What are the steps of a nuclear explosion?

Answer 1

I will give the full sequence of steps for a hydrogen fusion bomb, as the steps for a fission bomb are the same as the fission primary stage of the hydrogen fusion bomb.

1. rapid assembly of a subcritical mass of fissile material to a supercritical mass of fissile material; this can be done using either gun or implosion methods.
2. a neutron source fires a burst of neutrons through the supercritical mass of fissile material initiating the fission neutron chain reaction.
3. (optional, only in boosted fission devices) the fission neutron chain reaction produces heat (millions of degrees) which ignites fusion in a deuterium tritium boosting gas mixture in a sealed capsule at the center of the supercritical mass of fissile material, producing a very large burst of neutrons through the supercritical mass of fissile material rapidly accelerating the fission neutron chain reaction.
4. the fission neutron chain reaction completes in a period of time on the order of 10 microseconds, at which point the fissile material has expanded enough to have again become subcritical.
5. the blast wave of the fission bomb explosion expands away from the bomb (this is the end of the fission bomb's nuclear explosion).
6. x-rays (emitted by the fission primary stage beginning after step 2 and ending at the end of step 4) travel through a gap between the outside surface of the fusion secondary stage and the inside surface of the casing (called the "hohlraum" meaning cavity).
7. the outer surface of the fusion secondary stage is ablated by the x-rays, causing a "jet reaction" driven implosion of the fusion secondary stage.
8. as the fusion secondary stage implodes it compresses a plutonium rod (or pipe) called the "sparkplug" running along the center axis of the fusion secondary stage, very rapidly assembling the plutonium "sparkplug" from a subcritical mass to a supercritical mass.
9. neutrons (emitted by the fission primary stage beginning after step 2 and ending at the end of step 4) travel up the center of the imploding fusion secondary stage and into the now supercritical plutonium "sparkplug" initiating the fission neutron chain reaction.
10. the fission chain reaction in the "sparkplug" produces neutrons that breed tritium from the lithium in the lithium deuteride dry fuel in the fusion secondary stage; making a deuterium tritium fuel mixture in situ.
11. the fission chain reaction in the "sparkplug" produces heat (millions of degrees) and pressure as it pushes out against the x-ray ablation driven implosion in the fusion secondary stage; initiating fusion of the deuterium tritium fuel mixture now in the fusion secondary stage.
12. (optional, only in "conventional" hydrogen bombs this is left out in so called "clean" hydrogen bombs to reduce fallout) uranium-238, which cannot support a fission neutron chain reaction in the tamper of the fusion secondary stage is fissioned by the very high energy fusion neutrons produced by the fusion secondary stage; this can multiply the yield of the bomb by up to a factor of 10, providing about 90% of the total yield of the bomb.
13. fusion (and uranium-238 fission if step 12 is used) ends when the fusion secondary stage has expanded enough that the temperature and pressure have declined enough that fusion cannot be sustained.
14. (optional, only in high yield multistaged hydrogen fusion bombs) as many additional fusion stages after the fusion secondary stage may be added as desired (repeating step 6 through step 13 in each successive stage) with each stage being driven by x-rays and neutrons generated by the preceding fusion stage.
15. the blast wave of the hydrogen fusion bomb explosion expands away from the bomb (this is the end of the hydrogen fusion bomb's nuclear explosion).