[NonValueAdded]

Thanks for that answer. But couldn’t they test the explosive lenses all day long before a dress rehersal with fissile material?

[JasonC]

Until you have the radiation pressure of the critical-mass core pushing back, you don't face the actual conditions of the event.

The neutrons pass right through plasma, but heat the uranium tamper surrounding the whole shebang. The gamma rays strip off every electron from the incoming matter-wave until it is all pure ionized plasma. Then they hit the free electrons in that soup - all dumping their energy at short range.

The heating of the tamper outside, the momentum of the wavefront before the core gets going pushing back, and the short-range push-back of the gamma radiation, all matter.

It isn't a trivial calculation...

[JasonC]

If the previous still wasn't clear, remember this is about balancing forces to hold the core intact and concentrated as long as possible, for the maximum amount of fission to take place within it. Eventually the power of the fissioning core will overwhelm all other factors and blow everything apart violently. But when? Delaying the onset of that eventual winning-out, "just so", that is the trick of it...