As promised over the weekend, I have dug out the Jane's article I referred to in the thread on nuclear weapons. Its from Jane's International Defence Review, dated 18th December 2001. The pictuers following were in the article and are refered to in the text.
USA looks at nuclear role in bunker busting
Geoffrey Forden
Geoffrey Forden investigates thet echnical issues associated with the development of
low-yield nuclear weapons to destroy underground bunkers.
The USA is taking a renewed interest in developing low-yield nuclear weapons to meet a range of
security objectives.
Proponents of such systems say they are needed to destroy deep underground facilities that are
beyond the reach of conventional 'bunker buster' munitions. Such facilities might include storage and
production centres for weapons of mass destruction; command and control centres; and heavily
defended refuges fort errorist and other enemy leaders.
However, there remain question marks over the potential effectiveness of such weapons. One former
senior Pentagon official has said that such facilities might be buried under 300m of granite. Is that
within the destructive range of even a nuclear bomb? Furthermore, it might be impossible for
intelligence sources to pinpoint the precise position of the bunker inside a mountain. Satellite
surveillance can be used to estimate the length of a tunnel leading to thef acility by observing the
volume of material removed. They cannot, however,d etermine if a tunnel takes a sharp turn
somewhere along its path. Even human intelligence has limited utility for targeting since small errors
on the direction of a tunnel can lead to large discrepancies at its end. That means a nuclear weapon
must be capable of destroying an underground facility even if it misses by hundreds of metres.
In the 2001 Defense Authorization Bill, the US Congress required the Energy Department's weapons
laboratories to research these issues. However, enoughi nformation already exists in open literature -
mainly published in the 1960s as part of the Plowshare Program - to make a rough estimate of how
effective an uclear bomb would be at destroying buried facilities and how much radioactivity it would
release into the environment. The importance of this latter pointb ecomes more apparent each time a
conventional bomb misses its target in Afghanistan.
Underground deployment
There are limits to how deep a bomb can burrow underground. Even hardened steel bomb casings
eventually yield to the tremendous forces a projectileg enerates as it ploughs through earth or rock.
Increasing the length of the bomb can, to some extent, compensate for these effects. However,
practical considerations limit bomb lengths to about 3m and penetration depths to about 30m of rock.
Delivery of an earth-penetrating nuclear warhead to even 30 metres underground results in an
enormous increase in its destructive power against deeply buried facilities. This increase in lethal
range greatly reduces the need for highly precise intelligence about the location of a targeted facility.
The increased effectiveness is due to the shattering of the surrounding earth,c rushing of tunnels and
equipment nearby, and a vast and devastating shockwave capable of destroying equipment and
seriously injuring and killing people at even farther distances. If the weapon can be placed close
enough to the target of concern it may even be possible for the nuclear fireball to consume equipment
and material inside tunnels crushed by the blast, an important consideration when attacking
biological weapons facilities.
All this damage is possible because burying a nuclear bomb even a smalld istance below the surface
greatly increases the fraction of energy coupled to the earth. Almost all of the energy initially
released from a nuclear explosion is in the form of intense light and heat. In the case of an airburst,
roughly two thirds of this energy is eventually transformed into a shock wave of enormous extent and
power. It is the light that is the source of nearly all of the most destructive initial effects, including the
air blast and fires, associated with nuclear weapons. Such airbursts are not very effective atd
amaging underground bunkers because most of the air blast is reflected by the earth's surface. A
deeply buried burst solves this problem because the initially released intense light and heat from the
explosion is contained within thes urrounding earth, efficiently converting the energy into destructive
mechanical effects. The mechanical energy pulverises the region surrounding the bomb,g enerates
an intense shockwave that travels considerable distances, and causes severe ground motion that is
highly damaging to underground structures and their contents.
The required depth of burst to contain the light from a nuclear explosion, and change most of that
energy to ground motion, increases as the bomb's yield increases. Thus, the surrounding earth
absorbs nearly all of a one-kiloton (kT) bomb's initial pulse of intense light even if it is buried five
metres underground. Thirty metres depth is enough to absorb the initial radiation from a 200kT bomb.
However, even a 1kT bomb is far from being 'contained' at even 30m depth. Since the mechanisms at
work in such an explosion are important to an understanding of its destructive power and the amount
of radiation released into the surrounding area, the next section will briefly describe an undergroundn
uclear explosion.
