Secondary Stage

With regard to advanced nuclear weapons, it is possible to use the output of a nuclear
device, a "primary" to operate a so-called "thermonuclear second stage", or "secondary".
From these earliest of photographs it can be observed that the earliest radiation implosion designs
had a singular large cylindrical chamber which covered the primary and cylindrical secondary.
The casing was a hemisphere in shape at one end, where the primary sphere was located.
The thermonuclear weapon was inside this bomb casing.

Originally the Americans and British employed casings of steel, which were lead-lined with or
utilized a lead bismuth alloy to form the radiation case. The secondary pusher, which
composed the inner wall of the radiation channel, was of natural uranium or lead (possibly
as a lead-bismuth alloy). All operationalized weapons probably used uranium tampers to
maximize yield, however some test devices had lead tampers to limit yield and the
production of fallout. A radiation shield was located between the primary and secondary in
order to prevent fuel preheat by the thermal radiation flux.

The secondary stage consisted of a pusher or tamper, a standoff gap, and a cylinder which
was filled with fusion fuel. Lithium deuteride, which was highly enriched in Li-6, was the
most preferred fuel for maximum yield. However it is documented that early deficits of
lithium enrichment capacity led to the deployment of bombs which contained partially
enriched or natural lithium. Along the center of the fusion fuel cylinder was a spark plug rod
of plutonium or HEU.

A thick casing is necessary for radiation containment as a result of the massive energy
release from the primary, which makes the whole bomb very heavy. Although the primaries
employed in early two-stage bombs were a great improvement in comparison with early
fission designs, initially they were still relatively enormous. The first thermonuclear devices
were high yield but also massive. High yield weapons with greater yield-to-weight ratios,
which provided higher yields still in deliverable packages, were desirable.

By means of a rough rule, it can be stated that the amounts of energy necessary to implode
a secondary is proportionate to its mass, because the primary energy/secondary mass ratio
defines the implosion which can be achieved. The yield of the secondary should also be
approximately proportionate to its mass. Thus there is an approximately proportionate
relationship between the primary and secondary yields, if one uses similar design principles.
If the desire is to have a very large yield, then obviously a very large primary is necessary.
Large fission primaries are potentially unsafe as a result of the large amount of fissile
material which is present and very massive. Even in very heavy weapons, the yield of the
primary is limited to no more than a few hundred kilotons, which limits total yield to a
maximum of tens of megatons.

It seems that the high yield designs developed in reality mostly during the 1950s and the
early years of the 1960s employed refined versions of the basic thermonuclear weapon
design approach, as has been described above, but multiple staging to achieve even higher
yields has been added.

The Soviet Union developed the pinnacle of such design achievment which was 100 megatons,
which they tested in a 50 megaton configuration. This was the so called "Tsar Bomba". A
bigger driving explosion was necessary in order to implode the main fusion stage. This led to
the design of three stage weapons in which a thermonuclear secondary was the principal
driving force to implode a third stage.

However three stage designs provide other advantages. As a result of giving the weapon
designer more freedom to design, it can be useful even if the bomb is not particularly large.
For example, in optimizing a weapon to a minimize weight for a given yield, a designer can
consider which type of driver for the third stage is most appealing - one large fission primary
or a compact two stage device. If access to weapon-grade fissile material is restricted, then it
is possible to choose a two-stage driver for the third stage simply to conserve fissile material.