What is the smallest possible size to create nuclear fission and
produce an atomic bomb? Is it possible to make an explosion the size
of a house? That doesn't extend  beyond a tabletop? Microscopic?

And, secondarily, would the amount of radioactivity emitted by such a
small explosion be proportional?

Thanks!

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Clarification of Question by braitman-ga on 01 Jun 2005 15:59 PDT

I'm not trying to make a bomb! But I am trying to understand what the
practical limits are to size in creating nuclear fission. What is the
"smallest mass that will cause fission" and why is that so?

From what I'm hearing here is that it's not a matter of proportion;
there's a specific amount needed to make it work. So, there goes my
idea of developing fission on a microscopic scale! (or even in the
palm of my hand.)

I think you'd need to get a definition of a bomb and/or an explosion. 
Obviously you can get nuclear fission of a single atom but suppose
that would not count.
Assuming you wanted and got a supersonic shock wave I don't think
you'd confine the effects of that to the size of a house.

You need to get your fissile material well beyond critical (prompt
critical) and that requires a certain amount of it (which varies with
shape and what reflective wrapping it has).  Estimating this would
require details of material properties of Pu compressed by a
conventional explosion and of the rate at which your bomb comes apart
as it begins to work.

There has been work in recent decades in making them smaller; in part
because if you have the know-how to make a smaller bomb than your
enemy then they cannot use a bomb captured from you.  If you can
discover how to make a fusion weapon without starting it from a
fission explosion you might greatly reduce the mass needed (a fusion
chain reaction depends on density and temperature) and also remove
your reliance on scarce materials.

http://nuclearweaponarchive.org/Nwfaq/Nfaq0.html

>if you have the know-how to make a smaller bomb than your
enemy then they cannot use a bomb captured from you

Could you expand on that please  - I don't follow the logic.

Ian G.

It is very difficult to make a atomic bomb as you need highly enriched
materials that cost a lot of money to buy, If you do get some the
smallest mass that will cause fission will produce about the same
energy as 15,000-20,000 tons of TNT.
If you get some fuel rods from a nuclear power station they are no
good for making bombs,
If you want to make a bomb or to blow something up then just fill a
large plastic bag up with propane and oxygen mix,

If you're making bombs you want to make sure they aren't exploded on
the wrong occasions or by the wrong people.  Examples would include
your troops in a stressful situation deciding they could go ahead
without your order - or capture of a weapon in a base in cold-war
Europe or bad guys first to recover stuff fallen off an aircraft
carrier.

The gizmos for preventing unauthorized use are described:
main        http://www.cs.columbia.edu/~smb/nsam-160/pal.html
and also    http://www.cl.cam.ac.uk/~rja14/book.html

So if the enemy has captured a weapon and can't use the official means
to use it they've still got the chance of dismantling it and re-using
the material to make their own weapon.  Dismantling it without
damaging the SNM (special nuclear materials) is meant to be hard. 
Even if they get that far they might find they have too little SNM to
make the cruder kind of device they are capable of (perhaps all the
way back to 1940's design features).

That makes sense!  You've just got to make sure you don't lose 2 of them :-)

Ian G.

My old physics teacher said two 16lb hemispheres of uranium would do the trick

Add one drainpipe and dynamite at each end.

For a fission bomb you need at a critical mass of chain-reaction
capable material plus some device to keep it subcritical until a
detonation is desired.

The bare-sphere critical masses of some materials are:

plutonium-239: 10 kg
americium-242m: 9 kg
curium-247: 7kg
californium-251: 9kg

The smallest device (implosion type) to convert the critical mass from
a subcritical state to chain-reaction state is (with all overheads) of
the order of 20-40 kg. Thus the smallest fission bomb can be of the
order of 50 kg and the size of the order of 1 meter.

155mm weapons are documented; eg. here.
http://www.thebulletin.org/article.php?art_ofn=nd99norris_024

I don't know the exact size, but below a limit called the critical
mass the chain reaction will not have enough energy and the atomic
reaction will generate less energy than it takes to start it. I
believe these (atomic fission) bombs are called ?nucleonic? bombs
rather than nuclear. Incidentally I do roughly remember the weight -
1kg of nuclear material for a typical nucleonic bomb.

As for fusion bombs there is no limit at all really - except that you
have to have a detonation pulse of sufficient energy to start the
reaction. In theory you could have a nuclear bomb so small you could
detonate it in your hand and not be seriously hurt, you just need a
way to set it off.

There is a rumour about a tiny Russian neutron bomb called a Red
Mercury bomb that is only about the size of a tennis ball and has an
explosion radius of less than 20 feet. This used an ultra high energy
chemical/nuclear reaction to detonate the nuclear bomb - but it?s
almost certainly just a fantasy.

Thanks, folks, for all the good responses. I think I'm pretty fairly
satisfied with the discussion as an answer to my question. I suspect
there is ongoing research into "nanonuclear" fission, eh? Maybe still
just science fiction, but someday....?

You can infact do accelerator driven fission on the atomic scale. for
ease of the device you will need a magnetic pitfall for your atom
(Pu-239 or any fissiable one) and a neutron source. Have fun trying.

>Thus the smallest fission bomb can be of the
>order of 50 kg and the size of the order of 1 meter.

Some time back I was reading about Project Orion.

While the details are still classified they were of the opinion they
could build an atom bomb about 6" across with a sub-kiloton yield. 
This counts the explosives (and it's mostly explosives) but not the
electronics of the detonator.