Underground explosions
In the first fraction of a second, most of a nuclear explosion's energy is converted into heat and light,
predominantly soft X-rays. When the explosion is buried, these vapourise a significant quantity of the
surrounding earth andr ock, adding to the enormous pressures being generated by the vapourised
bomb material. This fireball expands until its pressure is balanced by the weight of earth and rock
above the explosion, creating a spherical cavity. A fireballg enerated by a 1kT nuclear explosion
roughly 20m deep in granite would just rise to the pre-explosion level of the ground. It would, of
course, have lifted most of the 20m of earth above this, much of which would be ejected from the
crater either to land nearby or be carried off. Increasing the depth of a 1kT bomb to 30m actually
widens and deepens the crater at the same time as it decreases the amount of radioactivity
released.
The fireball's expansion also forms a maze of cracks and vents in the region immediately surrounding
the initial cavity. This region, known as the rupture zone, is far from symmetrical. Instead, it extends
nearly three times as far horizontally as it does below the cavity. When examined after an
underground nuclear test explosion, these cracks and vents will often appear to be burnt and charred,
implying that an underground facility close enough to the detonation to lie inside the ruptured zone
might be consumed by the nuclear conflagration. (Figures 1a and 1b show the destructive distances
associated with this rupture zone horizontally and vertically respectively. Destruction zones for
granite, the other extreme 'soil' condition, are at most 20% smaller than those shown here).
A tunnel or extensive underground facility responds to mechanical stresses in the same way as the
surrounding medium. This can result in an increased range of destruction for loose soils such as
alluvium where the so-called plastic zone extends much farther out than the rupture zone. However,
not all materials are subject to extensive plastic deformation. Granite, which some analysts believe is
the most likely geologic formation for building underground facilities, does not produce extensive
zones of plastic deformation.
Equipment and personnel inside the structure can, however, be destroyed,k illed, or severely injured
far outside the plastic or rupture zones by thep owerful shockwave generated during the explosion.
Equipment is destroyed or disabled by being shaken until it breaks or deforms. Personnel can also
bei njured or killed by being shaken or possibly injured by bits of tunnel wall being broken off and
flying across the room as the shockwave passes in a process known as spallation. (The same
process, but on a much larger scale, contributes to surface crater formation for buried nuclear
explosions). Facilities can,h owever, be protected from spallation by lining the walls with sheet
metal.< P>Damage from shockwave-induced mechanical vibration critically depends on the
frequencies associated with the blast. Fortunately for this analysis, theP lowshare Project published
the vibrational spectrum associated with the 1961 GNOME shot together with its yield. This allows
us to estimate the damage to medium and light equipment (pumps, condensers, biological
centrifuges, small biological-agent fermentors), heavy equipment (large fermentors, generators, large
motors) and personnel.
Heavy equipment is particularly sensitive to vibrations around 10 cycles per second, which is close to
the frequency where most of the vibrations are generated by a nuclear explosion. If these vibrations
cause the equipment to accelerate at rates in excess of 20 times that caused by gravity, 20G, it will
cause significant damage. (It should be noted that this is a back and forthm otion and does not
require that the distances moved be more than a couple of centimetres maximum displacement).
Medium and light equipment, on the otherh and, are most sensitive to frequencies far from the peak
and also requirea ccelerations of 40G or greater for substantial damage. These two factors mean that
a nuclear bomb will have to explode much closer to light and medium equipment than it does to
heavy equipment to destroy it. In fact, a 1kT nuclear explosion, buried 30m underground, will destroy
heavy equipment within about 160m of its burst while light equipment will only be destroyed if the
tunnel is within the rupture zone.
Figure A shows the depths of destruction associated with a nuclear device buried 30m underground
for various yields for these various mechanisms. Horizontal destruction distances would be caused
by a tunnel rupturing and not by shockwaves since horizontal rupturing extends much farther than the
symmetric shockwave destruction, at least for the yields below 10kT considered here.S imilar
calculations have been made for human injury using data for seatedi ndividuals. While the resulting
distances for injury to humans are shown, the available data is not as readily applied to nuclear
explosions and that curve should be considered an estimate at best.
A facility can be hardened against the shockwave effects. For instance,p lacing all the equipment
and personnel on a platform suspended by shock absorbers hardens them to a point that they can
only be destroyed if the tunnel is in the rupture zone. This is the technique employed in Minuteman
launchc ontrol centres.
