# Question on supercritical mass of nuclear fuel with suboptimal geometry? **URL:** \ **Category:** Factual Questions\ **Created:** [September 11, 2002, 5:55pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994 "2002-09-11T17:55:24Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [September 11, 2002, 5:55pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/1 "2002-09-11T17:55:24Z")
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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 11, 2002, 8:56pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/2 "2002-09-11T20:56:05Z")
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? ---
**Author:** ![AndrewL](https://avatars.discourse-cdn.com/v4/letter/a/3d9bf3/32.png) [@AndrewL](https://boards.straightdope.com/u/AndrewL)\ **Post date:** [September 11, 2002, 9:32pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/3 "2002-09-11T21:32:49Z")
> [@](#): > > \*Originally posted by Chronos \* > \*\*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). \*\* 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. > [@](#): > > \*\*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. \*\* 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. > [@](#): > > \*\*By the way, a bit off topic, but where would this pseudoplant get the radioactive material? \*\* 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. ---
**Author:** ![Threadkiller](https://avatars.discourse-cdn.com/v4/letter/t/aca169/32.png) [@Threadkiller](https://boards.straightdope.com/u/Threadkiller)\ **Post date:** [September 11, 2002, 10:04pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/4 "2002-09-11T22:04:32Z")
See [this](http://www.military.com/Content/MoreContent1/?file=cw_nuclear_slotin) 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](http://newsdesk.inel.gov/press_releases/1999/Argonne_Nationa.htm). Mind you this is really low level stuff, but what is science fiction if not scaling up? ---
**Author:** ![femtosecond](https://avatars.discourse-cdn.com/v4/letter/f/f08c70/32.png) [@femtosecond](https://boards.straightdope.com/u/femtosecond)\ **Post date:** [September 12, 2002, 1:34pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/5 "2002-09-12T13:34:58Z")
A thread about Louis Slotin: [Did a professor die due to accidental fission?](http://boards.straightdope.com/sdmb/showthread.php?threadid=54076) > [@](#): > > _Originally posted by Chronos_ > **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 _addition_ to the Tokaimura incident? :eek: [http://www.aip.org/pt/dec99/toka2.htm](http://www.aip.org/pt/dec99/toka2.htm) > [@](#): > > Second, for convenience, they added the bucket contents to the precipitation tank rather than to the buffer tank. That was a key misstep, because the tall, narrow geometry of the buffer tank precludes criticality. Third, in filling the precipitation tank, the crew added seven buckets, or roughly seven times more uranium than permitted by the STA license. It was the seventh bucket that caused the mixture to go critical. > […] Judging from the levels of gamma and neutron radiation measured near the plant perimeter, the criticality excursion seems to have lasted about 20 hours; after that time, the radiation levels dropped below detection limits. 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. 🙂 ---
**Author:** ![femtosecond](https://avatars.discourse-cdn.com/v4/letter/f/f08c70/32.png) [@femtosecond](https://boards.straightdope.com/u/femtosecond)\ **Post date:** [September 12, 2002, 3:00pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/6 "2002-09-12T15:00:05Z")
Stupid hamsters forgot to update the forum list. Bump. ---
**Author:** ![femtosecond](https://avatars.discourse-cdn.com/v4/letter/f/f08c70/32.png) [@femtosecond](https://boards.straightdope.com/u/femtosecond)\ **Post date:** [September 12, 2002, 3:09pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/7 "2002-09-12T15:09:55Z")
I give up. :rolleyes: ---
**Author:** ![femtosecond](https://avatars.discourse-cdn.com/v4/letter/f/f08c70/32.png) [@femtosecond](https://boards.straightdope.com/u/femtosecond)\ **Post date:** [September 12, 2002, 3:22pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/8 "2002-09-12T15:22:12Z")
What if I’d take the “stupid” back? ---
**Author:** ![femtosecond](https://avatars.discourse-cdn.com/v4/letter/f/f08c70/32.png) [@femtosecond](https://boards.straightdope.com/u/femtosecond)\ **Post date:** [September 13, 2002, 10:17am UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/9 "2002-09-13T10:17:19Z")
Last try, when the board is faster. Do you know how difficult it was finding a goat to sacrifice? ---
**Author:** ![JohnM](https://avatars.discourse-cdn.com/v4/letter/j/848f3c/32.png) [@JohnM](https://boards.straightdope.com/u/JohnM)\ **Post date:** [September 13, 2002, 4:23pm UTC](https://boards.straightdope.com/t/question-on-supercritical-mass-of-nuclear-fuel-with-suboptimal-geometry/127994/10 "2002-09-13T16:23:29Z")
You might be interested in [evidence](http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/index.shtml) of naturally occuring critical reactions.