<P> The secondary is usually shown as a column of fusion fuel and other components wrapped in many layers . Around the column is first a "pusher - tamper", a heavy layer of uranium - 238 (U-238) or lead that serves to help compress the fusion fuel (and, in the case of uranium, may eventually undergo fission itself). Inside this is the fusion fuel itself, usually a form of lithium deuteride, which is used because it is easier to weaponize than liquefied tritium / deuterium gas . This dry fuel, when bombarded by neutrons, produces tritium, a heavy isotope of hydrogen which can undergo nuclear fusion, along with the deuterium present in the mixture . (See the article on nuclear fusion for a more detailed technical discussion of fusion reactions .) Inside the layer of fuel is the "spark plug", a hollow column of fissile material (plutonium - 239 or uranium - 235) often boosted by deuterium gas . The spark plug, when compressed, can itself undergo nuclear fission (because of the shape, it is not a critical mass without compression). The tertiary, if one is present, would be set below the secondary and probably be made up of the same materials . </P> <P> Separating the secondary from the primary is the interstage . The fissioning primary produces four types of energy: 1) expanding hot gases from high explosive charges that implode the primary; 2) superheated plasma that was originally the bomb's fissile material and its tamper; 3) the electromagnetic radiation; and 4) the neutrons from the primary's nuclear detonation . The interstage is responsible for accurately modulating the transfer of energy from the primary to the secondary . It must direct the hot gases, plasma, electromagnetic radiation and neutrons toward the right place at the right time . Less than optimal interstage designs have resulted in the secondary failing to work entirely on multiple shots, known as a "fissile fizzle". The Koon shot of Operation Castle is a good example; a small flaw allowed the neutron flux from the primary to prematurely begin heating the secondary, weakening the compression enough to prevent any fusion . </P> <P> There is very little detailed information in the open literature about the mechanism of the interstage . One of the best sources is a simplified diagram of a British thermonuclear weapon similar to the American W80 warhead . It was released by Greenpeace in a report titled "Dual Use Nuclear Technology". The major components and their arrangement are in the diagram, though details are almost absent; what scattered details it does include likely have intentional omissions or inaccuracies . They are labeled "End - cap and Neutron Focus Lens" and "Reflector Wrap"; the former channels neutrons to the U-235 / Pu - 239 Spark Plug while the latter refers to an X-ray reflector; typically a cylinder made out of an X-ray opaque material such as uranium with the primary and secondary at either end . It does not reflect like a mirror; instead, it gets heated to a high temperature by the X-ray flux from the primary, then it emits more evenly spread X-rays that travel to the secondary, causing what is known as radiation implosion . In Ivy Mike, gold was used as a coating over the uranium to enhance the blackbody effect . Next comes the "Reflector / Neutron Gun Carriage". The reflector seals the gap between the Neutron Focus Lens (in the center) and the outer casing near the primary . It separates the primary from the secondary and performs the same function as the previous reflector . There are about six neutron guns (seen here from Sandia National Laboratories) each poking through the outer edge of the reflector with one end in each section; all are clamped to the carriage and arranged more or less evenly around the casing's circumference . The neutron guns are tilted so the neutron emitting end of each gun end is pointed towards the central axis of the bomb . Neutrons from each neutron gun pass through and are focused by the neutron focus lens towards the centre of primary in order to boost the initial fissioning of the plutonium . A "Polystyrene Polarizer / Plasma Source" is also shown (see below). </P> <P> The first U.S. government document to mention the interstage was only recently released to the public promoting the 2004 initiation of the Reliable Replacement Warhead Program . A graphic includes blurbs describing the potential advantage of a RRW on a part by part level, with the interstage blurb saying a new design would replace "toxic, brittle material" and "expensive' special' material...(which require) unique facilities". The "toxic, brittle material" is widely assumed to be beryllium which fits that description and would also moderate the neutron flux from the primary . Some material to absorb and re-radiate the X-rays in a particular manner may also be used . </P>

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