Uranium bombs?

Factual Questions
1

Has any country ever built an enriched Uranium nuclear bomb? I know the first 2 atomic bombs the US built were Uranium, but all subsequent bombs (I assume) used plutonium. Has any country built enriched Ur bombs? Pakistan? India? N. Korea? USSR?

Not sure anyone can answer this in public, but is there ever a technical reason to use enriched Ur for a weapon?

The reason I ask is the current argument with Iran. They could be telling the truth and their enrichment program is completely for civilian purposes, but that doesn’t say anything about what will happen to the Pu produced by the those civilian reactors.

So in this case, the US has a goal of not controling the enrichment process, but controling the reprocessing of the nuclear fuel from the future Iranian reactors. If the fuel comes from overseas, then it can be reprocessed overseas. Hence the current stand-off. Iran is telling the truth, the US knows it doesn’t matter.

2

Yes - they are much easier to build and less likely to fail to go BOOM when you want them to. A perfect entry-level weapon - why do you think the first-ever bombs were of this design?

From Federation of American Scientists

South Africa built half-a-dozen. I believe the other declared powers went straight to plutonium. Israel and North Korea - don’t know.

3

I have no serious response to your question. I just wanted to let you know that I thought the title said “Uranium Boobs” and I immediately thought of Anna Nicole Smith.

4

Gun assembly devices, like artillery shells, require the use of U235 (HEU). Plutonium has a high background radiation rate that when combined with the slow assembly time of gun device, makes a “fizzle” (premature and weak detonation) likely. U235 can also be used in implosion devices. U235 is also much easier to work with. PU239 is weird and nasty. It has six solid phases and is very prone to corrosion and catching on fire. It also produces substantial decay heat.

5

I have always heard that the first bomb they dropped in Hiroshima was enriched uranium and the second on Nagasaki was plutonium.

6

Mmmm. I didn’t catch that in the OP, but I am pretty sure you are right. The first device detonated, the Trinity test, was a plutonium implosion device. The second, a gun-type Uranium device (Little Boy) was used over Hiroshima. The third, (Fat Man) was a Plutonium device.

I probably should’ve Googled this first. But I’m reasonably sure that if this is wrong, corrections will follow very shortly if I don’t have my facts straight. :wink:

7

Aside from the obvious one-off example of Little Boy as a gun-design, non-implosion enriched uranium device, this article by Jeremy Bernstein from this week’s NYRB suggests that the Chinese opted for an implosion design using enriched uranium:

As I’ve noted fairly recently on the Dope, the US also probably intended switching their enriched uranium supply to an analogous design immediately after Hiroshima for similar reasons, but the end of the war forestalled that plan and none of these were ever actually built.

8

The first atomic bomb ever exploded was a “pure” plutonium bomb. Trinity test, July 1945. An implosion device, in which a subcritical mass of Pu was compressed to a critical density. At its normal density, the Pu had insufficient density to start a chain reaction, but when compressed it reacted.

The first bomb dropped in war, at Hiroshima in August 1945, was a “pure” U235 bomb. A gun device, in which a sub-critical mass of U235 was fired at another subcritical mass to yield a critical combination.

The gun bomb wouldn’t work for Pu. The speed of the “bullet” had to be fast enough to cause an explosion before the energy melted it and the target; possible for U235 but not practical for Pu.

U235 was easy to explode but took forever to produce. Pu was hard to explode but easy to produce. But once they solved the implosion problem, they used it for all fission bombs.

Based on my reading of the literature, mixtures of Pu and U235 are used in fission devices. Mixtures of U235 and U238 are used in Navy ship reactors. The percentages in the mixtures are highly classified.

AFAIK, the US arsenal is composed solely of thermonuclear devices, which use a fission initiator to spark nuclear fusion reactions in lithium deuteride, which then fission plutonium fuel. Although the US has made these gadgets in a variety of yields over the years, the current “standard” seems to be a 550-kiloton warhead.

