What is the theoretical feasibility of micro-nuclear reactors? the same principle (pauli's exclusion) applies regardless the amount of U or Pu, eh? even 2 atoms with sufficient energy and angle combined together could trigger a cascading reaction, but they need not be that small. say, the size of the chip on your debit card? or the one in your phone? that size? scientifically possible?
For practical purposes, truly "micro"-nuclear reactors are impossible.
In order for a nuclear fission reactor to work, you have to be able to sustain a chain reaction. That means that every fission event needs to trigger, on average, at least one other fission event. Because the ability of materials to absorb neutrons is limited - neutron radiation is fairly penetrating, especially at the high energies emitted by fission events - this means you need a big enough chunk of uranium or plutonium that the emitted neutrons from one fission event will have a good enough chance of hitting and splitting another atom.
This creates a fundamental minimum limit on the amount of fissile material needed, known as the "critical mass." ("criticality" being the critical point where the nuclear chain reaction is self-sustaining, each fission event causing exactly one other on average.)
This can be improved with components like moderators and neutron reflectors, but ultimately you need a fairly large nuclear system to sustain a workable reactor.
Also, there's the problem of how to extract power - converting nuclear energy directly into electrical is nearly impossible, and so all existing reactors use thermal systems. These are also bulky.
However, if you aren't insisting on it being a *reactor*, you could use other nuclear energy processes like alphavoltaics or betavoltaics, harnessing the high-energy charged particles caused by nuclear alpha or beta decay to generate current directly. They're very inefficient, however.
The smallest reactors I've ever seen actually designed were for potential spaceflight applications; the smallest was the HOMER-15, a reactor producing 15 kW of thermal output (and 3 kW of electrical) that weighed just 214 kg and was 41 cm in diameter and 2.4 m tall.
A reactor designed for Earth usage could be made smaller, since you can use convection and conduction of heat to cool the reactor more efficiently, instead of having to radiate everything. However, you're not getting it too much smaller than that. Certainly not to credit-card size!
A note: Some fissile isotopes have much higher fission cross-sections, so you can make do with extremely small amounts of fuel.
"There is a growing need for very small nuclear reactors for space applications and as portable high-intensity neutron sources. This technical note investigates the question of what is the smallest possible thermal reactor. It was found that the smallest reactor is a spherically shaped solution of 242mAm(NO3)3 in water. The weight of such a reactor is 4.95 kg with 0.7 kg of 242mAm nuclear fuel. The radius of the reactor in this case is 9.6 cm."
>this means you need a big enough chunk of uranium or plutonium that the emitted neutrons from one fission event will have a good enough chance of hitting and splitting another atom.
right, but one can "increase one's chances" with precision; ie. finely and efficiently direct the energy transfer
>Also, there's the problem of how to extract power
the consideration before us, if I wasn't clear enough before, is weaponisation. the question isn't to harness the energy for anything other than human destruction
>This creates a fundamental minimum limit on the amount of fissile material needed
yes, there should be some theoretical minimum; what is it?
well, let's be realistic here; Homer-15 is ~14 year old technology. Contrast 2001 with today regarding computerisatin, and Homer-15 was the most advance unclassified project. do the math.
right; they can super-enrich the material as well; lowers the cost of entry, so to speak
interesting; we're getting smaller and smaller. and yet, that article is from 2006!
you see where this is potentially heading, gentlemen?
>you see where this is potentially heading, gentlemen?
Not where you think. The minimum fission reactor size (unless you want to go to elements with a high rate of spontaneous fission, at which point you're looking more at a radioactive sample than a controllable reactor -- there's no size limit on spontaneously radioactive samples) is about the size of a hardball.
See, a fission neutron has a fair amount of energy, and that makes it likely to fly by any given nucleus, without being drawn in or bouncing off. This is described as a small "cross section" for fission or moderation, since for the purposes of modeling, the neutron is treated as a point moving through space that might collide with various objects in its path. If the average neutron "sees" more than ~55% (depending on the number of neutrons produced per fusion) ways out of the reactor (and bear in mind that the ones released on the edges necessarily see a full hemisphere of total emptiness) a chain reaction can't be sustained.
