Thermal Radiation 

A conventional explosion produces thermal energy with temperatures of maybe 3,000 Celsius. In contrast, the fireball of a nuclear detonation will typically be as hot as the surface of the sun, of the order of 5,000 to 6,000°C.The centre of the fireball is many times hotter than the centre of the sun (15,000,000°C). 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% 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

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 re?ection 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 ?reball: 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 ?reball, 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 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 ?re 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.

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 frames. 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 of 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 ?reball. The indirect bums, otherwise known as secondary burns are referred to as contact burns or ?ame 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 classifed 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 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.

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 ?ames 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 ?ash 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.

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% 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.

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Fig.1 -Energy proportions
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Fig.1 -Comparison of the release of thermal energy by different power weapons.
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Fig.3 -The double thermal pulse
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Fig.1 -Thermal effects on bodies.