Nuclear Weapons
Effects
The effects of a nuclear war that
cannot be calculated are at least as important as those for which
calculations are attempted. Moreover, even these limited calculations
are subject to very large uncertainties.- The Effects of Nuclear War (1979) United States. Congress. Office of Technology Assessment.
This page considers the effects of nuclear weapons, it is much simplified for sake of brevity. More information is available elsewhere on this site, and more detailed accounts can be found in some of the sources referenced below are advised.
Explosions
An explosion results when a relatively large amount of energy is released in a relatively small volume, it doesn't much matter whether the explosion is caused by a chemical reaction as in a High Explosive (HE) detonation, or a nuclear reaction, the overall effects are much the same with the exception that nuclear weapons release nuclear radiation. An explosion produces large amounts of very hot gases, which expand rapidly, the major physical effects being due to both the heat and the rapid expansion. It is often suggested that a key difference is in the explosive power of the different weapon types, this is not strictly true. Whilst deliverable nuclear weapons have the potential to have greater powers than HE ones, there have been many HE weapons developed that are more powerful than smaller nuclear devices. It is also commonly said that only nuclear weapons produce electromagnetic pulse effects (EMP), this is not true, the causes may be different, but chemical explosives do produce a small EMP. A key difference between a conventional HE explosion and a nuclear explosion is that in the case of a chemical explosion all the atoms present prior to the explosion are present in the products. In the case of a nuclear explosion there are a whole range of elements and isotopes present in the products that were not there prior to the detonation.
Weapon sizes and bomb power/yield
The power of a nuclear weapon is the
total energy released during its explosion, including all forms of
energy. The units used are comparative. A 1 kiloton (1kt) unit is
equivalent to the energy released by the explosion of 1,000 tons of
T.N.T.1KT ~ 1012 calories
~ 4.2x1019 ergs
~ 4.184 terajoules (4.184×1012 J)
~ 3.1x1012 ft.lbs
~ 1.15x106 kwh
~ 1.8x109 btu
~ to the energy released by burning 350 tons of coal
and comes from the fission, in a uranium fueled weapon, of 1.45 x 1023 nuclei or about 25 grams of 235U.
The average person will have little idea of what a kiloton explosion means, the weapon dropped on Hiroshima is estimated to have had a yield of 15kt, and that on Nagasaki, 21kt. These early weapons were remarkably inefficient. The weapon used at Nagasaki, Fat Man, contained about 5kg of 239Pu of which only about 525 grams actually fissioned, equivalent to only 10.5% efficiency. In cold war terms, it was a relatively small weapon. In Civil Defence Corps training this is what was referred to as a 'nominal weapon'.
With the arrival of hydrogen or thermonuclear weapons, in the early 1950s, a new unit was needed, this was a thousand times greater than the kiloton and is known as the megaton (Mt). Weapons have now been tested up to and including a single bomb of 50 Mt (Tsar Bomba), although it is believed that the Soviet Union built bombs with potential yields as high as 100 MT.
Cube root law (of weapon power)
As a result of calculation, modified where necessary by the results of tests in the 1950s and 60s, various scaling laws have been developed by the relevant bodies in the UK and elsewhere, however most effects follow the cube root law.
The power of a weapon is the total energy released upon detonation, thus a 10 Mt weapon is 500 times as powerful as a 20 kt weapon, and releases 500 times as much total energy, including blast, thermal and radioactivity. The cube root of 500 or ∛500 is nearly 8 and it has been found that, if we compare these two weapons, the peak overpressure at distances is also different by a factor of approximately 8. Thus the peak overpressure at 1 mile from a 20kt weapon will be the same as that at 8 miles from a 10Mt device, assuming all other factors are equal. It is possible to use the same scaling law for many of the weapons effects, and indeed this is the basis of a number of weapons effects disk computers used for civil defence purposes.
