Hi /sci/ /pol/ here. What is the theoretical feasibility of

>>7640247
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.

>>7640275
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!

I wonder how small you could build a Farnsworth Fusor, but you're not gonna get any energy from that.

>>7640275
A note: Some fissile isotopes have much higher fission cross-sections, so you can make do with extremely small amounts of fuel.

http://thefutureofthings.com/3015-americium-power-source/


http://thefutureofthings.com/3015-americium-power-source/

>>7640308
thats not talking about a reactor. you still need over 10 kg for a critical system with Am

>>7640318
http://www.ans.org/pubs/journals/nse/a_2597

"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."

>>7640292
Tell that to this guy.
https://www.youtube.com/user/DCFusor/videos

>>7640275

>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?

>>7640288

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.

>>7640308

right; they can super-enrich the material as well; lowers the cost of entry, so to speak

>>7640308

hmm;

http://www.world-nuclear.org/info/non-power-nuclear-applications/radioisotopes/smoke-detectors-and-americium/

>>7640320

interesting; we're getting smaller and smaller. and yet, that article is from 2006!

you see where this is potentially heading, gentlemen?

>>7640414

http://www.cbsnews.com/news/exploding-head-syndrome-is-real-and-surprisingly-common/

https://www.youtube.com/watch?v=C0iBwm491zs

>>7640414
>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.

>>7640519

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

>>7640544
>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.

>>7640573

incorrect

see >>7640320

>>7640576
it hasnt changed, we just didnt know. there is a theoretical limit you mong, it doesnt just keep getting smaller and smaller

>>7640576
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.
https://en.wikipedia.org/wiki/Critical_mass#Critical_mass_of_a_bare_sphere

There's no trend of shrinking reactors.

>>7640584

with new discoveries comes new implementation

>it doesnt just keep getting smaller and smaller

and yet, it does

>>7640594
b8 or are you just fucking stupid?

>>7640576
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.)

>>7640320
>The radius of the reactor in this case is 9.6 cm.
diameter: 19.2 cm

>>7640593
>https://en.wikipedia.org/wiki/Critical_mass#Critical_mass_of_a_bare_sphere
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.

>>7640607
>fusion cross section
s/fusion/fission

>>7640593

>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.

>>7640600
see >>7640613

>>7640607

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

>>7640609

could fit in an iphone

>>7640613
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.

>>7640621
>could fit in an iphone
You have an iPhone that's bigger than a 19.2 cm sphere?

>>7640621
>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

>>7640320
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.

>>7640637

9.9cm

>>7640633

the metal depends on the heat output required

>>7640640

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?

https://en.wikiquote.org/wiki/Physics#/media/File:Optical-dispersion.png

>>7640647
there is a theoretical limit based on absorption cross sections and buckling. critical geometry doesnt change its a fucking theoretical limit you dense fuck.

>>7640647
>9.9cm
Do you know what a sphere is?

Can you guess why a 9.9 cm sphere can't fit in an iPhone?

>>7640662

since when?

>>7640672

look again

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

>>7640697
>since when?
since physics
>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

>>7640719
reported

>>7640702

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

>>7640697
>>Can you guess why a 9.9 cm sphere can't fit in an iPhone?
>look again
Is this trolling or some kind of tragic intellectual disability?

>>7640728

crybaby; report your own limitations then

or, report this;

https://www.youtube.com/watch?v=_gB2iWln0ls

https://www.youtube.com/watch?v=bWMw4vE3J8s

>>7640739

you're about 2 steps behind the conversation; keep up junior, or keep quiet

>>7640745
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!"

File: grigori-perelman-fields-medal-2006-and-millennium-prize-2010.jpg (26KB, 203x346px) Image search: [Google]

26KB, 203x346px

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?

>>7640748

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.

>>7640757
i hope you kill yourself

>>7640761

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

>>7640764
>I don't even study this shit, topkek
no shit but i do so jump off a fucking bridge you fucking faggot

>>7640764
>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?

>>7640780

by glimpses, darkly

>>7640781

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.
>2.6 years

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.

>>7640788
>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.

>>7640809

I did not have as much information as I do now. I am now aware that miniaturisation of the kind first mentioned is theoretically possible with sufficiently radioactive material

>>7640851

>this nigger doesn't know about https://en.wikipedia.org/wiki/List_of_radioactive_isotopes_by_half-life#106_seconds

>>7640851
>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

MODS
MODS
MODS

delete this dumb fucking thread and ban this stupid shitposting faggot NEET with nothing better to do

>>7640853
>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.

>>7640859
>https://en.wikipedia.org/wiki/List_of_radioactive_isotopes_by_half-life#106_seconds
All of the possible fissile isotopes in that list are in the critical mass table. There's nothing fissile that's lighter than uranium.

>>7640865
>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 [3], [8], or [9] is supported only by a dead link. Ref [10] 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.

>>7640909
There is nothing of value ITT.. You will learn nothing if you read anything but >>7640662

If your goal is to learn don't waste your fucking time on sci

>>7640922
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.

>>7640890
>https://en.wikipedia.org/wiki/List_of_radioactive_isotopes_by_half-life#106_seconds

>californium-253 17.81 days

253 > 238

you fail at basic math. which number is bigger? 'fuck outta here, back to preschool lil nigga

>>7640890

>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

>>7640909
>>7640922

>lack of any substantive counter-argument

>>7640985
>>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.

>>7640989
>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.

>>7640994

in any case cf-253 is heavier and easier to cascade; resistance is super low; like iron, somewhat (a star is exothermic until it; then, degenerate).

>>7640999

did the common american scientist know about the a-bomb before he read it in the papers, even tho it existed?

let those who have eyes see

/pol/ tier is a compiment