My understanding of the second stage of a thermonuclear bomb is as follows: X-rays from the first stage compress the "tamper", thereby igniting the fission sparkplug, and that the resulting combined pressure (from the tamper outside, and the sparplug inside), compress the fusion fuel so that temperature reaches the level at which fusion occurs. This then causes the fusion fuel to undergo fusion.

I'd like to check my understanding about this process:

  • Why do the X-rays exert enough pressure on the tamper to compress it enough to increase the sparkplug's pressure to cause fission, but not enough to break the radiation case (9)? Is the tamper really just being compressed by the X-rays itself (as wikipedia claims)? Intuitively it seems like X-rays alone wouldn't be enough to compress the tamper.

  • After the sparkplug ignites, the pressure on the fusion fuel from the inside should be much higher than from the outside, since the outside pressure only comes from X-rays that reflect from the radiation case, whereas the inside pressure is coming from a fission explosion. My understanding is that even though the tamper contains fission material, it doesn't actually undergo fission until after fusion occurs (also from wikipedia). Intuitively it seems therefore like this should push the fusion fuel out, even if no fusion were to occur. So I would guess that the fusion occurs during a very short interval before it had time to expand. Is this correct?

  • Why does the tamper contain fission material if it is not actually needed to create the pressure on the fusion fuel from the outside?

  • 1
    $\begingroup$ "Tamper", not "taper". As for the energy available in x-rays, your intuition is unlikely to be well calibrated for that regime, but it is, well, a whole lot of energy. And, yes, timing is everything... $\endgroup$
    – Jon Custer
    Commented Mar 23, 2023 at 12:50
  • $\begingroup$ Scott Manley has a Going Nuclear series of videos. See Going Nuclear - Nuclear Science - Part 5 - Hydrogen Bombs $\endgroup$
    – mmesser314
    Commented Mar 23, 2023 at 15:19

2 Answers 2


Firstly, its "tamper" not "taper". The tamper (very briefly) confines an atomic explosion in the same way the tamper in the shot hole when blasting rock in a quarry tamps the conventional explosion -- it holds back the expansion of the reaction products by virtue of its inertia. Secondly, while the pressure exerted by light at room temperature ($T_{\rm room}$ =300 K) is negligible, $P\propto T^4$ and $T_{\rm bomb}=300 \times 10^6$ K in the bomb. Now $(10^6)^4= 10^{24}$ and that gives a lot of $P$which starts by pushing the tamper inward compressing the fusion fuel.

One of the definitions of "tamp": Pack (a blast hole) full of clay or sand to concentrate the force of the explosion.

  • $\begingroup$ $300 \times 10^6$ K is $\approx 10 \times$ the temperature at which radiation pressure is enough to lift matter from the surface of a neutron star! $\endgroup$
    – John Doty
    Commented Mar 23, 2023 at 14:28
  • $\begingroup$ If the pressure on the tamper is so high, why don't the X-rays immediately break the radiation case apart immediately, thereby preventing any further reflection to the tamper? Or maybe this does happen, but the brief initial reflection is enough already to compress the tamper? $\endgroup$
    – user56834
    Commented Mar 23, 2023 at 14:57
  • $\begingroup$ Ok I had no idea that radiation pressure is proportional to T^4 that explains that part of it. I somehow assumed it was proportional to T, just like for gas pressure... What keyword should I google to figure out why $P \propto T^4$? $\endgroup$
    – user56834
    Commented Mar 23, 2023 at 14:58
  • $\begingroup$ Also, still curious why the tamper contains fission material $\endgroup$
    – user56834
    Commented Mar 23, 2023 at 15:03
  • $\begingroup$ @user56834 - Much of the energy of the explosion comes from neutrons from fusion igniting fission in the tamper. $\endgroup$
    – mmesser314
    Commented Mar 23, 2023 at 15:38

Is the tamper really just being compressed by the X-rays itself (as wikipedia claims)?

The article does not seem to make that claim. Looking at the table, it appears it is favoring the ablation of the tamper.

As this is also the mechanism used by ICF devices like NIF, which are largely systems to test these bombs, it may be the mechanism.

why don't the X-rays immediately break the radiation case apart

They do, for some definition of "immediately". It's just less immediate than the time it takes for the fusion mass to be compressed down.

The confusion likely occurs due to the diagrams in the wiki article, which show the tamper being much thicker than the reflective casing. As is the case for almost any bomb, including conventional, the opposite is true.

The outer casing is likely most of the mass of the bomb and getting all of that moving outward takes some time. And that time is much more than it takes for the shock wave created by the same reaction to compress the much lighter mass fusion fuel. The tamper may be relatively thin compared to the reflector and bomb case.

  • $\begingroup$ interesting. That would explain it. I guess it also doesn't matter if the reflective casing melts immediately? It maintains its reflectivity while that happens? $\endgroup$
    – user56834
    Commented Apr 18, 2023 at 20:29
  • $\begingroup$ Well again, its not "immediately". It's fast, but still slow compared to the time needed for the fusion to start. The x-rays would be ablating the tamper and bomb casing at roughly the same speed, but the resulting compression of the spark plug would, I presume, be much faster. $\endgroup$ Commented Apr 19, 2023 at 15:14

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