Nuclear Fusion
Nuclear energy
can also be released by fusion of two light elements (elements with low atomic
numbers). The power that fuels the sun and the stars is nuclear fusion. In a
hydrogen bomb, two isotopes of hydrogen, deuterium and tritium are fused to
form a nucleus of helium and a neutron. This fusion releases 17.6 MeV of energy.
Unlike nuclear fission, there is no limit on the amount of the fusion that can
occur.
The Hydrogen Bomb: The Basics
A fission bomb,
called the primary, produces a flood of radiation including a large number of
neutrons. This radiation impinges on the thermonuclear portion of the bomb, known
as the secondary. The secondary consists largely of lithium deuteride. The neutrons
react with the lithium in this chemical compound, producing tritium and helium.
This reaction produces the tritium on the spot, so there is no need
to include tritium in the bomb itself. In the extreme heat which exists in the
bomb, the tritium fuses with the deuterium in the lithium deuteride.
The Hydrogen Bomb: The Secret
The
question facing designers was "How do you build a bomb that will maintain the
high temperatures required for thermonuclear reactions to occur?" The shock waves
produced by the primary (A-bomb) would propagate too slowly to permit assembly
of the thermonuclear stage before the bomb blew itself apart. This problem was
solved by Edward Teller and Stanislaw Ulam.
Gamma Radiation
To do this, they introduced a gamma-ray absorbing material (styrofoam)
to capture the energy of the gamma radiation. As gamma radiation from the primary
is absorbed, radial compression forces are exerted along the entire cylinder at
almost the same instant. This produces the compression of the lithium deuteride.
Additional neutrons are also produced by various components and reflected towards
the lithium deuteride. With the compressed lithium deuteride core now bombarded
with neutrons, tritium is formed and the fusion process begins.
The Hydrogen Bomb: Schematic
The yield of a
hydrogen bomb is controlled by the amounts of lithium deuteride and of additional
fissionable materials. Uranium 238 is usually the material used in various parts
of the bomb's design to supply additional neutrons for the fusion process. This
additional fissionable material also produces a very high level of radioactive
fallout.
The Neutron Bomb
The
neutron bomb is a small hydrogen bomb. The neutron bomb differs from standard
nuclear weapons insofar as its primary lethal effects come from the radiation
damage caused by the neutrons it emits. It is also known as an enhanced-radiation
weapon (ERW).
The augmented radiation effects mean that blast and heat effects
are reduced so that physical structures including houses and industrial installations,
are less affected. Because neutron radiation effects drop off very rapidly with
distance, there is a sharper distinction between areas of high lethality and
areas with minimal radiation doses.
This was desired by the forces of the North Atlantic Treaty Organization
(NATO), since they have to be prepared to fight in densely populated
areas; any tactical nuclear explosion will endanger civilian lives and property.