And that's just to make the neutronics work. There's still no power conversion, control system, or shielding. If you want those things (especially shielding), you're looking at a much bigger device.
fact of the matter is sizes have been shrinking and shrinking while power has been increasing; it would be a fool to expect anything other than that trend to continue in the nuclear realm as well, just as it has in basic computerisation
>fact of the matter is sizes have been shrinking and shrinking while power has been increasing
Not true. The understanding of the minimum size of fission reactors hasn't changed since the 1960s.
>it would be a fool to expect anything other than that trend to continue in the nuclear realm as well, just as it has in basic computerisation
Nuclear reactors have NOT been miniaturizing. There's no trend to continue.
No one actually built that, it was just a theoretical exercise, and that's a larger volume than some bare critical spheres that have been known about for decades.
There's no trend of shrinking reactors.
As the guy who posted it, you're totally missing the point.
When that said "minimum possible size" they meant it. It's only possible to get that small at all because Am-242m1 has a ridiculously high fusion cross-section.
Also, while I can't find a free copy of that paper, judging by the given radius and mass that paper is literally *only* talking about the spherical blob of aqueous fuel solution itself. It's not including any form of power conversion device, shielding, cooling, or even the container to hold the solution in. (The abstract implies as such, and the given mass and radius suggest an overall density only barely above that of water.)
>The radius of the reactor in this case is 9.6 cm.
diameter: 19.2 cm
uranium-235 diameter: 17 cm
plutonium-239 diameter: 9.9 cm
The 19.2 cm exotic-fuel reactor is theoretical. These much smaller bare critical spheres of common nuclear fuels, or masses very similar to them, have been assembled in various nuclear weapon research and development programs.
>No one actually built that
no one you know of, anyway
nuclear weaponry was supposedly theoretical as long as nobody knew about it; you would have said in July '45 it's still theoretical, since the research was classified, but by then the weapons did exist.
point is the known minimum was larger until recently, several times over, as one would expect, and is easily observed in other related disciplines (eg. computing).
>It's not including any form of power conversion device, shielding, cooling, or even the container to hold the solution in.
already addressed; read the thread
could fit in an iphone
If anyone with the power to make it happen needed a tiny reactor that badly, it would be far easier to make a fast reactor, which could be more compact and would be made of much cheaper and more common materials.
Am-242m is basically unobtanium. Very difficult to produce.
>point is the known minimum was larger until recently, several times over, as one would expect, and is easily observed in other related disciplines (eg. computing).
no it fucking wasnt you stupid fuck. seriously gtfo you popsci faggot go read a fucking book
A note: Americium-241, the most prevalent isotope and the one found in smoke detectors, costs $1500/gram.
To convert it to Am-242m, you have to further irradiate it with neutrons in a nuclear reactor; only 10% will be transmuted to Am-242m, and most of the rest into ordinary Am-242.
And given that they have exactly the same mass, separating the 242 from the 242m is a bitch.
But even if you could separate them perfectly and there was no cost for dumping it back into a reactor and reprocessing, that means the fuel would cost - at minimum - $15,000/gram.
So the fuel alone for such a reactor would cost $10 million, best-case.
And for what? All you're getting out of the reactor is just a few kilowatts.
the metal depends on the heat output required
yah it was; advances in research lead to advances in actual physical technology
that's the way it's been from the beginning of recorded time. you think x is the best way until y comes along and it's better. you didn't know until you knew
this is basic fucking common sense; if you can't into that how the fuck you expect to do anything worthwhile?
>lowest is 6.9cm, and again, if they further enrich they will have a more reactionary product which, everything else being equal, results in smaller required sizes
>if they further enrich it
holy fuck get out those values are for 100% of each material at 100% theoretical density
best you got up there is a half-life of 2.6 years - YEARS
you get the fuck out. you don't think they can enrich to 70 days? how about 40 days? 20 days? free smart phones for everybody and free service! new obamaphone plan; it's several orders lower in time; imagine how that proportionately scales down in size
how about 2 days? really efficient shipping, amazon prime like speeds, doncha know
So we're all agreed that the "it can fit in an iPhone!" guy is a complete idiot who has, in the end, gone over the top ridiculous hoping to play it off as, "I was trolling all along!"
or maybe they calculated an asteroid into impact pacific and atlantic oceans
bye bye london, nyc.
think this guy could do it? how would you know either way?
no, you fail at reading comprehension
I never claimed to be trolling; you're either blind or dishonest. waste of time either way except as a foil
I've already explained none of the current (unclassified) research touches on any radioactive material more energetic than 2.6 YEAR half life. lulzok.wtf
just admit there's more behind the scenes than you seem to see.
