Thermal Effects
A conventional explosion produces
thermal energy with temperatures of maybe 3,000ºC (5,400°F). In
contrast, the fireball of a nuclear detonation will typically be as hot
as the surface of the sun, of the order of 5-6,000ºC (9,000 -
11,000°F).The centre of the fireball is many times hotter than the
centre of the sun, 15,000,000ºC (27,000,000°F). This immense amount of
heat energy causes energy to be radiated. Thermal radiation includes
light and heat. The thermal radiation travels with the speed of light;
indeed it behaves essentially like light in all respects. This energy is
so intense that it can fuse sand, blind people many miles away, burn
shadows into concrete, burn skin and ignite flammable materials at large
distances. Two pulses of thermal radiation emerge from the fireball. The
first pulse, which lasts about a tenth of a second, consists of
radiation principally in the ultraviolet region. Fortunately, the
ultra-violet rays, which are particularly injurious to living tissue are
strongly absorbed in the atmosphere so that at distances where people
are not killed outright by blast the thermal radiation consists almost
entirely of intense visible light and infra-red rays. The second pulse
which may last for several seconds, carries about 99 percent of the
total thermal radiation energy. The duration of the thermal pulse varies
with the weapon yield, for 1KT it is about 0.3 seconds, and for 10MT it
is about thirty seconds. It is this radiation that is the main cause of
skin burns and eye injuries suffered by exposed individuals and causes
combustible materials to break into flames.
Because thermal radiation is similar
to, and behaves in much the same way as light, it is difficult to
predict the effects of thermal radiation under any particular set of
circumstances. Fog, rain smoke and the like will inhibit the
transmission of thermal energy. Again the magnitude of the thermal
effects depend upon the size of the weapon. As previously mentioned
about 35% of the total energy released in a nuclear detonation is
emitted in the form of light and heat radiation which can cause fires and
skin burns out to considerable distances. To get an idea of the effects
that might be caused, a weapon of 100KT, exploded in a clear atmosphere,
will at a distance of 1.5 miles (2.4km), have fifty times the effect of
the sun.
Prompt thermal radiation is emitted in two pulses. The first is very
short in duration (micro - milliseconds) and contains only approximately
one percent of the total energy yield of the weapon. The second, longer
pulse, accounts for about one third of the total yield. The duration of
the two pulses is proportional to the weapon yield, ranging from less
than a half second for a 1 KT blast to around half a minute for a 10 MT
yield.
The intensity of the direct thermal radiation received at any location
may be enhanced, in a way similar to that of visible light, by reflection
and scatter from clouds or from fog and dust particles in the
atmosphere, or it may be reduced by absorption in passing through thick
fog or heavy atmospheric pollution.
In a clear atmosphere, the amount of heat which would fall on a person
exposed to radiation from a nuclear detonation would decrease rapidly
with their distance from the fireball: it would be decreased by a factor
which is the inverse of the square of that distance, i.e. if the
distance is trebled they would get only one-ninth as much radiation. In
practice, the atmosphere would contain some mist, dust and industrial
pollution; the actual conditions at the time and their position, in
relation to clouds of these substances in the air and to the fireball,
would determine whether they would receive more or less radiation than
would be calculated from the “inverse square" law. There is no simple
scaling law for determining the thermal effects produced by weapons of
different powers.Thermal double pulse
Thermal radiation contacting any object is either reflected from,
absorbed by, or transmitted through that object. The absorption is what
causes damage. Absorption levels depend upon the objects consistency,
colour, shape, etc. Dark objects will absorb more than light ones.
Smooth, highly polished objects are more reflective than rough surfaced,
porous materials, and so on. Absorption, put very simply, increases the
temperature of the object which is absorbing. Dark coloured objects are
more likely to catch fire than white or light coloured ones. Increased
temperatures can cause bums, ignite combustible materials, and melt some
materials. Other factors influencing the effects of thermal radiation
are the attenuation factors: absorption and scattering of the air. The
attenuation of thermal effects are related to the square of the
distance. For example, the thermal energy present 1 kilometre from a
nuclear explosion is four times greater than that felt at 4 kilometres.
Ultraviolet (UV) energy, because of its short wavelength, is especially
susceptible to being absorbed by atoms and molecules in the air.
Re-radiation of the UV is likely to occur after absorption. such
re-radiation is at longer wavelengths, chiefly in the infra-red range.