Attacking Tarhunah
It is clear from this analysis that even a 10kT nuclear weapon cannot destroy or even damage the
equipment in an underground facility buried 300m in granite. However, what about a facility not as
deeply buried but whose position is not accurately known? The alleged chemical weapons production
facility near Tarhunah in Libya makes an excellent example.
In the mid-1990s, the USA alleged that Libya had constructed an underground nerve-agent production
plant, buried under at least 18m of earth, 60km southeast of Tripoli. The main difficulty with attacking
this facility would not be its depth, which appears well within the reach of even sub-kiloton weapons,
butu ncertainty about its underground location. Publicly available details about this plant are sketchy,
but it appears that there are twin entrance tunnels, between 60m and 140m long. According to
eyewitness accounts, both tunnels go around a large rock formation near their entrance, purportedly
to defeat cruise missile attacks. However, it is not clear if they continue on in the samed irection or
make a sharp turn after passing this rock. After all, it is hard for an inexperienced observer to keep
track of directions passing throught unnels.
If the tunnels took up to a 60º turn around the rock formation, the main facility might be anywhere
inside a roughly rectangular area 80m wide by 240m long. The best course of action would place a
nuclear bomb in the centre of this rectangle and size its yield to guarantee destruction of any facility
within the rectangle, a maximum distance of 125m. If the facility does not have shocka bsorbers, a
half-kiloton nuclear explosion could destroy any heavy equipment inside it even at the far reaches of
the target area. If it is hardened with shock absorbers, or military planners decide it must be totally
destroyed, a 5kT explosive would have to be used. Other geologic formations in the area could
significantly reduce the effectiveness of such a nuclear weapon. For instance, deep crevasses, if they
lay between the explosion and the underground facility, would effectively neutralise the destructive
power of the bomb.
Radiation, fallout, and design
Nuclear radiation does not play a primary role in destroying deeply buried facilities. Prompt gamma
radiation, X-rays and neutrons are stopped and absorbed by the tens of metres of earth in the rupture
zone. However, relatively long-lived radioactive isotopes resulting from the bomb material and the
surrounding irradiated earth do pose a significant health hazard. It is likely that concerns for
maintaining any coalition in support of a war - includingc ountries neighbouring the target area - as
well as purely humanitarian concerns for innocent civilian populations, will compel low yields and
efforts to bury the warhead as deeply as possible, even if that does not produce significant increases
in destruction on deeply buried facilities.
How much radiation is acceptable to release into the environment is, ofc ourse, debatable. During the
era of US nuclear testing, explosions got progressively deeper as the acceptable amount of released
radiation was continually reduced. Eventually, nuclear devices were only exploded at depths greater
than 1,000 feet because of concerns over low-level seepage. Comparable depths for nuclear bombs
dropped from airplanes can never be achieved. Somea nalysts, however, might argue that such tight
standards are not required in a wartime setting.
Instead, those analysts would argue that the level of radiation could bes ufficiently reduced and
contained over a limited enough area so that a coalition would not be threatened. While determining
what level might be acceptable is a political question, it is still possible to estimate how much
radiation would be released for a given yield and penetration depth by using data released during the
Plowshare Project.
It is possible to reduce the radiation released into the environment byw rapping the 'physics package'
- the heart of the nuclear explosive - in materials that absorb the initial burst of neutrons. In the best
of cases, this eliminates fallout produced by the surrounding earth that would ordinarily get
bombarded by neutrons. (This does not weaken the destructive effect of the bomb since, as was
pointed out above, those neutrons do not contribute to destroying underground facilities). That leaves
only the fission products produced in the initial explosion as fallout. However, that is still a large
amount of radioactivity.
In the example above of attacking Tarhunah, a half-kiloton bomb would spread highly radioactive
debris over a circle of 300m diameter. The 5kT bomb would do the same over a 700m diameter circle.
Both bombs would also release significant fallout that could travel tens of miles before falling to earth.
Of course, the crater walls as well as the immediate debris field have also trapped a significant
amount of radioactivity that would otherwise land as fallout far from the explosion. If the fallout landed
uniformly over a one square kilometre area, the radiation from the half-kiloton explosion would
produce three rems per hour, 24-hours after the detonation, while the 5kT bomb would produce 50
rems per hour under the same conditions. These are significant amounts and threaten the health and
safety of the populations far from the target. An eight-houre xposure to the larger bomb's fallout would
kill about half the people exposed. It is also likely that the exposure levels would be higher from
breathing in or otherwise ingesting the fallout, causing even greater harm.
