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Given the fact that thermonuclear devices are a mix of fission through enriched uranium whose energy initiates the fusion in surrounding deuterium. Question is (how come) fission raises temperature so high that, instead of expansion of gases (possible at few hundred degree Celsius ) fusion in (,ready to expand,) gases is obtained?

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Given the fact that thermonuclear devices are a mix of fission through enriched uranium whose energy initiates the fusion in surrounding deuterium

That's not how they are built - and as you surmised, it is unlikely this would work.

When you read the following, try to imagine the thought process that one needed to figure out that this could even work, let alone actually build one!

The key to the h-bomb is the arrangement of the secondary fuel. In early models, this was formed as a cylinder and placed very accurately within the bomb case. When the fisson bomb goes off the first thing that comes out is a burst of x-rays, which bounce around the inside of the bomb casing at the speed of light. The result is that the entire inside of the case begins to glow x-ray hot.

The fuel cylinder is normally coated with a layer of the same material as the bomb casing, or some other heavy metal (often depleted uranium). When this is heated by the x-rays in the case, it begins to explode outward (basic gas law here). Newton's Third Law now comes into play - heavy metal is exploding outward so light lithium deuteride fuel is crushed inward.

In the very center of the fuel is a thin rod of plutonium. As the secondary fuel mass is crushed, its density goes up. A lot. Like hundreds (millions?) of times. What was previously very much less than crititcal mass at normal density suddenly becomes very much fissile. A burst of neutrons is released. These travel into the surrounding lithium deuteride fuel. Neutrons+lithium = tritium. Tritium+Deuterium = fusion + another neutron. Rinse, repeat.

For all of this to work, a couple of things have to be true. Work out from the middle:

  1. the rate that fission reactions in the "sparkplug" in the center of the secondary has to be rapid enough...
  2. that they cause tritium production so that it's available and...
  3. the fusion reactions have to occur fast enough...
  4. that their neutrons complete the production cycle...
  5. rapidly enough that the entire mass has time to explode...
  6. after being compressed...
  7. by the x-rays after they fill the casing...
  8. in the time between the fission bomb going off and the entire bomb being blown apart by its explosion!

Basically, the entire heating, compression and fusion side has to occur much more rapidly than the fission side or the fission bomb would simply blow it apart. The compression is the key, if you don't compress the entire thing down, the density is too low so the travel times of all the bits kills you.

I mentioned that the outside of the fuel cylinder is often covered by depleted uranium, as opposed to lead or steel or something. There's a reason: remember that the fusion process is proceeding because the neutrons from the reactions is creating tritium from lithium and that allows for more fusion. So the explosion proceeds from the center of the fuel outward.

But what happens when it gets to the outer edge? Well, you still have all these fusion neutrons, but no more lithium. Now D-T neutrons are 14.1 MeV, much higher than needed to spark fission in U238. And what happens when neutrons hit uranium? Boom! So by using leftover waste uranium - you don't need to use the good stuff, with these neutrons even rocks will fizz - you get more boom for the buck using a material you need to put there anyway.

I also mentioned early bombs were cylindrical. With modern computers, we can simulate this stuff in 3D so now you're seeing shapes of the secondaries that are more egg-shaped to deal with the fact that one end is slightly further from the primary than the other and use that to conserve the x-rays better so you need a smaller primary and the entire thing fits into a smaller case. This is how you get 10 warheads onto an MX.

It really is quite fascinating.

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This is possible because the fission reactions occur so fast that there is essentially no time for expansion to occur on the same time scale.

In addition, the X-ray-driven implosion reaction that is initiated by the radiation output of the initial fission process also happens so quickly that it too proceeds to completion (triggering the spark plug and hence the fusion processes) before the bomb case has a chance to expand.

In the case of a fission bomb, the expansion of the bomb case and tamper cause the density of the exploding core to fall enough to snuff out the fission, but by that time, the bulk of the energy in the bomb has already been unleashed.

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