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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.
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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.
