Thermonuclear Weapon Design

Fusion Boosting Fission Devices

Boosting is a technology which scientists use in order to increase the efficiency of fission
bombs by means of the introduction of a small amount of deuterium and tritium (typically
this contains 2-3 g of tritium) inside the core. At the same time as the fission chain reaction
proceeds, there is a point when the core temperature rises sufficiently for the fusion
reaction to begin to occur with a significant rate. This thermonuclear reaction inputs
additional neutrons into the core, thus the neutron population to increases faster than from
fission only.

The fusion neutrons are highly energetic - nearly an order of magnitude more energetic
than an average fission neutron. This causes them to increase the overall fission reaction
rate. This occurs as a result of several reasons. Fusion-born neutrons have a high velocity.
When such energetic neutrons strike a fissile nucleus, a much larger number of secondary
neutrons becomes released. In addition the fission cross-section is larger.

The fusion reaction rate is proportionate to the square of the density at a given
temperature. Thus it is important for maximizing the fusion fuel density. The higher the
density which becomes achieved, the lower the temperature which is necessary to initiate
boosting.

It is possible to achieve high fusion fuel densities by means of:

  • Using fuel which has a high initial density (for example highly compressed gas)
  • High compression during implosion
  • A combination of these factors above

Although liquid D-T has been used in the past, this is not a practical approach as a result of
the difficulty of achieving and maintaining cryogenic temperatures.

It is known that American nuclear weapons include tritium as a high pressure gas which is
kept in a reservoir outside the core (probably deuterium/tritium). The gas is forced into the
weapon core a short time before detonation as a part of the arming sequence. Initial
densities with a room-temperature gas (even one with a very high pressure) are much lower
than liquid density. The external gas reservoir has one important advantage, however - it
permits the use of a "sealed pit", a sealed plutonium core which does not need servicing. It is
easy to remove the tritium reservoir for replenishment (by means of removing the He-3
decay product and by means of adding tritium) without accessing the inside of the device.

Lithium hydrides have a density of hydrogen which is about 50% greater than the liquid
state, and because the hydride is considered a relatively stable inert solid it improves
handling. A large disadvantage is that it is necessary to permanently incorporate the hydride
into the core - complete core removal and disassembly are necessary to replenish the
tritium.

One possible location for the boosting gas is the cavity in the center of the weapon as the
core temperature is highest there and because this would maximize the probability of
neutron capture. In a levitated core design, this would make the levitated core into a hollow
sphere. This is undesirable from the point of view of efficient fissile material compression,
however, because a rarefaction wave would generate as soon as the shock reached the
cavity wall.

Siting of the fusion material between the outer shell and the core is one alternative path,
causing efficient compression of the gas to a thin very high density layer. There is evidence
that American boosted primaries contain the boosting gas in the external shell rather than
an inner levitated shell.

In real terms boosting is experienced when the fusion materials are sufficiently hot to
produce neutrons with a rate which is significant in comparison with the neutron production
rate which occurs only through fission. The D-T fusion rate increases very rapidly with
temperature in the temperature range where boosting occurs. Thus the boosting effect
rapidly strengthens at the same time as the core temperature increases.