Small Atomic Bomb ?

Factual Questions
1

Hello All
About a year or so ago I read an article in a magazine about a backpack sized A-bomb.
I also remember hearing about an atomic hand grenade, is such a thing possible?
Is there a minimum size limit on a bomb type chain reaction?
If so what would it be?

Thanks
Peace
t lion
P.S. If this question has been raised before any links to the topic would be appreciated.

2

Let’s just stop and think about that one a moment, shall we?

(Hint: How far can you throw?)

DHR

3

There is a minimum size that physicists postulate is required to ensure that the neutrons released in a fission reaction strike other uranium nuclei and create a spontaneous chain reaction.

I seem to remember from physics class that this amount is somewhere around 20 pounds of uranium 235.

If this amount is correct, then the backpack is a possibility but I doubt that a 22 pound handgrenade would be feasible. And even then, such a small package would be extremely dangerous to handle with such little protection from radiation.

4

I don’t think U235 has been used to make nuclear warheads since the bad old days of Hiroshima and Little Boy. Most nukes today use plutonium. Does anybody know if Pu has a lower critical mass than U?

5

In the book The Curve of Binding Energy by John McPhee, the life and times of Ted Taylor are described. Taylor was a nuclear weapon engineer. The book is fascinating, and while I haven’t read it in at least 12 years I recall the topic of ‘How small could a nuke be?’ coming up. My recollection of the answer was “You could certainly get down to briefcase size, but with a little more savvy you could be talking about grapefruit size.” It may have been that the briefcase size was unclassified knowledge, but that Taylor could not admit to grapefruit size nukes due to his security clearance - or something. My memory fuzzes over…

Has anyone got a copy of this book to look up the relevant passage?

6

Plutonium, P239, has a critical mass of about 10 kg. If I remember its density correctly, a sphere of very slightly less than critical mass would be about 10cm in diameter. This must be surrounded by several kilograms of shaped high explosive, and connected to an electronics package that will control the detonation.

All things considered, the smallest fission bomb I can envision would be about the size of a basketball, and weigh 20-25 kg. I wouldn’t exactly call it a hand grenade, but your average soldier could easily lug it around on his back.

Laugh hard; it’s a long way to the bank.

7

I just realized that you can have a fission reaction with a lot less than 10kg of plutonium. “Critical mass” is a misnomer, since it’s not the mass which is important; the chain reaction depends more on the density of the fissionable substance.

When a plutonium nucleus is hit by a neutron, it fissions, and releases a couple of new neutrons. In order for a sustained chain reaction to exist, one of those neutrons (on average) must impact another nucleus and cause another fission. This is most dependent on the density of the substance and the size of the chunk, or “pit”, of fissionable material. (More density means more nuclei per unit volume, so a neutron has more targets to collide with. A bigger pit means a neutron has a longer distance to travel before escaping, so it has more chances to collide with a nucleus.)

Density can be greatly increased by implosion. Controlled detonation of the conventional explosives generates a shock wave through the pit; at its very center, where the shock waves all converge, the fissionable material can be compressed by about a factor of two. This would help a lot at the very beginning (the first few hundred milliseconds) of a fission explosion.

As for increasing the size of the pit, you can’t physically do that without adding mass (or decreasing density). But you could have a similar effect-- keeping more neutrons inside the pit for a longer time-- by encasing the pit in a shell of neutron-reflecting material such as depleted uranium (U238). Even better, you could make that case out of a material that produces neutrons on its own; then, even if the pit lost slightly too many of “its own” neutrons, they would be replaced by ones emitted by the case, and the chain reaction would be able to sustain itself.

I don’t have enough data to figure this stuff out exactly, but I think a grapefruit-sized bomb could possibly work.

8

Wouldn’t dropping a critical mass of Plutonium into cold water start a “dirty” chain reaction?

The water would reflect a whole lot of neutrons back into the mass, sustaining an uncontrolled fission, would it not?

It wouldn’t destroy a city, but it’d make living there a bad idea.

9

Considering that a standard Howitzer can fire a shell that has an atomic bomb in it…I believe that the diameter of a Howitzer is about 5-6 inches? I know that the famous cannon fired bomb film was not of a howitzer launch, was with a larger cannon, but I’m pretty sure that I’ve seen reference to atomic shells for howitzers.

>>Being Chaotic Evil means never having to say your sorry…unless the other guy is bigger than you.<<

—The dragon observes

10

You could skip the whole part about having an actual nuclear explosion and just wrap a conventional explosive in radioactive anything. While a nuclear explosion might be flashy it’s the fallout that’s the real problem. You mix one of the nastier cobalt or potassium isotopes with plastique and detonate at above an agricultural region and you can render the area unfit for use for years.

