[Spaceships] Orion space drive: launch from underwater?

[lwcamp]

Originally Posted by Fred Brackin

Nope, If being hit by neutrons does not make a substance radioactive (and this is quite common) they you can't call it a radioactive contaminant.

Radiatioactivity isn't like being tainted by Original Sin.

Tungsten contains five stable isotopes: W-180, W-182, W-183, W-184, and W-186. Neutron capture by W-180 leaves you with W-181, with a 121 day half life (and which normally decays via electron capture to stable Ta-181, so it will only give off x-rays and thus is not that bad). Neutron capture on W-182 and W-183 just gives you another stable iostope of tungsten. Neutron capture by W-184 gives W-185, with a 75 day half-life that beta decays to stable Re-185. Neutron capture by W-186 gives W-187, with a half-life of 1 day which beta decays to stable Re-187. So neutron bombardment of tungsten will give rise to some residual activity. A more complete analysis would look at neutron capture cross sections and the energy spectrum of neutrons within the tungsten shell.

Brett didn't specify what form of fusion is being used, but D-T fusion produces neutrons with 14 MeV of energy - this is really quite energetic and can lead to various nuclear spallation processes as well, such as having the neutron knocking off another neutron or proton or alpha particle. These can make make other radioactive particles, which can be the head end of a decay chain of several isotopes before you reach something stable.

Lead will be similar in general scope to tungsten, although different in the particulars.

Luke

[Agemegos]

Originally Posted by lwcamp

Brett didn't specify what form of fusion is being used

Indeed not. I'm trying to cover all possible bases. For what it's worth, D-T seems most plausible. But you might use an expensive and bulky aneutronic launch unit to lob the spacecraft to a safe altitude where its cheap and cheerful D-T pulse units would be not to much of a problem.

[Fred Brackin]

Originally Posted by Agemegos

Indeed not. I'm trying to cover all possible bases. For what it's worth, D-T seems most plausible. But you might use an expensive and bulky aneutronic launch unit to lob the spacecraft to a safe altitude where its cheap and cheerful D-T pulse units would be not to much of a problem.

You might use an aneutronic reaction for other purposes. D-T is easiest to ignite but putting out 80% of its' energy in 6the form of neutrons makes it hard to sustain.

Indeed, in late generation thermonuclear warheads you start out with a minimal fission stage induced by neutrons which makes enough heat to cause a very small D-T fusion reaction to make more neutrons for more fission to make enough heat for a large fusion stage and then you put some cheap uranium in the outside casing to make use of those fusion stage neutrons.

You could also insert more fission-fusion stages to make very large bombs.

So fusion-only bombs would have to be significantly different. You would need a lot of energy input to bring the whole fusion mass up to temp and you'd really rather not have all those neutrons carrying off 80% of your energy.

Any reaction other than D-T needs more energy to start the fusion process but keeps much more energy in the fusing plasma. For fusing of large masses aneutronic might require a smaller net energy input.