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9 How Nuclear Weapons Work
about a million times as much energy per kg of material when we account for the
differences in atomic and molecular mass.
The release of so much energy in so short a period of time makes nuclear weapons
particularly devastating and a nuclear attack in a major city could kill hundreds
of thousands of people and destroy buildings over a large area; in Hiroshima and
Nagasaki a single weapon of, by today’s standards, relatively low yield destroyed the
better part of two mid-sized cities, killing or injuring two thirds of the inhabitants.
While state-developed nuclear weapons today are much more powerful than the
bombs dropped on Japan in 1945, it is not unreasonable to believe that a nuclear
weapon developed by a terrorist group could be comparable in yield to the devices
used in Japan.
9.2 How Nuclear Weapons Work
Understanding how nuclear weapons work—in general, as the specifics are highly
classified—is important if one is to understand why they are so devastating and why
they can cause not only physical destruction, but radiological dangers as well.
9.2.1 Criticality, Critical Mass and Critical Geometry
Nuclear weapons depend on a runaway series of nuclear fissions to achieve their
explosive yield. When a uranium atom fissions, it releases two or more neutrons, and
if one of those neutrons (on average) goes on to cause another fission then the material
is said to achieved criticality. Please note that “critical” in this usage is not intended
to indicate risk or anything other than a configuration that permits a constant rate of
fission over time. If more than one neutron (on average) goes on to cause a fission
then the mass will be supercritical and power will increase; the rate at which power
increases will depend on the average number of neutrons from fission that go on to
cause secondary fissions and on the amount of time required for each generation of
fissions. All else being the same, a larger mass of uranium (for example) will be able
to capture more neutrons and will experience a more rapid increase in power.
No nuclear weapon will function unless it has a critical mass of material that is
assembled in a critical geometry. Since neither of these factors are intuitively obvious
it is necessary to explain what the terms mean.
Critical mass refers to the smallest mass of material that will sustain a nuclear
chain reaction. This is based in part on the distance that a neutron from one fission
must travel before it is able to cause a secondary fission; if the fuel is physically
smaller than that distance then the neutron will escape from the mass of material
before it can cause a fission and that neutron is lost from the weapon. The mass
of material must be large enough so that enough neutrons from the majority of the
fissions are able to go on to cause another fission before they escape.
9 How Nuclear Weapons Work
about a million times as much energy per kg of material when we account for the
differences in atomic and molecular mass.
The release of so much energy in so short a period of time makes nuclear weapons
particularly devastating and a nuclear attack in a major city could kill hundreds
of thousands of people and destroy buildings over a large area; in Hiroshima and
Nagasaki a single weapon of, by today’s standards, relatively low yield destroyed the
better part of two mid-sized cities, killing or injuring two thirds of the inhabitants.
While state-developed nuclear weapons today are much more powerful than the
bombs dropped on Japan in 1945, it is not unreasonable to believe that a nuclear
weapon developed by a terrorist group could be comparable in yield to the devices
used in Japan.
9.2 How Nuclear Weapons Work
Understanding how nuclear weapons work—in general, as the specifics are highly
classified—is important if one is to understand why they are so devastating and why
they can cause not only physical destruction, but radiological dangers as well.
9.2.1 Criticality, Critical Mass and Critical Geometry
Nuclear weapons depend on a runaway series of nuclear fissions to achieve their
explosive yield. When a uranium atom fissions, it releases two or more neutrons, and
if one of those neutrons (on average) goes on to cause another fission then the material
is said to achieved criticality. Please note that “critical” in this usage is not intended
to indicate risk or anything other than a configuration that permits a constant rate of
fission over time. If more than one neutron (on average) goes on to cause a fission
then the mass will be supercritical and power will increase; the rate at which power
increases will depend on the average number of neutrons from fission that go on to
cause secondary fissions and on the amount of time required for each generation of
fissions. All else being the same, a larger mass of uranium (for example) will be able
to capture more neutrons and will experience a more rapid increase in power.
No nuclear weapon will function unless it has a critical mass of material that is
assembled in a critical geometry. Since neither of these factors are intuitively obvious
it is necessary to explain what the terms mean.
Critical mass refers to the smallest mass of material that will sustain a nuclear
chain reaction. This is based in part on the distance that a neutron from one fission
must travel before it is able to cause a secondary fission; if the fuel is physically
smaller than that distance then the neutron will escape from the mass of material
before it can cause a fission and that neutron is lost from the weapon. The mass
of material must be large enough so that enough neutrons from the majority of the
fissions are able to go on to cause another fission before they escape.