water as a radiation shield

[dragon]

Several times I here about the idea of using water tanks to protect crews against radiation dangers during a mission to Mars. And we know that when a proton from the radiation collides with the material of the craft, neutrons are produced. These neutrons (upon colliding with a hydrogen nucleus) liberate their energy. That I get get but what about the water is it still safe to use. Wasn't able to find much about that answer on line.

[fnord]

Warning: This is from an untrained amateur, so if you get a more authoritative answer, use that over this.

The water actually shields the crew by two mechanisms - the first is moderating the incoming neutrons down to lower (and less biologically damaging) energies through elastic collisions with the water nuclei, each collision knocking roughly 10% or so off the incoming neutron's energy until the incoming neutron reaches local thermal equilibrium. This can result in some fraction of the incident cosmic neutrons being scattered back outside, rather than into the crew.

Think about the speeds of the cue ball and the object balls before and after the cue ball hits them on a pool table.

Water can also absorb the incident neutron, forming small amounts of singly deuterated water (HDO), which is chemically identical to regular H2O. Some tiny fraction of the HDO wil absorb a second neutron to form D2O, aka heavy water, and so on - tritium (H-3) formation is quite small.

Since D's natural abundance is 1 in 6400 H atoms, a 100 kilo man (70 kg water) would naturally contain ~2 g of D, enough to make 11g of heavy water. Thus, there would be no additional hazard (above and beyond being aboard) from consuming the irradiated water.

[dragon]

cool thanks, gives me an idea in which direction to research.

[Zixinus]

How do the properties of water's radiation resistance change when turned into a solid, ie, ice? Would it be good to coat a spaceship in ice?

[RedImperator]

How do the properties of water's radiation resistance change when turned into a solid, ie, ice? Would it be good to coat a spaceship in ice?

Water's too heavy to use as armor, and there are better neutron absorbers out there. People talk about using water tanks for radiation shielding because you have to carry water anyway--for drinking, as a propellant, and in some designs (such as nuclear salt water) as the fuel. But a layer of ice armor wouldn't be worth anywhere near the mass penalty.

[Surlethe]

Besides, inside the orbit of Mars your spaceship would act like a little comet if you had a layer of ice armor, wouldn't it?

[starslayer]

Besides, inside the orbit of Mars your spaceship would act like a little comet if you had a layer of ice armor, wouldn't it?

Yup. Unless you have some magic way of cooling the ice down and dumping that heat in addition to all the heat you otherwise generate, the ice will sublimate as you get closer to the Sun.

[Fingolfin_Noldor]

Probably better off sandwiching lead between ceramic plates in addition to other metals to stop the radiation from coming in.

[RedImperator]

Probably better off sandwiching lead between ceramic plates in addition to other metals to stop the radiation from coming in.

That's an awful lot of mass to stop something which can be shielded by paraffin. The problem in space is more neutrons and charged particles than EM radiation.

[Akkleptos]

Orion, anyone? Carry whatever kind of radio-insulating material (as long as it's not liquid, because of liquid dynamics generating uncomfortable G-forces upon acceleration and braking) and nuclear-pulse out of the Earth's athmosphere and into Mars'. Thin lead plating doesn't sound as such a bad idea after all, unless, of course, you can use some other better, lighter material.

[fnord]

As has been said, you want low-Z shielding material for neutrons, and high-Z for gamma.

IIRC, Orion's pusher plate (what the nuclear initiation was directed against) doubled up as a shadow shield - it had to be massive to withstand the impulsive shock, sorting the gamma radiation, but requiring low-Z shielding material (neutron moderators and absorbers) to cut down on neutron irradiation, since the nuclear devices intended for use were apparently shaped charges, focusing their blast into a cone with half-angle 22.5 degrees.

[Simon_Jester]

Can they really shape a nuclear charge?

[dragon]

Cool thanks for all the info and ideas. Give's me a nice narrowing of the idea for my stupid paper. I hate writing

[RedImperator]

Can they really shape a nuclear charge?

The idea I've heard is to encase the bomb with an X-ray opaque material, leaving one small hole at the top. When the bomb goes off, the X-rays have nowhere to go but out the hole in a cone. The US government did a lot of work on this; quite a lot is still classified.

[Fingolfin_Noldor]

Can they really shape a nuclear charge?

The idea I've heard is to encase the bomb with an X-ray opaque material, leaving one small hole at the top. When the bomb goes off, the X-rays have nowhere to go but out the hole in a cone. The US government did a lot of work on this; quite a lot is still classified.

That assumes a lot of things, like the nuclear charge is set off in vacuum and the material (likely something similar to a optical fiber of sorts coated to handle X-rays somehow) survives long enough to direct the pulse.

But in any case, nukes aren't exactly the most destructive weapons out there in space since it can't send off shockwaves in space, unless the weapon penetrates the hull and set off the nuke inside the hull.

[RedImperator]

Can they really shape a nuclear charge?

The idea I've heard is to encase the bomb with an X-ray opaque material, leaving one small hole at the top. When the bomb goes off, the X-rays have nowhere to go but out the hole in a cone. The US government did a lot of work on this; quite a lot is still classified.

That assumes a lot of things, like the nuclear charge is set off in vacuum

...um, since it's being set off in space, why wouldn't it be set off in a vacuum?

and the material (likely something similar to a optical fiber of sorts coated to handle X-rays somehow) survives long enough to direct the pulse.

Ordinary U-238 is X-ray opaque.

But in any case, nukes aren't exactly the most destructive weapons out there in space since it can't send off shockwaves in space, unless the weapon penetrates the hull and set off the nuke inside the hull.

Well, first of all, the discussion is at hand is about the Orion nuclear rocket, which uses nuclear weapons for propulsion, not combat. And second, you can create a shockwave--or, more properly, a jet of plasma--by putting a disk of reaction mass at the open end of the bomb. It's far more efficient that just tossing nukes out the back and riding on the X-ray flash. I have no idea if you could turn that into a useful weapon, and I doubt anyone without a heavy-duty security clearance knows, either.