Weapons Effects
It must be understood that there are always difficulties regarding quantification of the effects of specific weapons detonations. It must be remembered that nuclear weapons have only been used in war twice, and that there were a number of characteristics which were similar in both cases. The first is that the weapons were of a similar size - 15kt & 21kt, both were low air-burst detonations - 580 metres & 500 metres. In neither case were the populations prepared in any way for what happened, although there had been major bombing raids producing firestorms, in the preceding weeks on other cities in Japan. The heights were about optimal to maximise blast and heat effects, but to minimise fallout.
Some of the effects of nuclear weapons are similar to those of conventional high energy bombs, namely blast and shock waves, and to a lesser extent light and heat flash, and even less so electromagnetic pulse.
The energy of a nuclear explosion is released in a number of different ways:
- an explosive blast, which is qualitatively similar to the blast from ordinary chemical explosions, but which has somewhat different effects because it is typically so much larger;
- direct thermal radiation, most of which takes the form of visible light;
- direct nuclear radiation;
- the creation of a variety of radioactive particles, which are thrown up into the air by the force of the blast, and are called radioactive fallout when they return to Earth;
- pulses of electrical and magnetic energy, called electromagnetic pulse (EMP).
The detonation of both fission and fusion weapons leads to the release of enormous amounts of energy in a very short time, and in a relatively small amount of matter. As a result the temperature of the bomb components, including all the products of the detonation, rises to a temperature much higher than the centre of the Sun, that is of the order of 10,000,000°C. Compare that with the temperatures reached in conventional explosions, which are in the region of 3,000 to 5,000°C. Because of the extreme heat all the materials that make up the weapon are converted to gas, but confined in a very small volume, this means that the pressures are enormous, maybe of the order of 1,000,000 times normal atmospheric pressure.
Burst classification
Because nuclear threat factors are a function of the height of burst, explosions are classified as one of the four: subsurface, surface, air, or high altitude. For example, blast, shock, and thermal threats are more significant from a surface burst than from a high altitude burst. EMP, on the other hand, is a greater concern as a result of a high altitude detonation.
- A subsurface burst is one in which the weapon is detonated beneath the ground or under the surface of water. A fully contained subsurface burst is one in which the fireball does not reach the surfacw.
- A surface burst is one which occurs either on the earth's surface or slightly above. The allowable distance above the surface which will differentiate between a surface burst and an air burst is determined by the size of the fireball.
- An air burst is one when the altitude of detonation is such that the burst is within the atmosphere (under 100,000 feet - approximately 19 miles or 30.5 kilometres), and the fireball, at its greatest intensity no longer touches land or water. A fireball can grow to over one mile across at its maximum brilliance, requiring a detonation altitude of over 2,500 feet to be an air burst.
- A high altitude burst is generally defined as one which occurs above 100,000 feet (above the altitude where there is any significant atmosphere).
Ground Zero
"Ground zero" refers to the point on the earth's surface immediately below (or above) the point of detonation. In some publications, ground zero is called the "hypocentre" of the explosion. Ground zero is commonly abbreviated as GZ.
Total casualties
Numerous attempts have been made to estimate the number of casualties likely in a nuclear attack on the UK, mostly during the Cold war for the purposes of civil defence planning. None of the figures can be said to be conclusive, but all are frightening. It is difficult to show the effects of an attack on the UK based on available data, as the various figures produced at different times were based on a wide range of different criteria. Even in the worst case scenario envisioned by the UK Home Office, there would still be a few millions of survivors, and that is with minimal civil defence type activity, some estimates suggest that the number of survivors could be tripled if there were an active level of civil defence such as there had been prior to 1968.
The effects of population distribution
For the majority of the Cold War, UK civil defence policy was predicated on the basis of not evacuating the population from areas at high risk of attack or of high population density, this was in contrast to WWII, when children were evacuated from London and other major cities, in the last few days prior to the outbreak of hostilities. The Home Office's own computer based predictions suggested that with limited dispersal, aimed at producing a more uniformly distributed population, in geographical terms, might result in some 9 million fewer casualties. This is a best case. The situation is complicated by the fact that the figures would be very different for a night time attack and a daytime one, city populations grow considerably during the daytime. Figures for London suggest that the population of London grows by about two million during the day, or about 1/5th, and that this growth is largely concentrated in Westminster, the City of London and Camden. The higher the density of population, the larger the number of casualties.
