Effects of Nuclear Weapons

3.1 : Introduction

It is a well-established fact that the explosion of a nuclear weapon causes damage through several effects: a powerful blast wave, a fireball causing intense heat and nuclear radiation, and from radioactive fallout. Finally there is a pulse of electromagnetic radiation although this is not thought to be harmful to living creatures.

Thermal radiation and blast are the inevitable consequences of such a near instantaneous release of immense energy in a small volume, characteristic to nuclear explosions. The first energy to be released in a nuclear explosion is the gamma rays produced by the nuclear reactions. These gamma rays have energies in the MeV range, and a significant amount escape from the bomb casing into the outside world at the speed of light.

By a chain of emissions and absorption's x-rays can escape from the hot centre of the bomb in a process called "radiative transport". These x-rays (particularly those at the upper end of the energy range) have substantial penetrating power into matter before absorption. The atoms of this matter will subsequently become excited when this absorption occurs and will therefore re-emit some of these x-rays at the lower end of the energy range. Since the emission and absorption process takes a certain amount of time, and the direction of re-emission is random (i.e as likely towards the centre of the bomb as away), the net rate of x-ray radiative transport is considerably slower than the speed of light.

The fireball is a hot ball of plasma (ionised gas) created when a nuclear explosion heats the bomb and the immediate surrounding environment to very high temperatures. The first observation an observer would make from a nuclear explosion would be a blinding flash of intense white light (enough to cause serious retinal damage), which in fact is emitted from this fireball. As this ball expands, a proportion of energy is radiated away as thermal energy in visible and ultraviolet light. The expansion of this plasma is considerably slower than the radiative transport of the x-rays. The two most important effects of thermal radiation will be to cause "flash" burns on exposed skin and also to ignite fires.

The x-rays heat everything to a near uniform temperature, (initially around 10 million degrees C) in an expanding bubble called the "iso-thermal" sphere. Due to these enormous temperatures, the sphere is incredibly brilliant, with a surface brightness trillions of times more intense than the Sun.

3.3 : Blast Wave Development and Thermal Radiation Emission

As the fireball expands it cools, thus causing the wavelength of the photons transporting energy to also drop. As the wavelength of these photon's increases their penetrating power before absorption drops. Thus the speed of energy transport also drops. When the iso-thermal sphere has cooled to a temperature of around 300,000 degrees C (with a corresponding surface brightness of 10 million times of the Sun), the rate of radiative growth becomes approximately equal to the speed of sound. At this point a shock-wave forms at the surface of the fireball as the kinetic energy of the fast moving ions starts transferring energy to the surrounding air. This is known as "hydrodynamic separation" and will typically occur about 100 microseconds after the initial explosion for a 20 kilo tonne nuclear device. A separate internal shock-wave (i.e internal to the fireball) caused by the rapidly expanding bomb debris may overtake and reinforce the surface shock-wave.

This shock-wave will travel at a velocity of around 30km/sec (approximately 100 times the speed of sound). This compresses and heats the air enormously, up to temperatures of around 30,000 degrees C (five times the Sun's surface temperature), and causing the air to become heavily ionised. Ionised gas is opaque to visible radiation, so therefore the shock-wave acts as an optical shutter to the iso-thermal sphere, causing its temperature to drop rapidly. This rapid drop in thermal energy (ten-fold) occurs approximately 10 milliseconds after the initial explosion for a 20 kilo tonne device, and about 100 milliseconds for a 1 Mega tonne device. However, this is only the "first pulse" which is estimated to emit 1% of the nuclear devices total emitted thermal energy. At this minimum, the fireball of a 20kt bomb is 180m across.

After approximately 15 milliseconds an effect called "breakaway" occurs when the shock wave expands, cools to around 3000 degrees C, and gradually also becomes transparent. The isothermal sphere at a temperature of around 8000 degrees C, becomes visible and both the apparent surface temperature and brightness of the fireball climb to form the "second pulse". This second peak occurs approximately 150 milliseconds after the initial explosion for a 20kt device, and 900 milliseconds for a 1Mt device. After the breakaway effect the shockwave and fireball have no further interaction. Once the fireball has cooled sufficiently it will transform itself into a mushroom shaped explosion cloud. This will occur after approximately one third of the explosive energy has been released as heat.