However, it would be in all directions thereby ‘diluting’ the
concentration in any given area. Attenuation of the UV is particularly
important to biological survival, as it is more harmful than the
infrared or visible light forms of energy. Attenuation by scattering is
simply the diffusion of the radiation as it encounters particles in the
air or obstacles between the blast and the area of concern. Attenuation
depends on the concentration and size of the particles and the wave-
length of the rays. UV, infrared (IR), and visible rays will all
attenuate differently, but for analytical purposes, a uniform
attenuation across the spectrum is assumed.
There is a misconception that visibility of an object from the point of
detonation is a prerequisite for thermal impact, it is stressed that
decreased visibility will attenuate, but not stop thermal radiation.
Even opaque materials can pass the harmful radiations. Also, even if
protected by an obstacle which does not pass any radiation, the
scattering of rays does attenuate the "straight line" strength but, at
the same time, allows the rays to "go around corners."
At a point at any specified distance from the point of burst the total
quantity of heat delivered is roughly proportional to the yield of the
weapon for the same atmospheric conditions. What happens to a surface
which receives thermal radiation depends upon how much thermal radiation
it receives and how fast it receives it.
Two groups of effects are normally considered when describing the
effects of thermal radiation. The first is the effects on buildings and
the like, and the second the effects on people. At Hiroshima and
Nagasaki, it was found that 20-30% of the deaths were due to heat flash,
that is the effects of the immediate thermal radiation, rather than due
to the effects of burns from fires caused by the weapons.
Thermal effects on structures
Thermal radiation from nuclear weapons
has the power to ignite flammable substances, because of the intensity
of the radiation this is a far greater hazard than is the case of
conventional weapons. Obviously material in the open is at greater
risk. Some simple measures can reduce the risks. Flammable material
within buildings can also be ignited through glass and open doors.
Painting windows and the like white significantly reduces the chance of
flash ignition, remember that the blast wave associated with an
explosion travels very much slower than the heat flash, and they will
still be intact for some time. Moreover the range at which thermal
radiation is hazardous is considerably more than that of blast.Inverse
Square Law
Primary fires
Primary fires
would result from heat flash through windows, open doors, etc., igniting
the combustible contents in buildings, wooden door and window frames,
and dry materials in the open such as crops. An obvious fire
precaution would be to rearrange the furnishings or equipment and to
remove all inflammable material out of the direct path of any heat rays
that might enter through windows or other openings. Another very
important precaution would be to whitewash windows and skylights as this
would keep out about 30% of the heat radiation. Whilst windows might be
broken by the blast wave it is important to remember that this travels
far more slowly and would arrive after the heat flash had passed, except
of course in the central area of complete destruction where it would be
of no consequence. These precautions apply to windows and other openings
with a direct view of some part of the sky. In a built-up area they
would apply more particularly lo the windows of upper floors: even from a
high air burst the buildings would have a considerable shielding effect
on one another.
Secondary fires
Secondary fires might be the
consequences of blast damage, scattering of domestic fires, rupture of
gas pipes or short-circuiting of electrical wiring. In the home these
risks could be reduced if simple precautions were taken on receipt of a
warning, such as shutting of stoves, covering open fires with sand or
earth and by turning off gas and electricity at the mains.
Fire storms
The chief feature of a fire storm is
the generation of high winds which are drawn into the centre of the fire
area to feed the flames. These in-rushing winds prevent the spread of the
fires outwards but ensure almost complete destruction by fire of
everything within the affected area. A fire storm inevitably increases
the number of casualties since it becomes impossible for people to
escape by their own efforts and they succumb to the effects of
suffocation and heat stroke. Fire storms occurred several times due to
HE bombing in WWII, and also in the case of the Nagasaki bomb, but not
in the case of Hiroshima. Fire storms seem to be unlikely in a nuclear
attack on the UK, largely because of the materials used in the
construction of modern city buildings.
Thermal effects on humans
Thermal radiation can cause burns
either directly, by absorption Thermal effects on humansof the radiant
energy by the skin, or indirectly by heating or ignition
of clothing, or as a result of fires started by the radiation. The direct
burns are frequently called “flash burns," since they are produced by the
flash of thermal radiation from the fireball. The indirect bums, otherwise
known as secondary burns are referred to as contact burns or flame burns;
they are identical with skin burns that result from touching a hot
object or those that would be caused by any fire. In addition,
individuals close to ground zero may be burned from hot debris,
gases, and dust.
