The critical device in this is the neutron source. These are the more highly classified parts of the bomb.
There have been a number of refinements on these weapons. The detonation involves a mix of HE explosives that surround the core. They are triggered by two charges at antipodal ends, where the explosive shock wave evolves through the HE in a way so that it is spherical by the time it reaches the core. The HE is then configured to have differential shock wave velocities by changing its formula.
To enhance the effectiveness of the Pu core the metal is now processed into a particular metallurgical phase. The crystal of atoms is arrayed so the implosion most effectively pushes the atoms together to enhance neutron interactions with the nucleus. This metallurgical phase is not stable and the Pu has to be reprocessed every 15 years or so. LC
On 10/19/2012 8:13 PM, Bob Zannelli wrote:Tayor’s point was that it’s not awfully difficult to make a primitive nuclear device and this is probably right. But I would like to expand the topic on the accidental detonation of a nuclear bomb. Whether or not it’s easy to set off a nuclear bomb by accident depends on the particular design.
There are two fundamental ways to trigger a nuclear explosion. The easiest and most primitive method is what is called the Gun trigger. Here you simply slam one piece of U235 into another so the combined mass is super critical. You have to do this very fast. So this is accomplished by shooting one piece of U235 at another with some type of low explosive. Usually Cordite or Ballastite. This design is horribly inefficient and an full nuclear yield in an accident is quite possible. The bomb dropped on Hiroshima was such a weapon , in order to make the take off of the plane safe the cordite charges were removed and replaced in flight. Both the Unites States and Russia have deployed gum type nuclear weapons. mostly for artillery shells. All US nuclear weapons must be one point safe , which means if they explode they aren’t suppose to produce any significant nuclear yield, I forget the number but its small. These inherently unsafe weapons were made one point safe by have removable obstructions between the projectile and target U 235 pieces. Later the US developed inefficient and low yield implosion nuclear artillery shells using what is called liner implosion. These ere safer. At the current time all these tactical weapons have been withdrawn from service.
The second method , the method used on all current nuclear weapon is implosion. The way this works is that a fissionable material either U 235 or PU 239 or more likely a composite structure of U235 and PU 239. In order for this to work the shock wave of the explosive has to be symmetrical to less than 5% a very challenging task. Early nuclear weapons used a 32 detonator system and what are called explosive lenses. An explosive lens is a shaped charge where two explosives are used, configured in such a way to turn a point detonation into a symmetrical shock front. The explosive is in two layers , a focusing layer and an strength equalizing layer.
Later nuclear weapons in the US were designed with 40, 60 and 92 detonators. This design is easy to make one point safe since all the detonators have to fire with 1 micro second of each to get the correct shock wave convergence. The first nuclear weapons compressed a solid sphere of plutonium or U 235. But is was it was quickly realized that it better to hammer a nail than push in into the wood. This resulted in the levitated core design where you have a space between an other layers of heavy metal ( usually U 235) and an inner solid core of PU 239.
Nuclear weapons need a pulse of neutrons after the core is assembled into a critical mass. Early nuclear weapons used what was called an urchin, a small sphere of polonium and beryllium separated by a partially serrated thin piece of light metal. When the core was compressed these two materials would be in contact causing the alpha rays of the polonium to produce neutrons when they were absorbed by the beryllium. However, this method of neutron generation was far from ideal. It was difficult to design it to produce neutrons at the exact right moment and the polonium has a very short half life and needed frequent replacement.
These urchins were replaced by external neutron pulse tubes which worked by accelerating deuterium ions into a tritium target to create very high energy neutrons which were fired into the core at its point of maximum density. These devices are called Zippers. Surrounding the core with a layer of beryllium enhancing the effectiveness of these Zippers as well as being an excellent neutron reflector for fission energy neutrons.
Another innovation , was the incorporation of a few grams of tritium or a mixture of tritium and deuterium in the space between the other shell and inner core. Since tritium has a short half life it is storied out side the core and is injected into the core during arming sequence. This is called boosting.
A major innovation in the science of mass murder was developed in the 1950’s. This was a new way to symmetrically implode a fission core using only two electronic detonators. This was called a Two point Air Lens Implosion. All US nuclear weapons in service today use this design.
The way this works is that the layer of slow explosive is replaced with an air gap. The implosion assemble including the strength equalizer retain a spherical shape. ( in most cases) But a somewhat foot ball shaped assemble is placed around this with electronic detonators at both ends. Inside this foot ball like assemble is hollow except for a layer of explosive coating on the metal on the inside. Surrounding the spherical assemble is a metal layer elevated away from the other layer of strength equalizer explosive.
When the two detonators are fired at the same time, the explosive shock wave travels around the thin layer of explosive at high speed and into the air gap at lower speed. The effect is to create a converging shock wave all along the elevated metal layer which smashes into the layer of equalizer explosive detonating it all around its surface, symmetrically compressing the fissionable core. At the point of max compression the Zipper fires a powerful pulse of neutrons into the compressed core and you get a nuclear detonation.
However , if you have a design that can imitate a nuclear explosion from only a two point detonation system you may have issue with one point safety. And in fact this is what happened. These new type of implosion
designs used on the early Polaris missile , the Mark 47 600 KT War head proved to fail the one point safety requirement by yielding a blast of 0.115 KT during a one point safety test. The fix to this was having a chain of neutron absorbing material stored between the other shell and solid core. This was removed by an electric motor ( it was wound up on a spool) and at the end of travel the empty volume was sealed to retain the tritium gas. But this fix proved to be a problem. Due to corrosion and a weak electric motor it was discovered that the Mark 47 war head wouldn’t arm after it was deployed for a short time. Modern nuclear weapons have made improvement in the core design so that they are now one point safe without any mechanical safing system.
Finally once you understand how these things work its obvious that getting a nuclear yield in an accident is pretty difficult. These devices are armed and fired by complex and high precision electronic firing systems. In addition US war heads have electronic coded locks called Permissive Action Links (PALS) which will disable the electronic components needed to detonate the war head if some one tried to arm the weapon with out the correct codes. The real danger of these war heads in an accident is the spreading of highly dangerous plutonium in the event the high explosive detonates. For this reason US nuclear war heads use an highly shock resistive explosive, PBX 9502 (Triaminotrinitrobenzene ) TATB.