material itself. Natural uranium nails, lined to the top of their head with copper, attached the radiation case to the ballistic case. The nails were bolted in vertical arrays in a double-shear configuration to better distribute the shear loads. This method of attaching the radiation case to the ballistic case was first used successfully in the Ivy Mike device. The radiation case had a parabolic end, which housed the COBRA primary that was employed to create the conditions needed to start the fusion reaction, and its other end was a cylinder, as also seen in Bravo's declassified film. The space between the uranium fusion tamper, and the case formed a radiation channel to conduct X-rays from the primary to the secondary assembly; the interstage. It is one of the most closely guarded secrets of a multistage thermonuclear weapon. Implosion of the secondary assembly is indirectly driven, and the techniques used in the interstage to smooth the spatial profile (i.e. reduce coherence and nonuniformities) of the primary's irradiance are of utmost importance. This was done with the introduction of the channel filler—an optical element used as a refractive medium, also encountered as random-phase plate in the ICF laser assemblies. This medium was a polystyrene plastic foam filling, extruded or impregnated with a low-molecular-weight hydrocarbon (possibly methane gas), which turned to a low-Z plasma from the X-rays, and along with channeling radiation it modulated the ablation front on the high-Z surfaces; it "tamped" the sputtering effect that would otherwise "choke" radiation from compressing the secondary. The reemitted X-rays from the radiation case must be deposited uniformly on the outer walls of the secondary's tamper and ablate it externally, driving the thermonuclear fuel capsule (increasing the density and temperature of the fusion fuel) to the point needed to sustain a thermonuclear reaction. (see Nuclear weapon design). This point is above the threshold where the fusion fuel would turn opaque to its emitting radiation, as determined from its Rosseland opacity, meaning that the generated energy balances the energy lost to fuel's vicinity (as radiation, particle losses). After all, for any hydrogen weapon system to work, this energy equilibrium must be maintained through the compression equilibrium between the fusion tamper and the spark plug (see below), hence their name equilibrium supers. Since the ablative process takes place on both walls of the radiation channel, a numerical estimate made with ISRINEX (a thermonuclear explosion simulation program)