All these calculations assume that the nuclear weapon will survive beingd riven deep into the earth. In
fact, a warhead will see, on average, forces well over 10,000 times the force of gravity as it ploughs
through the earth. The only US nuclear weapons publicly known to have been tested under such
extreme conditions are atomic artillery shells, which are no longer in the US arsenal. On the other
hand, if the development of a new weapon is undertaken, it might be possible to design new weapons
that use far less fissionable material and have far less fallout. Such a program would require a
considerable number of underground nuclear tests.
Would the reintroduction of an old design require additional tests of the nuclear explosive? Some
might argue that it does not even though the USA would employ new manufacturing techniques. This
weapon, they might point out, will not be an integral component of US general war plans and
therefore does not need the high reliability required of strategic nuclear weapons. They would be
willing to live with a small chance of having a weapon whose destructive power is reduced by some
unknown factor. Others might argue that it is even more important to test this weapon. Any nuclear
yield at all would release significant amounts of radioactive fallout into the environment and
proponents of testing would view it as politically catastrophic if the explosion failed to destroy its
intendedt arget and a second bomb had to be dropped.
If the USA decided to test this weapon, it would raise a plethora of politically charged questions long
before the weapon's combat use. When the US Senate refused to ratify the Comprehensive Test Ban
Treaty, it essentiallyr emoved any international obligation for the USA not to test. At the time of the
Senate vote, however, then-President Clinton promised to continue the unilateral moratorium on
underground nuclear testing. This might have limited the international damage created by the Senate
vote. A decision to conduct even one test of a low-yield weapon might cause a storm of international
protest andw eaken the international consensus on hindering nuclear proliferation. Other countries
than the USA are faced with regional challengers who increasingly use underground facilities. A US
decision to use nuclear weapons for destroying deep underground bunkers would legitimate the
proliferation of nuclear weapons by other states for this purpose.
Posts: 165
By: MikeP - 11th March 2002 at 17:05
As promised over the weekend, I have dug out the Jane's article I referred to in the thread on nuclear weapons. Its from Jane's International Defence Review, dated 18th December 2001. The pictuers following were in the article and are refered to in the text.
USA looks at nuclear role in bunker busting
Geoffrey Forden
Geoffrey Forden investigates thet echnical issues associated with the development of
low-yield nuclear weapons to destroy underground bunkers.
The USA is taking a renewed interest in developing low-yield nuclear weapons to meet a range of
security objectives.
Proponents of such systems say they are needed to destroy deep underground facilities that are
beyond the reach of conventional 'bunker buster' munitions. Such facilities might include storage and
production centres for weapons of mass destruction; command and control centres; and heavily
defended refuges fort errorist and other enemy leaders.
However, there remain question marks over the potential effectiveness of such weapons. One former
senior Pentagon official has said that such facilities might be buried under 300m of granite. Is that
within the destructive range of even a nuclear bomb? Furthermore, it might be impossible for
intelligence sources to pinpoint the precise position of the bunker inside a mountain. Satellite
surveillance can be used to estimate the length of a tunnel leading to thef acility by observing the
volume of material removed. They cannot, however,d etermine if a tunnel takes a sharp turn
somewhere along its path. Even human intelligence has limited utility for targeting since small errors
on the direction of a tunnel can lead to large discrepancies at its end. That means a nuclear weapon
must be capable of destroying an underground facility even if it misses by hundreds of metres.
In the 2001 Defense Authorization Bill, the US Congress required the Energy Department's weapons
laboratories to research these issues. However, enoughi nformation already exists in open literature -
mainly published in the 1960s as part of the Plowshare Program - to make a rough estimate of how
effective an uclear bomb would be at destroying buried facilities and how much radioactivity it would
release into the environment. The importance of this latter pointb ecomes more apparent each time a
conventional bomb misses its target in Afghanistan.
Underground deployment
There are limits to how deep a bomb can burrow underground. Even hardened steel bomb casings
eventually yield to the tremendous forces a projectileg enerates as it ploughs through earth or rock.
