Nuclear Weapon Design

To design a nuclear weapon means to have a great understanding of and experimentation
on real explosives and nuclear materials. It also means great expertise in the field of numeric
modelling of hydrodynamic, radiation and neutron transport effects.

Nuclear weapon designs are highly classified and are not available to the public. However, it
is possible for general concepts and a reasonable scientific or engineering understanding to
surprisingly bring some understanding of this extremely secret technology.

Design Objectives

Regardless of the method of achieving a viable nuclear device, it is necessary to meet the
following objectives:

  • The device must remain subcritical before operation
  • It is necessary to assemble the supercritical mass in a neutron-free enviroment in
    order to minimize the chance of pre-detonation (the weapon detonating before
    the ideal time)
  • It is necessary to have exactly timed injection of neutrons at peak supercritcality in
    order to maximize performance at the same time as minimizing variability
  • It is necessary to contain the fissioning core as long as possible

It is possible only to achieve the second objective if there is appropriate fissile material (in
order to spontaneous fission), and if the assembly or implosion implosion timescale is short in
comparison with the production of background neutrons, through either cosmic rays or
spontaneous fission.

Of course it is necessary for the weapon to be deliverable. In this sense it should physically
fit inside a bomb casing, cruise missile, re-entry vehicle of a ballistic missile, etc. and be able
to survive extreme evironments. These environments are physical and potentially nuclear
from other nuclear weapon use in proximity.

It is necessary for materials to be easily available. This likely provides a proliferant nation
with its biggest challenge. It is also necessary to obtain confidence in a final design.
Historically scientists achieved this by means of full yield nuclear tests, which were above
and below ground. However, if fissile material availability is limited, or if the advancement of
a particlar design is to be achieved secretly, full yield nuclear tests become less attractive.

Originally scientists developed nuclear weapon designs by means of very limited computing
capability. In recent decades, computing power has increased at the same time as the costs
have decreased. The publication of previously classified information and the
easy availability of information on the internet mean this task is only becoming more
straightforward - this should be a warning for any responsible government with nuclear weapons.