A skin burn is an injury caused by an increase in skin temperature
resulting from direct absorption of thermal radiation. For example, a
skin
temperature of 70°C for a fraction of a second will produce the same
type of burn as a temperature of 48°C for a few minutes.The severity of
burns depends on the amount of the temperature increase and on the
duration of that increase. The colour of skin is also a factor, darker
skin absorbs more of the incident energy.
Skin burns are generally classified as first, second, or third degree, in
order of increasing severity of the burn. This is a somewhat limited way
of characterising burns since it is not possible to draw a sharp line of
demarcation between first-and second-degree, or between second-degree and
third-degree burns, as it makes no reference to the area of the body
surface involved, and hence the degree of incapacity of the
victim. Within each category the burn may be mild, moderate, or
severe, so that upon preliminary examination it may be difficult to
distinguish between a severe burn of the second-degree and a mild
third-degree burn. Subsequent pathology of the injury, however, will
usually make a distinction possible.
It is obvious, that the duration of the heating is as important as the
total amount of heat in causing skin burns since the temperature of a
surface will not increase if its rate of dissipating heat is greater
than the rate of heating, and that a burn affecting a large area is of
more significance than one affecting a more limited area. It is
therefore necessary to consider three important factors:
- the total amount of heat,
- the area on which it falls and
- duration of application of this
quantity of heat to the surface. The pain associated with skin burns
occurs when the temperature of certain nerve cells near the surface
is raised to 43°C or more. If the temperature is not
sufficiently high or does not persist for a sufficient length of
time, pain will cease and no injury will occur. The degree of pain
is not directly related to the severity of the burn injury, but it
can serve a useful purpose in warning an individual to evade part of
the thermal pulse from a nuclear explosion.
First-degree burns
First-degree burns, are characterized
by immediate pain and by ensuing redness of the affected area. The pain
continues even after the temperature of the skin has returned to normal.
The first-degree burn is regarded as a reversible injury; that is to say,
healing is complete with no scar formation.
Second-degree burns
Second-degree
burns, result from skin temperatures that are higher and/or of longer
duration than those causing first-degree skin burns. The injury is
characterized by pain which persists, and may be accompanied either by
no immediate visible efiect or by a variety of skin changes including
blanching, redness, loss of elasticity, swelling, and development of
blisters. After 6 to 24 hours. a scab will form over the injured area,
The scab may be flexible and slightly pigmented, if the injury is
moderate, or it may be thick, stiff, and dark, if the injury is more
severe. Such wounds heal within one to two weeks unless they are
complicated by infection. Second-degree burns do not involve the full
thickness of the skin, and the remaining uninjured cells may be able to
regenerate normal skin without scar formation.
Third-degree burns
Third-degree burns, result from even
higher skin temperatures or those of longer duration Pain is
experienced at the peripheral, less injured areas only, since the nerve
endings in the centrally burned areas are damaged to the extent that
they are unable to transmit pain impulses. Immediately after suffering
the burn, the skin may appear either normal, reddened, or charred, and
it may lose its elasticity. The healing of third-degree burns is more or
less protracted and will always result in scar formation unless new skin
is grafted over the burned area. The scar results because the full
thickness of the skin is injured, and the skin cells are unable to
regenerate normal tissue.
The depth of the burn is not the only factor in determining its effect
on the casualty, the extent of the area of the skin which has been
affected is also important, as is the length of exposure. A first-degree
burn over the entire body may be more serious than a third-degree burn
covering a small area. The larger the area burned, the more likely is
the appearance of symptoms involving the whole body, including surgical
shock and organ failure. There are also certain critical, local regions,
such as the hands, where almost any degree of burn will incapacitate the
individual to some degree.
Persons exposed, in the open, even at a considerable distance from
ground zero would be liable to burns injuries, in the case of a 1MT
explosion this might be up to to 20 kilometres. Burns would be
predominantly to the face and hands. Simple precautions could result in
the reduction of burns casualties, sheltering behind even a flimsy
structure would cause a significant reduction in risk, provided that it
was not, in itself, flammable, likewise keeping as much of the body
covered with clothing would have considerable benefits, and lighter
coloured clothing being the best.
Skin burns under clothing, which depend on the colour, thickness and
nature of the fabric, can be produced in the following ways: by direct
transmittance through the fabric if it is thin and merely acts as an
attenuating screen; by heating the fabric and causing steam or volatile
products to impinge on the skin; by conduction through the hot fabric to
the skin; or the fabric may ignite and hot vapours and flames will cause
burns where they impinge on the skin some textiles such as nylon will
also melt and adhere strongly to the skin. These burns generally involve
deeper tissues than the flash burns produced by the direct thermal pulse
on bare skin. Flame burns caused by ignited clothing also result from
longer heat application, and thus will be more like burns due to
conventional conflagrations.
