Richard Rhodes ' book '' Dark Sun '' stated that a layer of plastic foam was fixed to the lead liner of the inside of the Ivy Mike steel casing using copper nails . Rhodes quotes several designers of that bomb explaining that the plastic foam layer inside the outer case is to delay ablation and thus recoil of the outer case : if the foam were not there , metal would ablate from the inside of the outer case with a large impulse , causing the casing to recoil outwards rapidly . The purpose of the casing is to contain the explosion for as long as possible , allowing as much X-ray ablation of the metallic surface of the secondary stage as possible , so it compresses the secondary efficiently , maximizing the fusion yield . Plastic foam has a low density , so causes a smaller impulse when it ablates than metal does
Ivy Mike
Ivy Mike
Thermonuclear weapon