2 more questions

[Lowly Uhlan]

My SH game will finally commence this Friday and I've got 2 questions for the science-minded.

 

1 Where in our Solar System can you find deuterium? It will be used to fuel fusion reactors in my game and I need to know where it would be mined. I'm thinking Jupiter, Saturn, Uranus but am not sure. Getting it from Earth isn't an option (Dead World Syndrome).

 

2 What materials would be used to build efficient radiation sheilding? The type to be used on spacecraft to protect the crew from solar radiation effects during interplanetary flight. I would rather not tell the players"You're protected by 2 meters of lead"

 

And thanks to the users of this forum for answering past questions

[Talon]

Re: 2 more questions

 

From http://herschel.jpl.nasa.gov/science/solarsystem02.html, apparently the expectation is that Uranus and Neptune would be expected to have more deuterium than Jupiter and Saturn; however, given the sizes of these planets, I'm sure you could stop off at any of them and get more than you could use.

 

It seems that deuterium is found wherever hydrogen is, so anywhere that has hydrogen bound in some form will have deuterium as well, so icy asteriods, gas giants, and the Oort Cloud should all be dandy candidates.

[Intrope]

Re: 2 more questions

 

2 What materials would be used to build efficient radiation sheilding? The type to be used on spacecraft to protect the crew from solar radiation effects during interplanetary flight. I would rather not tell the players"You're protected by 2 meters of lead"

 

And thanks to the users of this forum for answering past questions

Depleted Uranium is used as radiation shielding as I understand it. As I understand it, any of the really dense elements will work, unless the element is already seriously radioactive. I'd expect Osmium to work well, if you want something that sounds cool.

 

Rich/Decadant folks might go in for Gold or Platinum shielding, too.

[Lowly Uhlan]

Re: 2 more questions

 

Depleted Uranium is used as radiation shielding as I understand it. As I understand it, any of the really dense elements will work, unless the element is already seriously radioactive. I'd expect Osmium to work well, if you want something that sounds cool.

 

Rich/Decadant folks might go in for Gold or Platinum shielding, too.

 

Osmium does sound cool.

 

Thanks for the link as well, Talon.

 

Anyone else?

[Dr. Anomaly]

Re: 2 more questions

 

Deuterium and Tritrium are both isotopes of hydrogen, with one and two neutrons in the nucleus respectively ("regular" hydrogen has none, just a proton). If you've ever heard of the "heavy water" used in atomic reactor experiments, it's referring to water in which some significant portion of the molecules have an atom of deuterium or tritrium in place of one of the "ordinary" hydrogen atoms.

 

Pretty much anything that has hydrogen in it will have some fraction of that hydrogen as deuterium. Deuterium is relatively rare, though, because it's not produced in atomic reactions since the Big Bang -- it's consumed in them, since it fuses easier than plain hydrogen. When stars first begin their nuclear burn, the deuterium in them is the first thing to go.

 

The real problem is that while it's present in anything containing hydrogen, it's such a small percentage of the total hydrogen content that it makes "mining" it out of the substance (water, methane, whatever) a laborious task. You may want to consider these options:

 

1) Use ordinary hydrogen instead of dueterium. Much more plentiful and easier to get, though it would require a more advanced reactor design to make it fuse.

 

2) Set up deuterium "refineries" in low orbit around the gas giants which do nothing but skim the atmospheres and sort out the deuterium, ejecting the "waste hydrogen" back into the planet.

 

3) Set up deuterium "manufacturing plants" that turn ordinary hydrogen into deuterium by adding a neutron to the nucleus (or that splits a helium nucleus into two deuterium atoms). Either option would require a lot of power, so the "plant" would probably be set up near the orbit of Mercury so that large solar collectors could provide the power to run the plant. This option is a lot like the way our current fossil-fuel infrastructure works today...taking energy from the sun and "storing" it in fuel. (The energy used to create the deuterium is released when the deuterium is fused in reactors.)

 

4) Switch to a two-part fuel for your reactors: Helium-3 and Hydrogen (the oridinary kind). You can fuse those together one-for-one to get helium-4 (the "ordinary" kind of helium) and get energy out of the process. Since helium-3 can be collected from the solar wind, a "refinery" near the orbit of Mercury, using huge, diffuse magnetic fields as "sweeps", could gather the helium-3 and plain old hydrogen is available pretty much anywhere. Or you could just fuse helium-3 with helium-3. The reaction isn't as effiecent as using the deuterium-deuterium chain, but the reaction is easier to initiate and sustain. Plus, it may be easier to get helium-3 than deuterium, since you can mine for helium-3 on the Moon. Several billion years of bombardment by the solar wind are supposed to have left a sizeable amount of helium-3 "trapped" in the lunar rocks, a bit like water "trapped" in a sponge.

