Implosion

Theory

For a gun device, it is possible to assemble a supercritical mass as a result of two or more
subcritical masses at nominal density. However, the criticality does not only depend on the
mass and geometry, but also on the density of the material - this is the characteristic which
scientists exploit for an implosion device. The "core" of an implosion device is subcritical.
This is then compressed by use of high explosives. For a sufficient increase in the density of
the fissile material, it is necessary that the implosion is maximally smooth and symmetrical.
For this, simultaneous initiation of the high explosive on the outer surface is necessary. The
resulting convergent shock wave compresses the fissile core, and as a result there is supercritical
assembly.

The convergent shock wave of an implosion can compress solid uranium or plutonium with a
result of a factor of 2 to 3. The compression occurs very rapidly and the window for neutron
initiation lasts only a few microseconds. For maximum performance, the time of the neutron
initiator must be under a microsecond. For internal neutron initiators, the initiation time is
close, or even simultaneous with the maximum compression. For further information, see
the section "neutron initiator".

A solid core design was employed for the "Gadget" device, which was the first nuclear
explosive which was tested at the "Trinity" test, and "Fat Man", the atomic bomb, which was
dropped on Nagasaki.

The implosion not only generates the supercriticality, but the extreme compression also
reduces the neutron mean free path (inversely proportional to density). This process reduces
the time between successive neutron generations and causes a faster reaction, which
progresses further in the time before the device disassembly. An implosion device employs
not only less fissile material than a gun assembly device (in absolute respects), but it also
more efficiently uses the material.

The main advantages of implosion are:

  • high assembly speed - this permits the use of materials with high spontaneous fission
    rates (i.e. plutonium)
  • high density becomes achieved. This leads to a very efficient burn-up of fissile
    material
  • Low (less than a critical mass of) quantities of fissile material are employed for each
    device
  • There is potential for small and light weight designs

Disadvantages:

  • Spherical instead of cylindrical shape (in comparison with gun assembly device) can
    provide integration problems (this depends on the system)
  • Greater complexity - more components
  • Increased demand for exactness of detonation and implosion
  • More demand for exactness of manufactured components

The main components of an implosion device are: the shockwave generator; the high
explosive system which generates an initial shock wave of appropriate shape and the
implosion hardware: the materials which the shock wave compresses. This consists of the
fissile materials and additional inert materials which may be included.

For the "Fat Man bomb" the high explosive system was unconfined. However it is possible to
obtain greater explosive efficiency as a result of placing a tamper around the explosive. The
explosive gases become trapped which pushes in as the gases expand.

It is possible for a high performance explosive to generate shock wave pressures of hundreds
of kilobars. By means of implosion convergence it is possible to raise this to several
megabars. This pressure can double the density of the confined fissile core, or possibly even
greater.

Fissile cores have different configurations: the most usual are solid cores or hollow shells.

Solid Core Designs

The first Soviet and American implosion designs employed a solid core configuration. The
imploding convergent shockwave, which the symmetrical detonation of the high explosives
generated, is transmitted into the core, which is then compressed. Although this is
straightforward, it is not necessarily the most effective as a method.

Bearing in mind that the fissile material is approximately one order of magnitude denser
than the explosive, shock reflection at high impedance interfaces occurs. It is possible to
augment this by means of the addition of one or more layers of materials of increasing
density between the explosive and the dense tamper/fissile material in the center.

The ratio of the fissile core radius to the outer radius of the implosion system limits the
shock convergence. The increase in pressure is approximately proportional to this ratio. The
effect of convergence increases at the expense of system size.

It is also possible that solid cores can suffer as a result of the Taylor wave phenomenon. The
Taylor wave is a sharp drop in pressure with increasing distance behind the detonation front.
This creates a sharp step change in the shock profile at the peak, and it is followed by a more
gentle relaxation to zero pressure a little distance behind the shock front. This effect is
worse with regard to a convergent system. If the Taylor wave is not suppressed, the outer
portions have possibly already expanded to their original density by the moment the shock
reaches the center of the fissile mass.

Levitated Core Designs

It is possible to alleviate these limiting phenomena, such as Taylor waves for solid core
designs by means of a shell or hollow core.

Usually the shell is usually comprised of an outer tamper material and inner fissile material.
When the implosion shock wave arrives at the inner surface of the shell, the shock-compressed
material accelerates in and expands into the void. The compression energy becomes converted
into kinetic energy. This also minimizes the loss of energy by the outward expansion of
material in the Taylor wave.

Although it is possible to achieve reasonable compression as a result of a collapse of a shell
alone, it is more effective when the collapsing shell strikes a so-called "levitated core" in the
center. The impact of the shell on the core generates two shockwaves: one is transmitted
into the core and causes it to accelerate inward, the other is a reflected shock which slows
the shell. Because the pressure between these shocks is constant, the core witnesses
reasonably smooth and efficient compression.

When the shock converges at the center of the device, it reflects. This out-moving shock
accelerates material away from the center and creates an expanding low density region
surrounded with a layer which is compressed to an even bigger degree than in the initial
implosion.

When one raises a core inside a shell, it is important that the support structure does not
interfere with the implosion symmetry. Usually it is possible to achieve this by means of
truncated aluminum hollow cones or supporting wires under tension.

Trinity and RDS-1

The first American and Soviet implosion devices were "Trinity"/"Fat Man" and RDS-1, and
they were very similar in their design. It is possible to find accurate drawings which show the main
concepts as we discuss above on the Internet:

The Russian RDS-1 was very similar to Fat Man's design. The designs both employ internal
neutron initiators, solid plutonium cores, aluminum pushers, uranium tampers, high
explosive main charge and lenses.