“All I say here is by way of discourse and nothing by the way of advice. I should not speak so boldly if it were my due to be believed.” ~ Montaigne

11

A 16" atomic shell was designed and built for the Iowa class battleships back in the late 40’s/early 50’s. Not quite something you can haul around and toss at your foe, but it was 16" long, 5’ tall, and weighed something like 1900lbs.

I also remember seeing the infamous “atomic cannon” footage. Again, though, I don’t know what size the shell was that they lobbed. In that same show, they talked about an atomic howitzer shell, and, as was said above, I’m sure a 6" to 8" shell must have been floating around at some point in time, something big enough for 1 or 2 people to stuff into said howitzer. Then again, what do I know.

Jeremy…

Anyone who says they don’t like children obviously isn’t cooking them properly.

12

<font face=“tempus sans itc” size=4>In theory , Californium can be used to create a bomb of roughly the same size as a rifle bullet, and with an explosive yield of about 10 tons. No, not kilotons; just tons. I cite the text “Future War”.</font>

Is an appreciation of beauty a function of the human soul?

13

AuraSeer wrote:

In Starship Troopers (the Heinlein novel, not that Melrose-Place-ish movie), Rico had a rocket launcher that fired a “pee wee” nuclear round – it was capable of producing an explosion on the order of a kiloton or two by “forcing a chain reaction with subcritical mass.”

Was this convergence of shockwaves at the center of the pit what Heinlein was talking about?

14

In the mid to late sixties, the U.S. Navy and Marines practiced deployment of The Special Atomic Demolition Munition (SADM), an atomic weapon small enough to be deployed by a parachutust or a swimmer. A RealVideo clip demonstrates how this weapon was deployed.

On May 25, 1953, as a part of Operation UPSHOT/KNOTHOLE at the Nevada test site, the GRABLE test utilized a Mark 9 fission device fired from 280mm gun, detonated in the air with a yield of 15 kilotons. A RealVideo clip of this atomic artillery test is included.

Not exactly nuclear satchel bombs, but it was as close as I could find.

TT

“Believe those who seek the truth.
Doubt those who find it.” --Andre Gide

15

You’d still wanna keep your distance when one of those suckers goes off :slight_smile:

Semirelatedly, here’s the info I can find on small (actual) nukes in the (past) US inventory:

W-51: No size available, but described as “Small and spherical”. Yield 22 tons. Produced in 1959. Unclear if this was ever in the active inventory, or was just an experimental device. However:

W-54: Updated version of the W-51. Size 10.75 inches by 15.7 inches. Weight 50 lbs. Yield 250 tons. Manufacturered 61-65, retired by 72.

W-48: Artillery shell, 6.1 inches by 33 inches. 118 lbs. Yield 72 tons. Produced between 63 and 68, retired in 1992.

–
peas on earth

16

Oh, here’s another one:

Mk-54 (different than the W-54): 10.75 inches by 17.6 inches. 50 lbs. Yield 10 or 20 tons (variable). Made 61-65, retuired in 71. For the “Davy Crockett M-388 recoiless rifle”, with a “compact spherical implosion plutonium warhead.”

That (10 tons) is the smallest yield I see as ever having been in the US inventory.

–
peas on earth

17

You can reduce the amount of mass required substantially with a lot of exotic tricks, including putting neutron-reflective casings around the bomb, and a tamper at the center (I think a common tamper is made of beryllium).

18

I would like to thank everybody for taking the time to reply to this topic.
I have read all the replys so far and they go a long way towards filling in the blank holes in my knowledge.
I will try to reply in more detail when time permits

Thanks Again
Peace
t lion

19

Unless I misunderstand “critical mass” (mine is a ‘common knowledge’ understanding), the mere fact of having assembled a critical mass (the actual amount of which depends upon configuration and composition, no?) has its own consequences. Which is to say, I doubt you’d really have time to “drop” it anywhere, and it would be plenty dirty (and noisy, and bright) regardless of environment.

Or is this just evidence that age has done my memory in?

20

Despite Daniel’s font size, he has a strong point. While U-235 and Pu-239 require fairly big things for critical mass, Americium and Californium’s critical mass is substantially smaller. I recall seeing an article several decades ago forecasting the possibility of (large caliber) bullets that would cause “pony nuke” explosions using Am or Cf as the actinide-of-choice.

A note on the hand grenade question: a grenade launcher could probably land a grenade-sized atomic “bomb” at a distance where the explosion would be “bad for them, good for us,” as opposed to “mess them up at the cost of our deaths.”