Synergism (combined effects of injury):
In other related pages each of the causes of injury and death (blast, nuclear radiation, and thermal radiation) has been considered in isolation. When calculating the numbers of casualties it is customary to consider for any given range, the effect most likely to kill people and its consequences are calculated, while the other effects are ignored. It is obvious that combined injuries are possible, but there are no generally accepted ways of calculating the probability of the outcome. What data do exist seem to suggest that calculations of single effects are not too inaccurate for immediate deaths, but that deaths occurring some time after the explosion may well be due to combined causes, and hence are omitted from most calculations. Some of the obvious possibilities are:
Nuclear Radiation Combined With Thermal Radiation:
Severe burns place considerable stress on the blood system, and often cause anemia. Nuclear radiation reduces the ability of the haematopoietic tissues to produce sufficient blood cells. A sub-lethal radiation dose could make it impossible to recover from a burn that, without the radiation, would not cause death. It must be remembered that in the event of nuclear attack that there would be insufficient blood stocks for transfusions, even if the medical and technical staff were available to do them, there are times during peace when the NHS gets very low on certain blood types.
Nuclear Radiation Combined With Mechanical Injuries.
Mechanical injuries, the indirect results of blast, take many forms. Flying glass and the like will cause puncture wounds. Winds may blow people into obstructions, causing broken bones, concussions, and internal injuries. Persons caught in a collapsing building can suffer many similar mechanical injuries. There is evidence that all of these types of injuries are more serious if the person has been exposed to radiation, particularly if treatment is delayed. Damage to the circulation will clearly make a victim more susceptible to blood loss and infection. The number of prompt and delayed (from radiation) deaths both increase over what would be expected from the single effect alone.
Thermal Radiation and Mechanical lnjuries.
There is little information available about the effects of this combination, beyond the common sense observation that since each can place a great stress on a healthy body, the combination of injuries that are individually tolerable may subject the body to a total stress that it cannot tolerate. Mechanical injuries should be prevalent at about the distance from a nuclear explosion that produces sub-lethal burns, so this synergism could be an important one.
In general, synergistic effects would be most likely to produce death when each of the injuries alone is quite severe. Because the uncertainties of nuclear effects are compounded when one tries to estimate the likelihood of two or more serious but (individually) nonfatal injuries, there really is no way to estimate the number of victims. A further dimension of the problem is the possible synergy between injuries and environmental damage. To take one obvious example, poor sanitation (due to the loss of electrical power and water pressure) can clearly compound the effects of any kind of serious injury. Another possibility is that an injury would so incapacitate a victim that they would be unable to escape from a fire.
Sources
Advisory Committee on Human Radiation Experiments - Final Report (April 1994)
DOE Document NV 0060036: Effects Of Nuclear Weapons Chapter IX Residual Nuclear Radiation (1960 Chapter Revision)
DTIC AD0476572: Biological and Radiological Effects of Fallout From Nuclear Explosions. Chapter 1: The Nature of Fallout. Chapter 2: Formation of Fallout Particles: Defense Technical Information Center.
Nuclear Weapons Effects Canada Emergency Measures Organization (1963)
DTIC AD0410522: Fallout and Radiological Countermeasures, Volume 1: Defense Technical Information Center
DTIC AD0439332: Protection Factors of Emergency Shelters in a British Residence: Defense Technical Information Center
Glasstone Samuel & Philip. J. Dolan: The Effects Of Nuclear Weapons (1977)
British Medical Association's Board of Science Education: The Medical Effects of Nuclear War (1983)
UK Home Office: Civil Defence Handbook No.10 Advising the Householder on Protection Against Nuclear Attack (1963)
