Can urine be used for radiation shielding?

Question

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".

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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.

Answer 5

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.

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.

Answer 6

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.

Answer 7

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.

http://www.nuclear-power.net/nuclear-power/reactor-physics/atomic-nuclear-physics/radiation/shielding-of-ionizing-radiation/

Answer 8

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.