Increasing the length of the bomb can, to some extent, compensate for these effects. However,
practical considerations limit bomb lengths to about 3m and penetration depths to about 30m of rock.
Delivery of an earth-penetrating nuclear warhead to even 30 metres underground results in an
enormous increase in its destructive power against deeply buried facilities. This increase in lethal
range greatly reduces the need for highly precise intelligence about the location of a targeted facility.
The increased effectiveness is due to the shattering of the surrounding earth,c rushing of tunnels and
equipment nearby, and a vast and devastating shockwave capable of destroying equipment and
seriously injuring and killing people at even farther distances. If the weapon can be placed close
enough to the target of concern it may even be possible for the nuclear fireball to consume equipment
and material inside tunnels crushed by the blast, an important consideration when attacking
biological weapons facilities.
All this damage is possible because burying a nuclear bomb even a smalld istance below the surface
greatly increases the fraction of energy coupled to the earth. Almost all of the energy initially
released from a nuclear explosion is in the form of intense light and heat. In the case of an airburst,
roughly two thirds of this energy is eventually transformed into a shock wave of enormous extent and
power. It is the light that is the source of nearly all of the most destructive initial effects, including the
air blast and fires, associated with nuclear weapons. Such airbursts are not very effective atd
amaging underground bunkers because most of the air blast is reflected by the earth's surface. A
deeply buried burst solves this problem because the initially released intense light and heat from the
explosion is contained within thes urrounding earth, efficiently converting the energy into destructive
mechanical effects. The mechanical energy pulverises the region surrounding the bomb,g enerates
an intense shockwave that travels considerable distances, and causes severe ground motion that is
highly damaging to underground structures and their contents.
The required depth of burst to contain the light from a nuclear explosion, and change most of that
energy to ground motion, increases as the bomb's yield increases. Thus, the surrounding earth
absorbs nearly all of a one-kiloton (kT) bomb's initial pulse of intense light even if it is buried five
metres underground. Thirty metres depth is enough to absorb the initial radiation from a 200kT bomb.
However, even a 1kT bomb is far from being 'contained' at even 30m depth. Since the mechanisms at
work in such an explosion are important to an understanding of its destructive power and the amount
of radiation released into the surrounding area, the next section will briefly describe an undergroundn
uclear explosion.
Underground explosions
In the first fraction of a second, most of a nuclear explosion's energy is converted into heat and light,
predominantly soft X-rays. When the explosion is buried, these vapourise a significant quantity of the
surrounding earth andr ock, adding to the enormous pressures being generated by the vapourised
bomb material. This fireball expands until its pressure is balanced by the weight of earth and rock
above the explosion, creating a spherical cavity. A fireballg enerated by a 1kT nuclear explosion
roughly 20m deep in granite would just rise to the pre-explosion level of the ground. It would, of
course, have lifted most of the 20m of earth above this, much of which would be ejected from the
crater either to land nearby or be carried off. Increasing the depth of a 1kT bomb to 30m actually
widens and deepens the crater at the same time as it decreases the amount of radioactivity
released.
The fireball's expansion also forms a maze of cracks and vents in the region immediately surrounding
the initial cavity. This region, known as the rupture zone, is far from symmetrical. Instead, it extends
nearly three times as far horizontally as it does below the cavity. When examined after an
underground nuclear test explosion, these cracks and vents will often appear to be burnt and charred,
implying that an underground facility close enough to the detonation to lie inside the ruptured zone
might be consumed by the nuclear conflagration. (Figures 1a and 1b show the destructive distances
associated with this rupture zone horizontally and vertically respectively. Destruction zones for
granite, the other extreme 'soil' condition, are at most 20% smaller than those shown here).
A tunnel or extensive underground facility responds to mechanical stresses in the same way as the
surrounding medium. This can result in an increased range of destruction for loose soils such as
alluvium where the so-called plastic zone extends much farther out than the rupture zone. However,
not all materials are subject to extensive plastic deformation. Granite, which some analysts believe is
the most likely geologic formation for building underground facilities, does not produce extensive
zones of plastic deformation.
Equipment and personnel inside the structure can, however, be destroyed,k illed, or severely injured
far outside the plastic or rupture zones by thep owerful shockwave generated during the explosion.
