9.2 How Nuclear Weapons Work
85
But having a critical mass of material is not enough to ensure a chain reaction
will occur. Picture a critical mass of plutonium that is hammered out into a flat sheet
a few mm thick. In this configuration a neutron must be emitted within the plane of
this sheet in order to cause a fission; if it is emitted at even a small angle it will likely
escape the sheet before it can cause a fission. In this case there is a critical mass of
material, but it is not in a critical geometry.
Now picture this sheet of plutonium being crumpled up like a sheet of paper.
At some point it will reach a compact, nearly spherical configuration in which the
neutrons are able to be captured before they escape—now we have a critical mass of
plutonium that is in a critical geometry.
Thus, to construct a nuclear weapon one needs to have a critical mass of fissile
material that can be assembled into a critical geometry (Note: there is a difference
between the terms “fissionable” and “fissile.” Fissile materials will undergo fission
when struck by neutrons of any energy while fissionable materials require neutrons
above a threshold energy level).
9.2.2 The Fission Process
If we begin with a single fission and assume that two neutrons from each fission go
on to cause a secondary fission then in ten generations there will be over 1000 atoms
fissioning, releasing about 32 mJ of energy. Another ten generations of doubling will
increase the number of atoms fissioned by another factor of 1000, producing about
32 J. The next ten generations will increase the energy released to 32 kJ and ten more
will bring this up to 32 MJ, about as much energy as about seven kg of TNT. Another
ten generations will increase this yield to seven kt of TNT, the yield of a small nuclear
weapon. Thus, 50 generations of fission with the number of fissions doubling each
generation will bring the energy production from a level that can be measured only
with specialized instruments to a point that will level a small city. Assuming a fission
generation lasts about 0.01 µs [6], these 50 generations of doubling will take place
in less than 1 µs.
The amount of energy released by a nuclear detonation depends on the number
of fissions that take place so, in general, a larger amount of fissionable material will
produce a larger number of fissions and a larger explosion. As noted above, a weapon
must contain a critical mass of fuel in order to achieve a nuclear yield. A critical mass
of fissionable uranium (
235 U) is about 49 kg, and a critical mass of
239 Pu is about
17 kg as bare metal, although these masses can be reduced using various techniques
[7].
With about 200 million electron volts (MeV) of energy released for each fission,
the fission of every atom in a critical mass of
235 U (about 5 × 10
25 atoms) would
release the equivalent of nearly 400 kilotons (kt) of TNT (one kt is the amount of
energy released by the detonation of one ton of TNT and is equal to about 3.8 ×
10
15 J). In reality, this amount of uranium is more likely to release about 10–20 kt of
energy, because a large fraction of the atoms will not fission—the energy released
85
But having a critical mass of material is not enough to ensure a chain reaction
will occur. Picture a critical mass of plutonium that is hammered out into a flat sheet
a few mm thick. In this configuration a neutron must be emitted within the plane of
this sheet in order to cause a fission; if it is emitted at even a small angle it will likely
escape the sheet before it can cause a fission. In this case there is a critical mass of
material, but it is not in a critical geometry.
Now picture this sheet of plutonium being crumpled up like a sheet of paper.
At some point it will reach a compact, nearly spherical configuration in which the
neutrons are able to be captured before they escape—now we have a critical mass of
plutonium that is in a critical geometry.
Thus, to construct a nuclear weapon one needs to have a critical mass of fissile
material that can be assembled into a critical geometry (Note: there is a difference
between the terms “fissionable” and “fissile.” Fissile materials will undergo fission
when struck by neutrons of any energy while fissionable materials require neutrons
above a threshold energy level).
9.2.2 The Fission Process
If we begin with a single fission and assume that two neutrons from each fission go
on to cause a secondary fission then in ten generations there will be over 1000 atoms
fissioning, releasing about 32 mJ of energy. Another ten generations of doubling will
increase the number of atoms fissioned by another factor of 1000, producing about
32 J. The next ten generations will increase the energy released to 32 kJ and ten more
will bring this up to 32 MJ, about as much energy as about seven kg of TNT. Another
ten generations will increase this yield to seven kt of TNT, the yield of a small nuclear
weapon. Thus, 50 generations of fission with the number of fissions doubling each
generation will bring the energy production from a level that can be measured only
with specialized instruments to a point that will level a small city. Assuming a fission
generation lasts about 0.01 µs [6], these 50 generations of doubling will take place
in less than 1 µs.
The amount of energy released by a nuclear detonation depends on the number
of fissions that take place so, in general, a larger amount of fissionable material will
produce a larger number of fissions and a larger explosion. As noted above, a weapon
must contain a critical mass of fuel in order to achieve a nuclear yield. A critical mass
of fissionable uranium (
235 U) is about 49 kg, and a critical mass of
239 Pu is about
17 kg as bare metal, although these masses can be reduced using various techniques
[7].
With about 200 million electron volts (MeV) of energy released for each fission,
the fission of every atom in a critical mass of
235 U (about 5 × 10
25 atoms) would
release the equivalent of nearly 400 kilotons (kt) of TNT (one kt is the amount of
energy released by the detonation of one ton of TNT and is equal to about 3.8 ×
10
15 J). In reality, this amount of uranium is more likely to release about 10–20 kt of
energy, because a large fraction of the atoms will not fission—the energy released