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10 Physical Effects of Nuclear Weapons
can be dangerous in and of itself, but the neutron activation products tend to be shortlived and are not dangerously radioactive. And, while the gamma radiation dose is
quite high, the radius at which the gamma dose rate is potentially fatal is smaller
than the radius at which the blast and thermal effects will be fatal. More important,
especially for a surface burst, is the radiation emitted by radioactive fission products
that settle to the ground.
When uranium or plutonium atoms fission they each produce two radioactive
atoms called fission fragments. Assuming an average fission energy of 200 meV we
can calculate that a 10 kt explosion results from the fission of about 10
24 atoms and
produces about 2 × 10
24 radioactive fission product atoms. In a surface burst, these
radioactive atoms will attach themselves to debris swept up into the mushroom cloud
or to the vapors of refractory materials that begin to condense as they rise into the sky.
These will fall to the ground, forming the “footprint” of a fallout plume. An airburst,
on the other hand, will not involve surface materials and there will be nothing to
condense and scavenge the fission products. In an airburst, the fission products will
remain dispersed in the atmosphere, drifting downwind and settling more slowly due
to the smaller particle size; this results in a more tenuous plume, lesser deposition,
and lower radiation exposure to those exposed.
What is the gamma dose 1 km from a 10 kt explosion?
A single fission releases about 200 meV of energy and 10 kt is the equivalent
of about 2.6 × 10
26 meV. Thus, a 10 kt explosion involves the fission of about
10
24 atoms and releases an average of 7 meV of gamma energy [1] per fission.
Thus, the total energy of the gammas from fission is about 7 × 10
24 meV.
This energy will be emitted isotropically, passing through the surface of a
sphere with an area of 4πr
2 . A sphere with a radius of 1 km will have a surface
area of 4 × π x (10
5 cm)
2 , or about 1.26 × 10
11 cm
2 . Dividing the total gamma
energy emitted by the surface area of this sphere gives an energy fluence of 5.6
× 10
13 meV cm
−2 , or about 9 J cm
−2 .
By definition, 1 Gy is equal to the deposition of 1 J of energy per kg of
absorber so the gamma radiation dose from the detonation itself will be about
9 Gy at a distance of 1 km from the bomb at the time it detonates, assuming that
all of the gamma radiation is absorbed in water (a reasonable approximation of
human tissue). The dose at other distances can be calculated using the inverse
square law.
This value is comparable to the conclusions of the Radiation Effects
Research Foundation’s Dosimetry Study, which concluded that the gamma
dose rate at a distance of 1 km from the Hiroshima weapon’s detonation point
was about 10 Gy with a neutron dose that was lower by about a factor of 10
(Figs. 10.1 and 10.2).
10 Physical Effects of Nuclear Weapons
can be dangerous in and of itself, but the neutron activation products tend to be shortlived and are not dangerously radioactive. And, while the gamma radiation dose is
quite high, the radius at which the gamma dose rate is potentially fatal is smaller
than the radius at which the blast and thermal effects will be fatal. More important,
especially for a surface burst, is the radiation emitted by radioactive fission products
that settle to the ground.
When uranium or plutonium atoms fission they each produce two radioactive
atoms called fission fragments. Assuming an average fission energy of 200 meV we
can calculate that a 10 kt explosion results from the fission of about 10
24 atoms and
produces about 2 × 10
24 radioactive fission product atoms. In a surface burst, these
radioactive atoms will attach themselves to debris swept up into the mushroom cloud
or to the vapors of refractory materials that begin to condense as they rise into the sky.
These will fall to the ground, forming the “footprint” of a fallout plume. An airburst,
on the other hand, will not involve surface materials and there will be nothing to
condense and scavenge the fission products. In an airburst, the fission products will
remain dispersed in the atmosphere, drifting downwind and settling more slowly due
to the smaller particle size; this results in a more tenuous plume, lesser deposition,
and lower radiation exposure to those exposed.
What is the gamma dose 1 km from a 10 kt explosion?
A single fission releases about 200 meV of energy and 10 kt is the equivalent
of about 2.6 × 10
26 meV. Thus, a 10 kt explosion involves the fission of about
10
24 atoms and releases an average of 7 meV of gamma energy [1] per fission.
Thus, the total energy of the gammas from fission is about 7 × 10
24 meV.
This energy will be emitted isotropically, passing through the surface of a
sphere with an area of 4πr
2 . A sphere with a radius of 1 km will have a surface
area of 4 × π x (10
5 cm)
2 , or about 1.26 × 10
11 cm
2 . Dividing the total gamma
energy emitted by the surface area of this sphere gives an energy fluence of 5.6
× 10
13 meV cm
−2 , or about 9 J cm
−2 .
By definition, 1 Gy is equal to the deposition of 1 J of energy per kg of
absorber so the gamma radiation dose from the detonation itself will be about
9 Gy at a distance of 1 km from the bomb at the time it detonates, assuming that
all of the gamma radiation is absorbed in water (a reasonable approximation of
human tissue). The dose at other distances can be calculated using the inverse
square law.
This value is comparable to the conclusions of the Radiation Effects
Research Foundation’s Dosimetry Study, which concluded that the gamma
dose rate at a distance of 1 km from the Hiroshima weapon’s detonation point
was about 10 Gy with a neutron dose that was lower by about a factor of 10
(Figs. 10.1 and 10.2).