I don't even study this shit, topkek
my 1st cousin teaches at MIT doe
you're probably a public school brat
40 years ago you would be manning some machine or cleaning something son
fuck back off to whatever video game you were playing before you decided to log on to 4chain and pretend to be smart
>I don't even study this shit, topkek
Well exactly. You don't study it, you're not familiar with basic considerations, you don't know what half-life means or how it matters, you think a grapefruit fits in an iPhone.
>my 1st cousin teaches at MIT doe
Does he come visit you in the care home for the developmentally challenged?
I have studied and more importantly comprehended enough to understand basic considerations; more basic than you can see
>you think a grapefruit fits in an iPhone.
like, what don't you understand about 2.6 years? are you unaware that we have nuclear material with much, much shorter half-lives; in the hours, even. and concomitantly these materials require much less total size to sustain the critical reaction
basic fucking physics
>hey sci why come you can't have really small nuke-yuh-lar reactors?
>nuh uh! my phone is smaller than it used to be. that means things have to miniaturize
Holy shit. Why did you even post the question? You obviously had your mind made up before you even started the damn thread.
>are you unaware that we have nuclear material with much, much shorter half-lives; in the hours, even. and concomitantly these materials require much less total size to sustain the critical reaction
Except that's something you just made up on the spot, guessing because of a weak correspondence in the critical mass table I showed you.
There's no strong relationship between fission cross-section and half-life. The critical masses of Pu-239 and Pu-238 are roughly equivalent, despite Pu-239 having a half-life 300 times longer. Pu-241's half-life is less than a thousandth of Pu-239's, but Pu-241 has a larger critical mass.
The source for the Cf-252 figure explains that it's a rough estimate, and goes on to explain:
"The estimation model used tends to underestimate critical mass sizes" It also gives an estimate for Cf-251 of 1.94 kg, which is a considerable underestimate relative to other sources (the wikipedia page shows 5 kg).
It's unlikely that Cf-252 actually has a critical mass less than Cf-251. None of the Californium figures are trustworthy because nobody has had Californium samples anywhere near those sizes.
And there'd be no point in building a nuclear reactor with a material that has a half-life in hours. Half of the fuel would decay every few hours anyway. Producing more than a few atoms of an exotic isotope isn't a fast process. It would be basically impossible to assemble such a reactor.
>There's no strong relationship between fission cross-section and half-life.
can't be fucking serious;
somebody graph these values https://en.wikipedia.org/wiki/Critical_mass#Critical_mass_of_a_bare_sphere for this mathematically illiterate nigger
>I am now aware that miniaturisation of the kind first mentioned is theoretically possible with sufficiently radioactive material
Well no. You are now imagining that it's possible, based on unreliable information and very little understanding.
All of the possible fissile isotopes in that list are in the critical mass table. There's nothing fissile that's lighter than uranium.
>somebody graph this for me, I don't know how, help
...and I've already explained why the lowest values are untrustworthy: nobody has samples of those materials near the critical mass to work with.
Also: anything with ref , , or  is supported only by a dead link. Ref  uses a critical mass estimate method known to underestimate critical mass.
I can't tell the difference between intentional shit posting and honest-to-goodness stupidity. Thank god for you fucks who are willing to sit down and explain this shit. At least I get to learn something in the process.
Every once in a while, some interesting discussions actually pops up, like that group that posted designs for the hyperloop competition. They just get buried in the endless NJ Wildburger, "engineers suck cock", and "what is the evolutionary purpose of x" threads.
You're right. I should be learning something.
>californium-253 17.81 days
253 > 238
you fail at basic math. which number is bigger? 'fuck outta here, back to preschool lil nigga
>nobody has samples of those materials near the critical mass to work with.
of course they have it; it's the government. they have what they will.
the links are peripheral. if you really want to check out the theory/math it's available online too, if you know where to look
>lack of any substantive counter-argument
>>There's nothing fissile that's lighter than uranium.
>californium-253 17.81 days
>253 > 238
Yes? That's not lighter than uranium. Anyway, the lightest fissile isotope of uranium is 233, 238 is not fissile.
>of course they have it; it's the government. they have what they will.
Sure. They've got cures for aging and every kind of cancer, too. Anything they want, they just have it.
Truly, this is /pol/-tier.