We know that for a nuclear bomb to explode optimally, every thing has to be exactly right - the fuel has to go from subcritical to supercritical and be triggered to detonate with millisecond timing. If anything goes wrong, you get a fizzled explosion with much less yeild than optimal.
My question is, what happenes if you take subcritical masses of nuclear fuel and arrange them into a supercritical mass, with no particular care for timing or speed, and no external triggering neutron source? Say you’ve got a bunch of plutonium nuggets of about half critical mass each. Say you dig a hole in the ground and drop them in one at a time. Will the plotonium merely get hot, melt, or actually explode and spray radioactive material around?
This information is for a futuristic-themed RPG I run, where the heroes have discovered a non-carbon-based plant-like life-form which uses low-yeld nuclear explosions as a seed-dissemination method, and I’m trying to get the science reasonably correct.
First of all, you’re going to get a ton of radiation. Maybe this isn’t harmful to your pseudo-plant, but it almost certainly will be a concern for your characters (unless they’re also some weird non-carbon life form).
I think that what will happen is that you’ll get a small explosion of some sort, sufficient to spread out the reactant enough that it’s no longer critical. This is an extremely inefficient and “dirty” process. “Critical mass” isn’t just a matter of mass. It’s possible to have a subcritical amount of radioactive liquid in a container, which goes critical just by being poured into a different container of a different shape (I believe this actually happened at Oak Ridge during the Manhattan Project). In general, the closer to spherical you are, the lower the mass you need for “critical mass”.
By the way, a bit off topic, but where would this pseudoplant get the radioactive material?
Characters are mostly normal humans. The goal is for them to figure out what’s going on, and prevent the criticality from occuring in the first place.
That’s what the goal is - the seeds of the pseudoplant prefer to germinate in transuranics-enriched soils. I’m just trying to figure out how fast the explosion would occur, how much warning you’d have as the critical mass built up, and just how large the resulting blast radius would be. I realize it’s heavily dependant on the geometry of the material as well.
The mature plants are capable of forced nuclear fusion to generate heavier elements. Yeah, yeah, I know - it’s a piece of magic technology I’ve introduced into the campaign. The “plant” is actually a piece of nanotechnology left over from a long-vanished alien race.
See this site for info on a hazardous experiment gone horribly wrong. I do take exception about the “took the full brunt of a nuclear detonation right in his stomach” part of it.
Talking with the Nuke Ph.D. down the hall confirms that the thing wouldn’t “blow.” Certainly not a nuclear explosion (Hiroshima) or even a fizzle where stuff would be “spewed” out. His guess (my guess too, but he confirmed it) is that it would seriously overheat, melt and render itself into a subcritical configuration.
As for the reasoning of the OP: have the seeds ride the resulting thermal out of the short-lived criticality (makes a nice phoenix metaphor assuming the original “plant” died).
As for collecting the material. There are plants that are used to concentrate nuclear material. Mind you this is really low level stuff, but what is science fiction if not scaling up?
If pouring it by the bucket ‘just’ creates a long and nasty fizzle, I don’t think a plant’s metabolism just collecting it can do anything spectacular.
There is no clear threshold of going ‘boom’, it fissions before, just slower. You can’t accumulate a supercritical mass over time without it dissipating its energy before it can do something ‘big’ like exploding. The slower it comes together, the more time there is to spread the resulting heat over, so you need to do it quickly. Preferably by shooting many little pre-collected subcritical pieces together with explosives. Inside of a hollow seed pod would be perfect, and I always wondered what plants use the ammonium nitrate fertilizer for.