Equipment is destroyed or disabled by being shaken until it breaks or deforms. Personnel can also
bei njured or killed by being shaken or possibly injured by bits of tunnel wall being broken off and
flying across the room as the shockwave passes in a process known as spallation. (The same
process, but on a much larger scale, contributes to surface crater formation for buried nuclear
explosions). Facilities can,h owever, be protected from spallation by lining the walls with sheet
metal.< P>Damage from shockwave-induced mechanical vibration critically depends on the
frequencies associated with the blast. Fortunately for this analysis, theP lowshare Project published
the vibrational spectrum associated with the 1961 GNOME shot together with its yield. This allows
us to estimate the damage to medium and light equipment (pumps, condensers, biological
centrifuges, small biological-agent fermentors), heavy equipment (large fermentors, generators, large
motors) and personnel.
Heavy equipment is particularly sensitive to vibrations around 10 cycles per second, which is close to
the frequency where most of the vibrations are generated by a nuclear explosion. If these vibrations
cause the equipment to accelerate at rates in excess of 20 times that caused by gravity, 20G, it will
cause significant damage. (It should be noted that this is a back and forthm otion and does not
require that the distances moved be more than a couple of centimetres maximum displacement).
Medium and light equipment, on the otherh and, are most sensitive to frequencies far from the peak
and also requirea ccelerations of 40G or greater for substantial damage. These two factors mean that
a nuclear bomb will have to explode much closer to light and medium equipment than it does to
heavy equipment to destroy it. In fact, a 1kT nuclear explosion, buried 30m underground, will destroy
heavy equipment within about 160m of its burst while light equipment will only be destroyed if the
tunnel is within the rupture zone.
Figure A shows the depths of destruction associated with a nuclear device buried 30m underground
for various yields for these various mechanisms. Horizontal destruction distances would be caused
by a tunnel rupturing and not by shockwaves since horizontal rupturing extends much farther than the
symmetric shockwave destruction, at least for the yields below 10kT considered here.S imilar
calculations have been made for human injury using data for seatedi ndividuals. While the resulting
distances for injury to humans are shown, the available data is not as readily applied to nuclear
explosions and that curve should be considered an estimate at best.
A facility can be hardened against the shockwave effects. For instance,p lacing all the equipment
and personnel on a platform suspended by shock absorbers hardens them to a point that they can
only be destroyed if the tunnel is in the rupture zone. This is the technique employed in Minuteman
launchc ontrol centres.
Attacking Tarhunah
It is clear from this analysis that even a 10kT nuclear weapon cannot destroy or even damage the
equipment in an underground facility buried 300m in granite. However, what about a facility not as
deeply buried but whose position is not accurately known? The alleged chemical weapons production
facility near Tarhunah in Libya makes an excellent example.
In the mid-1990s, the USA alleged that Libya had constructed an underground nerve-agent production
plant, buried under at least 18m of earth, 60km southeast of Tripoli. The main difficulty with attacking
this facility would not be its depth, which appears well within the reach of even sub-kiloton weapons,
butu ncertainty about its underground location. Publicly available details about this plant are sketchy,
but it appears that there are twin entrance tunnels, between 60m and 140m long. According to
eyewitness accounts, both tunnels go around a large rock formation near their entrance, purportedly
to defeat cruise missile attacks. However, it is not clear if they continue on in the samed irection or
make a sharp turn after passing this rock. After all, it is hard for an inexperienced observer to keep
track of directions passing throught unnels.
If the tunnels took up to a 60º turn around the rock formation, the main facility might be anywhere
inside a roughly rectangular area 80m wide by 240m long. The best course of action would place a
nuclear bomb in the centre of this rectangle and size its yield to guarantee destruction of any facility
within the rectangle, a maximum distance of 125m. If the facility does not have shocka bsorbers, a
half-kiloton nuclear explosion could destroy any heavy equipment inside it even at the far reaches of
the target area. If it is hardened with shock absorbers, or military planners decide it must be totally
destroyed, a 5kT explosive would have to be used. Other geologic formations in the area could
significantly reduce the effectiveness of such a nuclear weapon. For instance, deep crevasses, if they
lay between the explosion and the underground facility, would effectively neutralise the destructive
power of the bomb.
