[Spaceships] rad weapons and radioactive cargo

[mlangsdorf]

So the merry band of smugglers in my Spaceships game agreed to freight some 50 tons of radiation shielded casks carrying an unspecified but substantial amount of highly enriched U-235. It's enough to be worth shipping, but not enough to blow up or to be a noticeable radiation hazard while in the casks.

Now they find themselves facing some space pirates, who are possibly going to fire some 30 MJ particle beams at them. Which have the rad modifier, and are apparently capable of delivering around 100 rads per hit past the ship's radiation protection.

I'm reasonably sure that subcritical masses of U-235 will not undergo fission chain reactions without blowing themselves apart before making much of a boom, but not much of a boom is a cold comfort if the boom is happening inside a cargo bay.

Does anyone with a better understanding of nuclear physics have any suggestions for the likelihood of big or small explosions in this scenario and the expected damage?

I guess the easiest answer would be to treat the cargo hold as volatile (Spaceships 1 p 62) but I'm not at all sure if that's appropriate.

[Anthony]

Radiation weapons are generally not producing neutrons or cosmic rays; bombardment with electrons or moderate energy nuclei isn't likely to trigger fission. On the other hand, if you're in a game with space pirates, realism may not be your highest priority.

[Varyon]

I could be mistaken, but I believe particle beams use charged particles (protons and electrons), not neutrons, and you need to hit U-235 with neutrons to get it to undergo fission "early" - basically, a critical reaction involves one atom of U-235 undergoing random radioactive decay, which releases 3 neutrons, one of those neutrons hits another atom of U-235 and causes it to decay, and this continues (supercritical is when, on average, more than one neutron from each reaction goes on to cause more reactions; this is where nuclear warheads come from). So, my layman's guess is that a particle beam hitting a container of U-235 wouldn't have any special effect... although the damage to the container is likely to result in a radiation hazard.

[David Johnston2]

Ehn. Your cargo isn't going to explode. Creating a nuclear explosion requires pretty particular conditions. But you could end up with a cargo hold that would very dangerous to go into.

[Fred Brackin]

As a technical possibility if you hit the uranium directly with antiprotons you might get fission with secondary neutrons to start a chain reaction. Possibly even in non-critical masses. Antimatter catalysis can apparently trigger fission in smaller masses than conventional fission bombs.

Spaceships keeps particle and antiparticle beams as separate items so if your space pirates really do have _particle_ beams nothing is going to happen.

[Varyon]

Of course, as an aside, because fission basically destroys some of your product, I think the U-235 would be shipped mixed with graphite or some other neutron absorber, so that you don't end up with an accelerated decay rate from having a subcritical mass of U-235. As a side effect, this may reduce the amount of radiation shielding your containers need (the neutron absorber serves as a sort of internal shielding). I may be mistaken, however.

[David Johnston2]

Originally Posted by Fred Brackin

As a technical possibility if you hit the uranium directly with antiprotons you might get fission with secondary neutrons to start a chain reaction.

To be clear, the chain reaction would cause the cargo to heat up and generate rads but not actually explode.

[Fred Brackin]

Originally Posted by David Johnston2

To be clear, the chain reaction would cause the cargo to heat up and generate rads but not actually explode.

If they've done something sane like reduce the 235 to 1 gram pellets and disperse them through some neutral material to separate them then probably not.

If they've done something notably less sane like keep the 235 in the form of 5 kg bars (still only 10% of critical mass) then there might be enough material in one place for an antimatter catylzed explosion.

[Anthony]

Originally Posted by Fred Brackin

If they've done something notably less sane like keep the 235 in the form of 5 kg bars (still only 10% of critical mass) then there might be enough material in one place for an antimatter catylzed explosion.

If you've got 10% of a critical mass it means each regular fission produces an average of 0.46 follow-up fissions (critical mass is when that number is 1; antimatter-triggered fissions are neutron-heavy so they'll produce slightly more in the first generation). This gives you a modestly bigger boom if the beam directly hits the fissionables, but it's mostly just going to hit packing. To actually get a large magnification from antimatter catalyzed fission, the initial explosion has to compress the fuel into a critical mass, which isn't likely for random weapon hits.

I would note that, depending on the quality of the packaging and the relevant freight handling skills, it's possible (if unlikely) that weapon hits could shift the cargo from a safe configuration to a critical mass. This is not particularly dependent on the type of weapon.

[Varyon]

Originally Posted by Anthony

I would note that, depending on the quality of the packaging and the relevant freight handling skills, it's possible (if unlikely) that weapon hits could shift the cargo from a safe configuration to a critical mass. This is not particularly dependent on the type of weapon.

"Kill confirmed, captain - the pirates are dust."

"Good, but we may not be out of trouble yet. Damage report."

"One of our tertiary batteries is offline, and the aft cargo bay took a nasty hit, but it looks like the emergency net deployed quickly enough we shouldn't have lost any cargo. Checking the math - yes, mass loss negligible, probably from some of our armor getting burned off. Aside from that it looks like... Captain! There's a radiation spike from the aft cargo bay!"

"Get me visual, main screen."

*The screen reveals the carefully-packaged crates torn open, and disks of uranium piled atop each other, held to the floor by the vessel's thrust. They and the surrounding area of the floor are glowing with heat.*

"Mother of God..."

*commercial break*