|
|
Post by zuthal on Jan 20, 2018 0:05:05 GMT
Everyone likes to talk about (or at least, it has become a sort of meme on the Discord server) how our lithium-6 radiation shields are quite unrealistic - but are they really? The most common objection to them is that they would be ablated, turning into a mixture of helium and tritium, by the neutron flux - but that is not something that happens extremely quickly. Another objection is the heat, which is an issue - especially as the reaction between Li-6 and a fission neutron, itself already having about 1 MeV of energy, releases another 4.8 MeV. This, if you limit your shield to being solid and radiating off both faces, limits the neutron flux it can withstand without melting to about 5.1*10^15 m^-2 s^-1 - at which rate, it would take 343 years to ablate away 1 mm of lithium.
However, there is no reason a neutron shield has to be solid. It can, in fact, be a liquid - and at 1 atm of pressure, lithium boils at about 1600 K. This means that, for each 1 m^2 of radiator area, you can absorb a neutron flux of 4*10^17 s^-1 per m^2 of radiator area - or, in more typical CoaDE reactor terms, a reactor radiation hazard of 63.7 kW, assuming 1 MeV neutrons. The liquid radiation shield also allows an easy way to deal with the gas that is produced - you simply include a centrifugal gas separator into your line somewhere, where the helium bubbles out (the tritium reacts with the hot lithium to lithium tritide, and eventually decays to helium-3). And this system wouldn't use up that much lithium - at a flux of 4*10^17 s^-1, it consumes about 125 g of lithium-6 per year.
|
|
|
|
Post by n2maniac on Jan 20, 2018 17:09:29 GMT
A good rule of thumb when worrying about whether material will be used up to nuclear reactions: nuclear reactions produce a LOT of energy, so if the thing is going to be used up quickly it is going to be exceedingly hot.
|
|
|
|
Post by AdmiralObvious on Jan 20, 2018 17:53:49 GMT
I mean, this would he a good reason to actually use a neutron reflector. Making a similar issue regarding "fuel will be used up by 6 months" we could probably add in "radiation shield will decompose by 6 months, radiation will exceed (I forgot the magic #)".
|
|
|
|
Post by Enderminion on Jan 20, 2018 19:54:48 GMT
I mean, this would he a good reason to actually use a neutron reflector. Making a similar issue regarding "fuel will be used up by 6 months" we could probably add in "radiation shield will decompose by 6 months, radiation will exceed (I forgot the magic #)". 50 miliserveits |
|
|
|
Post by zuthal on Jan 20, 2018 21:16:59 GMT
I mean, this would he a good reason to actually use a neutron reflector. Making a similar issue regarding "fuel will be used up by 6 months" we could probably add in "radiation shield will decompose by 6 months, radiation will exceed (I forgot the magic #)". True, but the amount of Li you are using will be pretty low compared to the shield - a 1 cm thick Li-6 shield is about 5 kg/m^2 or so, and while absorbing 63.7 kW of neutron radiation, you as I said only consume 125 g of Lithium-6 per year... so really, you just have a tiny reserve tank for your liquid lithium rad shield system. |
|
|
|
Post by n2maniac on Jan 21, 2018 22:08:22 GMT
I mean, this would he a good reason to actually use a neutron reflector. Making a similar issue regarding "fuel will be used up by 6 months" we could probably add in "radiation shield will decompose by 6 months, radiation will exceed (I forgot the magic #)". "Radiation shield will melt/vaporize under the applied radiation load" |
|
|
|
Post by tangentialthreat on Jan 22, 2018 6:22:23 GMT
Isn't the extra 4.8 MeV in gamma rays?
Isn't this kind of bad?
|
|
|
|
Post by AdmiralObvious on Jan 22, 2018 6:46:46 GMT
Isn't the extra 4.8 MeV in gamma rays? Isn't this kind of bad? Gamma rays are pretty easily stopped by most things. An inch of steel can usually resist gamma radiation pretty well. Long term exposure (which is probably what we'll get), we aren't too sure. |
|
|
|
Post by The Astronomer on Jan 22, 2018 8:18:00 GMT
Isn't the extra 4.8 MeV in gamma rays? Isn't this kind of bad? Gamma rays are pretty easily stopped by most things. An inch of steel can usually resist gamma radiation pretty well. Long term exposure (which is probably what we'll get), we aren't too sure. I thought gamma rays are the tough one to stop? |
|
|
|
Post by jtyotjotjipaefvj on Jan 22, 2018 12:57:34 GMT
Isn't the extra 4.8 MeV in gamma rays? Isn't this kind of bad? Gamma rays are pretty easily stopped by most things. An inch of steel can usually resist gamma radiation pretty well. Long term exposure (which is probably what we'll get), we aren't too sure. An inch of steel is not exactly light compared to a few millimeters of lithium though. Might as well use something else for the neutron shield if you need such a heavy secondary shield. |
|
|
|
Post by AdmiralObvious on Jan 22, 2018 17:53:29 GMT
Gamma rays are pretty easily stopped by most things. An inch of steel can usually resist gamma radiation pretty well. Long term exposure (which is probably what we'll get), we aren't too sure. I thought gamma rays are the tough one to stop? Yeah, I got it mixed up. Still, you can block gamma radiation reasonably well with most of our standard crew compartments. |
|
|
|
Post by zuthal on Jan 23, 2018 0:31:56 GMT
Isn't the extra 4.8 MeV in gamma rays? Isn't this kind of bad? The extra 4.8 MeV should be (I think) mostly in the kinetic energy of the reaction products, i.e. tritium and helium-4 nuclei. I am not sure how much of the energy will be either immediately released as gammas, or turned into xrays due to bremsstrahlung. |
|
|
|
Post by treptoplax on Jan 23, 2018 16:14:34 GMT
At the radiation levels we're running embrittlement of reactor structural components are likely an issue... What are you going to use as casing for your liquid Li6, though; will that hold up?
i wonder if it could be feasible to kill two birds with one stone; just bathe the whole reactor core in Li6 and use that for the inner heat-exchange loop. It's probably not particularly good for the purpose thermally, but...
|
|
|
|
Post by newageofpower on Jan 26, 2018 20:20:28 GMT
At the radiation levels we're running embrittlement of reactor structural components are likely an issue... What are you going to use as casing for your liquid Li6, though; will that hold up? i wonder if it could be feasible to kill two birds with one stone; just bathe the whole reactor core in Li6 and use that for the inner heat-exchange loop. It's probably not particularly good for the purpose thermally, but... Liquid lithium has pretty decent thermal properties, the problem is that the neutron absorption characteristics (good for shielding) will interfere with fission processes. |
|
|
|
Post by apophys on Jan 27, 2018 2:15:54 GMT
Worse, the vapor pressure of lithium at 2500 K is somewhere around 1,000+ atmospheres (extrapolating from Wikipedia data). You're going to have issues containing that.
Also, if you plan to circulate it in the inner loop, know that lithium's critical temperature is 3223 K. A liquid loses a lot of its thermal conductivity as it approaches its critical point.
|
|
