Submitted by L0RD_E t3_11kx8rs in askscience
On the internet there is almost no information about these, everyone talks about the uranium based ones..
Edit: I said chemical reaction because, in my understanding (which is very limited and probably wrong), a chemical reaction has an input and an output (it's probably like Plutonium+neutron-> ??+energy+neutron(s))
Edit2: I will not build an atom bomb, I would've just looked up a tutorial on youtube if that was the problem.
Thanks. That's what I was looking for
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My understanding of the issue of a critical mass and nuclear fission with Plutonium is that you need Pu239. The nuclear reaction to create Pu239 also creates Pu240. Pu240, being only one mass number difference is very hard to separate from Pu239. A gun design is not fast enough to initiate a fission reaction with Pu, thus an implosion design is needed.
Basically, to generate the heat, pressure, and neutron flux to ignite a fusion reaction, takes a fission reaction first.
That's mostly correct. You can't breed Pu-239 without also breeding a little Pu-240, and you basically can't get it out of the Pu-239. Pu-240 has a very high spontaneous fission rate (atoms occasionally just fall apart, often releasing a neutron or two). At the levels present in a few kilograms of Pu, there's a random neutron every microsecond or so.
For a gun-type design, it spends a fraction of a millisecond in a configuration where a chain reaction is possible but generates a dud. And the Pu-240 spontaneous fission makes it very likely that such a dud chain reaction will happen.
Isn't it the main purpose of explosive lens? To compress separate sub critical masses into critical mass?
the explosive lens is a "lens" in the sense that it can focus shock waves. To compress a sphere into a tiny ball, you have to have continuous equal pressure over the whole surface or else it will deform and splash into a complicated shape.
You cover the plutonium sphere with an shell of explosives (or a shell of some dense metal that is then covered with a shell of explosives.) If you start to detonate the explosive shell using a detonator at one location, the shock wave will hit the plutonium directly under that location first, starting a dimple. The plutonium on the other side isn't being compressed because the explosive there hasn't started to explode because the shock wave hasn't arrived yet.
So you use a bunch of detonators all over the surface. That's better, but it still creates an uneven pattern of pressure -- now maybe you get 20 or 60 dimples forming symmetrically but it's still not going to result in a compressed ball. There will always be locations on the surface of the plutonium where the shock wave is pressing the material sideways instead of inward.
The explosive lens uses two materials with two different speeds at which the shock wave can travel. The shock wave directly under the detonator is going through the slow stuff but the shock wave spreading sideways from the detonator is going through the fast stuff. If you shape these in the right pattern, using curved interfaces, the effect is just like light passing through a curved lens. The shock waves are bent into a pattern that is almost equal pressure everywhere at the surface of the plutonium.
The general concept is the same. You make a sphere, typically hollow, surrounded by chemical explosives designed to compress the sphere to become critical and add a neutron source that triggers at the time of maximal criticality.
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drhunny t1_jba3f78 wrote
There is almost no difference between how a Pu vs U fission bomb works. There are several minor differences in their chemical and nuclear properties that affect the engineering details.
The explosion in both cases is not chemical but nuclear. A lot of regular chemical explosives are used to suddenly change the shape of the plutonium or uranium mass in such a way that it goes from being a shape that absolutely can't generate a nuclear explosion to a shape that easily can. And then a tiny particle accelerator (basically similar to a Tesla coil but about the size of your finger) turns on and sprays the new shape with a bunch of neutrons which kick-start the nuclear reaction. The timing is important because it's in the ideal shape for less than a thousandth of a second.
The particle accelerator acts just like a spark plug in a car's engine. A car engine that uses gasoline (petrol) has a mix of gasoline and air inside a cylinder. That mix changes shape (the cylinder compresses it) and when it's at just the right shape, the spark plug ignites the mix.
But think about a diesel engine. Those don't have spark plugs. The mix is compressed down so much that it ignites itself. That can also happen in a gasoline engine if the fuel is bad or the design is wrong -- the mix can ignite before the spark plug fires. Similarly, the only key difference between Pu and U bombs is that Pu has a high possibility of accidentally starting the nuclear explosion a bit early, before it's at the ideal shape (and without needing the particle accelerator).
As a result, you can't use the simplest engineering design with plutonium. You can make a uranium bomb where the starting shape is a rod and a hollow cylinder and use an explosive to shoot the rod into the cylinder. Then the particle accelerator is turned on right when the rod is perfectly lined up with the cylinder. But if you try this with plutonium, there's a really high chance that before the rod gets lined up, the nuclear reactions start in the plutonium. The nuclear reactions are so fast that basically the plutonium rod melts and expands into a blob and hits the cylinder instead of sliding right into the center. It's still a nuclear explosion, but it's a dud because instead of getting the equivalent of thousands of tons of TNT explosive power, it only generates maybe the same as a few tons of TNT before it shatters and stops the nuclear reaction. So for plutonium you have to use a much harder to engineer design where it starts as a large hollow sphere and the explosives compress it into a small solid ball.