenPechtel, heavy water doesn't have an LD-50 because it isn't directly poisonous. Reactions in heavy water proceed at a slightly different rate than reactions in regular water; heavy water kills from protracted consumption by replacing enough of the regular water in your body that your metabolism no longer runs at the right speed. The lethal concentration is somewhere around 25% to 50%. $\endgroup$
    – Mark
    Sep 7, 2017 at 20:43

The space radiation consists almost exclusively of charged particles slamming with high velocity against the hull. When they are deaccelerated, they will emit x-rays.

Water and everything with high water content (food, flesh) and therefore urine has excellent shielding properties against charged particles.

The thing is that water allows in contrast to heavy elements that the charged particle loses their energy more gradually. Heavy elements are stopping charged particles fast and this causes strong X-rays...Bremsstrahlung.

This is not good, so the perfect shielding a thin outer hull as container, then, erm...water as particle shield and then a heavy element shielding to reduce the remaining x-ray radiation. Space agencies are well aware of the problem so the idea of using water as shield are not new.


A variation of what you are looking at has been conceptually designed. This is the "Spacecoach", and is built around massive water balloons which encases the manned portions in order to provide radiation shielding, thermal buffering and a massive store of life support materials.

enter image description here

One conceptual design for the Spacecoach

The basic Spacecoach can use its vast store of water as a "once through" system, and the waste water is put through the engines, which resemble overpowered microwaves, and proivide an ISP of between 800 and 1200 seconds, similar to nuclear thermal engines, but with less thrust due to a much smaller mass flow.

As noted, it is crazy to think that you can have an advanced spaceship without already understanding recycling and closed or semi closed life support systems, and even the Spacecoach would probably work to reclaim water, possibly using wastewater (suitably treated) to run a hydroponic farm, for example. Any accidents or problems in space could take a long time to resolve (assuming they are survivable), so husbanding your resources is always wise.

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    $\begingroup$ This reply shows well, through what it does for the "SpaceCoach" link, exactly what DPT's answer didn't do for the "Packing For Mars" link: the difference in quality, interest and usefulness is tangible. $\endgroup$ Sep 6, 2017 at 16:09

This has been covered in a book called Packing for Mars. IIRC the food is the shield on the outward leg and the waste is the shield coming back.

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    $\begingroup$ Interesting book. Can you tell us more from the book? Is it effective? Don't post just a link and point people to read the book without giving the important part here that solves the question. $\endgroup$
    – Vylix
    Sep 6, 2017 at 1:14
  • $\begingroup$ Yes, the book was written by a human being and may have no relevance to the world that is being built by the questioner. $\endgroup$
    – SFWriter
    Sep 6, 2017 at 1:22
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    $\begingroup$ While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review $\endgroup$
    – Vincent
    Sep 6, 2017 at 1:36
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    $\begingroup$ The essential part was included. Food is used for the outward journey, waste for the return journey. I could have down-voted the original question as being very google-able, bur thought promoting reading was preferable. Loss of the link wouldn't matter since the title of the book was included. $\endgroup$
    – SFWriter
    Sep 6, 2017 at 1:49
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    $\begingroup$ See Are answers solely referencing novels/movies/etc. okay? Based on the guidance in the highest voted answer to that question, this does look like an answer to me, although rather low quality because it doesn't really go into any detail on the viability of such a design, which appears to be the main focus of the question. As for @Vincent's comment above, that's a canned message from the review queue which obviously isn't going to be tailored to the specific answer each time. $\endgroup$
    – user
    Sep 6, 2017 at 5:54

To an extent, yes.

There are several types of radiation:

  • Alpha and beta radiation can be stopped by a thin sheet of metal, so it is not a big issue.

  • Gamma rays are very penetrating and cannot be stopped entirely but the more mass you put between the source and yourself, the better. In nuclear power stations they have thick walls made out of heavy concrete, but really it's just a question of how much mass you have between the source and yourself. Heavy materials can be packed closer to the core where they will block more radiation due to being closer, but if you have many tons of something (eg water) it makes sense to place it between the reactor and the crew.

  • Neutrons are effectively blocked by light elements but not heavy elements. Since a water molecule has two hydrogen elements, it is quite effective at blocking neutrons.

    What you probably want to do is have a long spacecraft with the crew at one end and the reactor at the other. That way, you have the mass of everything else blocking the gamma rays. You also benefit by having more distance between the crew and the reactor, which is a good form of protection in itself.



This is what might be called a closed ecology kind of question. For a journey of the length you are talking about, you would need to decide whether you are going to add massive amounts of mass to take things like drinking water with you, or are you going to recycle as much as you can in order to reduce mass.

If this trip is 34 years, you are going to want to look at as much recycling as possible, just from a practical standpoint. That means everything possible. Any salts removed from the Urine get used somewhere else. Solid waste gets used for something. anything you vent to the outside is lost from your system and cannot be recovered, so you need to bring the replacement for the human body with you, which will add mass you have to move. This also goes for anything you put in a position to be unuseable, such as using urine for radiation shielding.

It was mentioned that matter used as radiation shielding will become radioactive over time. That means in a closed system using urine as a shield, you will not want to recycle that liquid. 34 years continuous exposure to even low level radiation is going to give your passengers all kinds of nasty tumors and cancers. It may even be lethal over time.

All of this doesn't even address the question of what was the shielding before the urine tank filled up?

So the short answer is Yes, urine will make a workable radiation shield. It's just a really Bad Idea to do so.

Now, If you are really attached to the pee shield idea, you might make some adjustments. Perhaps the astronauts fill it up during the initial phase then go into some sort of Cryo-Sleep for the bulk of the journey. this is going to dramatically reduce the amount of stuff you have to bring with you, and reducing the requirement for a nearly 100% efficient closed system. They wake at the destination, then go back into Cryo for the return.

Your only other option, really, is to build a much, much larger ship and bring the extra mass and food as you need for a 34 year voyage. Adjusting of course for whatever recycling capacity you have.


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