Whilst simple first aid procedures will improve the survival rate of
burns casualties, it is unlikely that those with third degree burns
would survive even with medical intervention due to the lack of blood
and plasma for transfusion. Casualties with second degree burns covering
even a relatively small area of the body are unlikely to survive without
medical treatment.
Burns of certain areas of the body, even if only relatively minor, will
frequently result in incapacity because of their critical location. Any
burn surrounding the eyes that causes occluded vision, e. g., because of
swelling of the eyelids, will be incapacitating. Burns of the elbows,
knees, hands, and feet produce immobility or limiting of motion as the
result of swelling, pain, or scab formation, and will cause
ineffectiveness in many cases. The occurrence of burns of the face,
neck, and hands are probable because these areas are most likely to be
unprotected. Second-degree or third-degree burns in excess of 20%
of the body's surface area should be considered major burns and would
require special medical care normally, this would be unlikely to be
available under conditions of nuclear attack. If the nose and throat are
seriously involved and obstructive oedema occurs, breathing may become
impossible and tracheotomy may be required as a life-saving measure.
Range of
heat effects on people exposed in the open, radii in miles
(kilometres)
|
Weapon
power
|
20
KT
|
100
KT
|
500
KT
|
1
MT
|
2
MT
|
5
MT
|
10
MT
|
|
Ground burst weapons |
3rd degree burns,
charring of skin
|
0.875
(1.4)
|
1.5
(2.4)
|
3.5
(5.6)
|
4.75
(7.6)
|
6.25
(10.1)
|
9.5
(15.3)
|
13
(20.9)
|
2nd degree burns, blistering
of skin
|
1(1.6)
|
2
(3.2)
|
4
(6.4)
|
5.5
(8.8)
|
7.5
(12.1)
|
11
(17.7)
|
15
(24.1)
|
| 1st
degree burns reddening of skin |
1.5
(2.4) |
3
(4.8) |
5.5
(8.8) |
8
(12.9) |
11
(17.7) |
15
(24.1) |
21
(33.8) |
|
Air burst weapons |
| 3rd degree burns,
charring of skin |
1.375
(2.2)
|
2.5
(4)
|
6
(9.7)
|
8
(12.9)
|
11
(17.7)
|
16
(26)
|
22
(35)
|
| 2nd degree burns, blistering
of skin |
1
(1.6)
|
3.5
(5.6)
|
7
(11.3)
|
9
(14.5)
|
13
(21)
|
18
(29)
|
25
(40)
|
| 1st degree burns, reddening
of skin |
2.5
(4)
|
5
(8)
|
9
(14.5)
|
13
(21)
|
17
(27)
|
25
(40)
|
35
(56)
|
It is not a simple matter to predict distances at which burns of
different types may be expected from a given explosion. Apart from
radiant exposure, the probability and severity of the burns will depend
on several factors. One of the most important is the absorptive
properties of the skin for thermal radiation. ln a normal population,
the fraction of the radiation energy absorbed may vary by as much as 50
percent because of differences in skin pigmentation. For thermal
radiation pulses of 0.5 second duration or more, meaning those from
weapons in excess of 1 kiloton, the energy absorbed by the skin, rather
than the radiant exposure, determines the extent of the burn injury. The
spectral absorption of the skin, i.e., the fraction of the incident
radiation energy (or radiant exposure) that is absorbed, depends on the
skin pigmentation. In fact, people with very dark skins could
receive burns from approximately two-thirds the incident radiant energy
that will cause similar burns in very light-skinned people.
Shock
Shock describes a generalized state of
serious circulatory inadequacy, resulting from a variety of injuries. If
serious, it will result in incapacitation and unconsciousness and if
untreated may cause death. Third-degree burns of 25 percent of the body
and second-degree burns of 30 percent of the body will generally produce
shock within 30 minutes to 12 hours and require prompt medical
treatment. Such treatment is complicated and causes a heavy drain on
medical personnel and supply resources, consequently it is unlikely to
be available under attack conditions.
'Square Leg'
The 1980 Home Office 'Square Leg'
exercise was based on an attack covering both military and civilian
targets involving 125 detonations totaling 196.5MT, and predicted 2.5
million deaths due to burns. The figure in 2024 would be 3 million.