It is extremely difficult to pinpoint an exact time for the cut-off for the second pulse as the thermal emission rate gradually declines over an extended period. However rough estimates give that 50% of the total thermal emission has been emitted by 300 milliseconds for a 20kt device (1.8 seconds for 1Mt). The rate of emission at this time has also dropped to 40% of the second peak. By around 750 milliseconds for a 20kt device (4.5 seconds for 1Mt) 75% of the total thermal radiation has been emitted. The second pulse, despite not being as bright as the first pulse, still emits 99% of the total thermal radiation simply because of its extended period of emission.

Nuclear explosions produce four types of ionising radiation that can cause significant injury: alpha particles, beta particles, gamma rays, and neutrons. They all share the same basic mechanism for causing injury, mainly through the creation of chemically reactive compounds called "free radicals" which disrupt the normal chemistry of living cells. These radicals are produced when the energetic radiation strikes a molecule in the living tissue, breaking it into ionised fragments. Radiation is produced directly by the nuclear reactions that generate the explosion, and also by the decay of the radioactive products left over, i.e fission debris, or induced radioactivity from captured neutrons.

Surprisingly the initial gamma ray burst (100 nanoseconds after explosion) contributes little to the overall radiation hazard as the bomb casing before it is blown apart absorbs them. The gamma rays that do escape however can cause a great deal of damage as gamma rays are an extremely penetrating type of radiation. They can travel through hundreds of metres of air and through walls of houses. A small part of the gamma ray energy is converted to electromagnetic energy through interaction with the surrounding air. The strong electromagnetic that is briefly developed (less than a millisecond) will cause serious damage to any electronic equipment. Tenth-value thickness for gamma rays (i.e thickness of material to reduce intensity by one tenth) is steel 8-11cm and concrete 28-41cm. After the initial explosion there are a number of fission products with very short half-lives (milliseconds to minutes). The decay of these isotopes can generate intense gamma ray emission that is emitted directly from the fireball. This process will generally be completed within 10 seconds of the initial explosion. The bulk of gamma ray production is produced from the decay of the fission products, particularly so with the larger nuclear devices (i.e1Mt and above).

The initial explosion also emits a brief burst of neutrons at the same time as the gamma ray emission. The neutrons being more penetrating also have a greater chance of escape. Neutrons can cause the "free radicals" disruption of the other three radiation types, but they can also transmute ordinary atoms into radioactive isotopes, thus creating even more ionising radiation inside the body. As the neutrons travel through the air they are slowed by collisions with air molecules and are eventually captured. As the neutrons are slowed by collisions they will lose some of their kinetic energy which is converted into damaging gamma rays. The capture of nitrogen-14 also produces gamma rays.

Radioactive decay is the sole source of alpha and beta particles. Beta and alpha particles have such a short range (beta has several metres in air before absorption, and alpha has just a few cm) that their contribution to radiation only becomes important when considering nuclear fallout.

Fallout is a complex, lethal combination of many different isotopes, of which the composition changes as each isotope decays into another. Fallout originates from the mushroom cloud that is formed when the fireball cools down, sucking a huge column of dust and smoke to form the stem of the mushroom. The cloud is fully developed after a period of around 10 minutes, with a radius of approximately 2-3km, approximately 4km high, and approximately 6km above the ground. Many of the radioactive particles in the mushroom cloud do not constitute a health hazard until they are deposited on the ground as fallout. This process can range from a day to a few years.

As the cloud developed from the fireball it is logical therefore that it should contain most of the fission products that the fireball contained from the initial explosion. The combined radioactivity of these fission products one hour after the explosion is equal to several thousand tons of radium. This activity however declines rapidly, and after two weeks the activity is one thousandth of the activity after the first hour. However, some of the nuclides may have half-lives which can range from thousands to hundreds of millions of years.

Some of the more immediate fallout originates from when the bomb explosion injects thousands of tons of soil into the hot vapours of the fireball. Large particles (radii of 0.5mm) carry a significant part of the residual radioactivity and fall to the ground within a matter of minutes or at most a few hours. This creates intense radioactive fallout that can potentially give lethal radiation doses to people over an area of 50-100km². This effect occurs far more predominately when the nuclear weapon is exploded near to the ground as opposed to an airburst.

It is a well-known fact that any nuclear explosion, even a small one such as the device used on Hiroshima (13kt) would cause an immense amount of casualties both short and long term. Although it is possible to analyse in great detail the effects of a nuclear exchange on a super-power, it is estimated that around 500,000 to 1,000,000 civilians would die if a device the size used on Hiroshima was used on New York. Of these around 200,000 would be killed instantly, whilst the rest would die from radiation poisoning.