Radiation, fallout, and design
Nuclear radiation does not play a primary role in destroying deeply buried facilities. Prompt gamma
radiation, X-rays and neutrons are stopped and absorbed by the tens of metres of earth in the rupture
zone. However, relatively long-lived radioactive isotopes resulting from the bomb material and the
surrounding irradiated earth do pose a significant health hazard. It is likely that concerns for
maintaining any coalition in support of a war - includingc ountries neighbouring the target area - as
well as purely humanitarian concerns for innocent civilian populations, will compel low yields and
efforts to bury the warhead as deeply as possible, even if that does not produce significant increases
in destruction on deeply buried facilities.
How much radiation is acceptable to release into the environment is, ofc ourse, debatable. During the
era of US nuclear testing, explosions got progressively deeper as the acceptable amount of released
radiation was continually reduced. Eventually, nuclear devices were only exploded at depths greater
than 1,000 feet because of concerns over low-level seepage. Comparable depths for nuclear bombs
dropped from airplanes can never be achieved. Somea nalysts, however, might argue that such tight
standards are not required in a wartime setting.
Instead, those analysts would argue that the level of radiation could bes ufficiently reduced and
contained over a limited enough area so that a coalition would not be threatened. While determining
what level might be acceptable is a political question, it is still possible to estimate how much
radiation would be released for a given yield and penetration depth by using data released during the
Plowshare Project.
It is possible to reduce the radiation released into the environment byw rapping the 'physics package'
- the heart of the nuclear explosive - in materials that absorb the initial burst of neutrons. In the best
of cases, this eliminates fallout produced by the surrounding earth that would ordinarily get
bombarded by neutrons. (This does not weaken the destructive effect of the bomb since, as was
pointed out above, those neutrons do not contribute to destroying underground facilities). That leaves
only the fission products produced in the initial explosion as fallout. However, that is still a large
amount of radioactivity.
In the example above of attacking Tarhunah, a half-kiloton bomb would spread highly radioactive
debris over a circle of 300m diameter. The 5kT bomb would do the same over a 700m diameter circle.
Both bombs would also release significant fallout that could travel tens of miles before falling to earth.
Of course, the crater walls as well as the immediate debris field have also trapped a significant
amount of radioactivity that would otherwise land as fallout far from the explosion. If the fallout landed
uniformly over a one square kilometre area, the radiation from the half-kiloton explosion would
produce three rems per hour, 24-hours after the detonation, while the 5kT bomb would produce 50
rems per hour under the same conditions. These are significant amounts and threaten the health and
safety of the populations far from the target. An eight-houre xposure to the larger bomb's fallout would
kill about half the people exposed. It is also likely that the exposure levels would be higher from
breathing in or otherwise ingesting the fallout, causing even greater harm.
All these calculations assume that the nuclear weapon will survive beingd riven deep into the earth. In
fact, a warhead will see, on average, forces well over 10,000 times the force of gravity as it ploughs
through the earth. The only US nuclear weapons publicly known to have been tested under such
extreme conditions are atomic artillery shells, which are no longer in the US arsenal. On the other
hand, if the development of a new weapon is undertaken, it might be possible to design new weapons
that use far less fissionable material and have far less fallout. Such a program would require a
considerable number of underground nuclear tests.
Would the reintroduction of an old design require additional tests of the nuclear explosive? Some
might argue that it does not even though the USA would employ new manufacturing techniques. This
weapon, they might point out, will not be an integral component of US general war plans and
therefore does not need the high reliability required of strategic nuclear weapons. They would be
willing to live with a small chance of having a weapon whose destructive power is reduced by some
unknown factor. Others might argue that it is even more important to test this weapon. Any nuclear
yield at all would release significant amounts of radioactive fallout into the environment and
proponents of testing would view it as politically catastrophic if the explosion failed to destroy its
intendedt arget and a second bomb had to be dropped.
If the USA decided to test this weapon, it would raise a plethora of politically charged questions long
before the weapon's combat use. When the US Senate refused to ratify the Comprehensive Test Ban
Treaty, it essentiallyr emoved any international obligation for the USA not to test. At the time of the
Senate vote, however, then-President Clinton promised to continue the unilateral moratorium on
underground nuclear testing. This might have limited the international damage created by the Senate
vote. A decision to conduct even one test of a low-yield weapon might cause a storm of international
protest andw eaken the international consensus on hindering nuclear proliferation. Other countries
than the USA are faced with regional challengers who increasingly use underground facilities. A US
decision to use nuclear weapons for destroying deep underground bunkers would legitimate the
proliferation of nuclear weapons by other states for this purpose.
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