Russia, China, France, and UK all have thermonuclear weapons. India and Pakistan definitely have fission weapons. Israel and South Africa probably have fission weapons (add Tom Lehrer’s “Who’s Next?” song here, complete with lyrics “when Alabama gets the bomb! Who’s Next, Who’s Next, Who’se Next?”

I don’t know about India, but I’m almost certain that Pakistan’s bomb is enriched U235. That’s because it apparently produced its bomb material with ultrasophisticated centrifuges. You don’t need to do that with plutonium; you can separate plutonium chemically.

Again, it’s a tradeoff. Before the centrifuge idea was worked out, you needed Oak Ridge-sized separations systems costing billions of dollars, but the raw material was just uranium. You could steal the plutonium, or buy it, and then just machine it; making it required a large atomic reactor and sophisticated remote-control systems.

For the 3rd world country that wants to put itself on the map (insert plot of “What’s Up Tiger Lily” here) by joining the bomb club, it’s probably just as easy to buy the centrifuge technology on the black market (or is that a radioactive glow market?) and whip up their own U235 as it is to buy and move around Pu.

9

According to Wikipedia, India has thermonuclear technology as well. They definitely have a fission implosion device based on plutonium, see this article. The plutonium apparently came from a reactor system given to them by Canada!

India tested their fission device in 1974. They tested a fusion device in 1998.

For more information about the fusion device, see this article.

Pakistan’s arsenal is apparently based on enriched uranium from gas centrifuges, although they may also be producing plutonium. For more information, see this article.

All of these are Wikipedia articles; caveat emptor.

Wikipedia also says that the IAEA confirmed that South Africa had gun-type atomic weapons but dismantled them. One certainly hopes so!

10

Thanks
I knew Little Boy was a U235 device, and I assumed that the very first test was also U235. It appears that my assumption is incorrect. Thanks for the update.

11

Thanks
The way I was thinking about it is that it is expensive slow and vulnerable to “disruptions” to build and operate large enrichment plants. It seemed to me logical that any country would want to use the easier to obtain Pu for their devices-once they figured out the admittedly hard problem of building an implosion device. Pu is obtainable from any reactor. But to fuel the reactor you need Uranium. Either you use moderately enriched fuel like almost all power reactors in the world or you use U238 fuel in the CANDU reactors-but that requires a large supply of heavy water as moderator. Hard to get. The Pu is the same either way. But the key to your supply of Pu is to have control over the spent fuel from your peaceful power reactor. N. Korea wasn’t subtle about this step-they waited until enough Pu had formed in their donated reactor, kicked the IAEA out of the country and reprocessed the fuel. I was wondering whether any country after the US and USSR had bothered with the HEU step. From what I gather so far, S. Africa did so.

My conclusion is kind of nitpicky. I am wondering whether in practice both Iran and the US are correct in their statements. Iran very well may not have any kind of nuclear weapon program under way now. They don’t need one. What they need is the complete fuel cycle so that when all that work is done, they control the spent fuel. With that their options are open. Which is exactly what the US is saying. Kind of a nitpick, but an interesting situation where both sides are correct even though one side is saying exactly the opposite of the other.

12

I have to say, I do kind of wonder why the Iranians don’t just use the Canadian design of natural-uranium reactor, which would sidestep the need to build expensive controversial uranium-enrichment plants (suitable for U235 nukes) and expensive controversial reprocessing plants (suitable for Pu nukes). Unless, of course, nukes were a major consideration in running of the program, in which case it makes perfect sense.

13

to do that they would have to build an expensive and large plant to produce heavy water. I don’t see a net benefit for any nation in doing that, no matter whether that nation is interested in nuclear weapons or not. The CANDU design is an interesting design, but IIRC it’s primary advantage is that it is safer to operate. And when has that ever been a primary consideration in building large plants?

14

A heavy water plant is expensive, but so is a uranium enrichment plant. Purchasing heavy water is not a problem. Purchasing enriched uranium is a very big problem. CANDUs are proven and efficient. I think that Iran is trying to build a bomb.