 

 

Also, a note about radiation shielding: sufficiently strong magnetic fields also work as radiation shields. A strong enough one will even deflect neutrons, despite the fact they are electrically neutral; being composed of quarks, which do have electric charges (albeit fractional ones) a neutron does have a magnetic moment, and thus can be affected by magnetism. It's just that they interact so weakly with magnetism that you need a really strong field to affect them.

[Lowly Uhlan]

Re: 2 more questions

 

You rock, Doc.

[DigitalGolem]

Re: 2 more questions

 

Dr. A. was pretty freakin' thorough, so I'll just add a couple of comments.

 

Deuterium may also be found in the solar wind, probably in greater quantities than He-3, so if you're setting up shop near Mercury, you can get both. Also, the Deuterium-Helium-3 reaction produces only charged particles, which greatly reduces the need for shielding, and therefore reduces the mass of the power plant. As I understand it, this is also one of the "easiest" reactions to initiate. (I've heard that nuclear physicists say this reaction has a very high "cross-section", whatever that means. Anyone grok that?)

 

I'd recommend Transhuman Space for a good rpg-oriented discussion of all the ideas in this thread. And The Starflight Handbook by Mallove and Matloff offers a more rigorous look at this subject.

 

thanx heaps,

 

DGv3.0

[Dr. Anomaly]

Re: 2 more questions

 

The cross section of a reaction is an expression of the probability that the particles participating in the event will engage in a particular physical or nuclear reaction.

 

In other words, the higher the cross section the more likely it is to happen, which generally means the reaction is easier to induce/produce.

 

Sometimes you hear a specific type of cross section listed for a given material -- for example, graphite has a relatively high neutron cross section, meaning it soaks up neutrons handily, which is why it's used in the damper rods in a fission reactor (soak up free-flying neutrons, fewer reactions get triggered).

[DigitalGolem]

Re: 2 more questions

 

Thanks for clarifying that, Dr. A!

[Dr. Anomaly]

Re: 2 more questions

 

Glad to be of service!

[theltemes]

Re: 2 more questions

 

Getting my MS in Nuclear Engineering this spring, thought I'd chime in.

 

You can use pretty much any material you want to for shielding. For gamma rays (from the nucleus) and x-rays (from the electrons of the atom) you want a "High Z" material - basically Tungsten or some other dense material. For neutrons and charged particles you want to use hydrogen for shielding - which one reason most power reactors use either light or heavy water. Thick shielding is better than thin shielding because of "build-up" flux, basically cascade reactions that happen at the surface being irradiated (thick shields attenuate out the build-up flux).

 

For shielding bases on other planets 2-3 meters of regolith over the habitat should suffice for all but the most intense solar flares (assuming you are basing on the moon or mars).

 

For D-3He reactions, keep in mind that you are making some tritium and neutrons in the D-D side reactions at about 1%-0.1% of the total fusion reaction rate, so there is radiation present.

 

A sufficiently advanced society would probably use 3He-3He fusion exclusively. It produces 2 protons and an atom of regular Helium-4. Proton cycle fusion wouldn't be feasible because it requires a carbon catalyst and produces a lot of gamma radiation, and requires a lot more energy to get going.

 

Another advantage is that 3He from the solar wind is relatively abundant in the first 3 meters of rocky bodies that have no atmosphere. It is also present in Jupiter and Saturn because their gravitational fields are strong enough to retain helium in the atmosphere.

[Dr. Anomaly]

Re: 2 more questions

 

I've heard that any material with a lot of hydrogen in its structure is good for soaking up neutrons, so parafin wax, for example, is a good barrier to neutrons.

[theltemes]

Re: 2 more questions

 

Water and parafin wax are neutron moderators - they slow down the energetic neutrons to room temperature velocities ~2200m/s. To absorb these slow neutrons, you want a material with a high neutron absorption cross-section. Boron has a great absorption cross section at thermal energies and nuclear reactors use a form of borated concrete to increase shielding capability. So a layer of water in front of boron-carbide ceramic should remove most of the neutrons.

 

Keep in mind though, that whenever one deals with free neutrons, whatever you are shielding with will eventually become activated because of the neurtron captures going on in the material. This is a big problem for fission reactors and schemes using Gen. I (D-T, D-D)and Gen. II (D-3He) fusion reactions.