Spark image

Thermonuclear weapons

Much useful energy could be generated by the use of fission and fusion power but there is no doubt that the misuse of the power could see the end of our planet as we know it. I refer to the atomic and thermonuclear bombs. The first atomic bomb was developed at Los Alamos in the United States by an international team of scientists and exploded in the Alamogordo desert in New Mexico on 16th July 1945. A leading statesman at the time quoted from the book of Hosea "They have sowed the wind, they shall reap the whirlwind".

Fortunately for our world only two atomic bombs have ever been dropped in war, on the cities of Hiroshima and Nagasaki in Japan. Recently it has been revealed that during the Cuban missile crisis of 1963 the Cubans were ready to use tactical nuclear weapons against the United States troops should they have launched an attack on the Cuba. It is thought that this would have plunged the world into nuclear war.

Statistics often mean little and sources do not always agree but it seems likely that about 100000 people died at 8.10 am on 6th August 1945 when the first atomic bomb was dropped on Hiroshima, and many thousands more died later as a result of the burns and radiation that they received. Three days later a second bomb was dropped on Nagasaki - 70 000 people died.

The bomb dropped on Hiroshima was 0.71m in diameter, 3m long and had a mass of 4000 kg and a destructive power of 20 000 tons of high explosive. It was exploded 300 m above the ground for "maximum effect".

To get some idea of the power of such weapons just think about the speed at which the fireball from a nuclear explosion travels. It reaches a diameter of 150 m in 0.006s (25 000 ms-1), 300m in 0.016s and 1 km only 1s after the explosion. The temperature of the Hiroshima fireball reached 7000oC - hotter than the surface of the Sun - wooden buildings five kilometres from the explosion were completely obliterated. Solid materials on the ground immediately below the burst reached surface temperatures of up to 4000 oC and even 1000m away they were 1750 oC.

It is true that terrible destruction occurred in Coventry, Munich and Dresden but that was due to continuous bombing. The cities of Hiroshima and Nagasaki were each destroyed by just one bomb and the nuclear stockpile of the early 1990s was some 50 000 warheads, equivalent to 1.5 million Hiroshima bombs. This represented the equivalent of about four tons of high explosive for every man, woman and child on the planet.

Atomic and thermonuclear weapons produce a high degree of fallout, that is, material that has been irradiated and then sucked up by the great up rush of hot air from the explosion and is then deposited over a wide area of the Earth's surface.

One further possible and sobering result of a nuclear war has been suggested by scientists only in the last few years. They suggest that the huge clouds of dust that would be thrown high in the stratosphere would reduce the amount of sunlight reaching the Earth and plunge us into what has become known as the Nuclear Winter. It has been estimated that the mid-summer temperature in the latitude of London would drop below 0oC and even the deserts would eventually freeze. These conditions could be brought about by the detonation of only 10% of the present stockpile of warheads and would last from six months to three years. During this time the sunlight would be reduced to 1% of its present amount, the sea would start to freeze, forests would die and much of the Earth's oxygen producing plants would be destroyed. Life as we know it would cease.

Most modern nuclear weapons are thermonuclear devices - so called H bombs. They rely on a fission bomb to raise hydrogen atoms to a very high temperature in a split second so that they produce a fusion reaction. Much greater destructive powers are possible with such weapons, they are measured in many megatons compared with the 20 kilotons of the Hiroshima and Nagasaki bombs.

What follows is the approximate data for a 10 megaton weapon. Consider a 10 Megaton bomb exploded five miles (8km) away at the optimum burst height. The maximum temperatures, as in all fission devices, just after the explosion would reach several tens of millions of degrees, releasing 4.18 x 1016 J of energy!
The air pressure would rise as the blast wave hit reaching almost double normal air pressure. Such an increase in pressure would cause severe damage to concrete structures and produce lung haemorrhage.

The maximum wind velocity would reach 330 mph (150 ms-1), the blast wave arriving 21s after the explosion. A 75 kg person standing in the open would be blown off their feet, reaching a maximum velocity of 30 ms-1 (66 mph) in the first 3.3 m of travel. (A person hitting a flat vertical surface at only 14 ms-1 would have a 99% chance of being killed.)

The fireball formed would have a maximum radius 1.7 miles and for a person just within the edge of this fireball (1.5 miles from the explosion) the effects would be even more horrific. The blast would reach them in 2.7 s with a maximum wind velocity of 2000 mph. They would literally be blown to pieces.

Anyone within 2 miles of a 10 Megaton airburst would be dead within 2 days due to the effect of the ionising radiation produced. Someone reading a newspaper 12 miles from a 10 Megaton airburst would see the paper catch fire!

If the bomb is detonated on the ground then large craters can be formed. For a 10 Megaton bomb detonated on the ground the crater would be roughly 579m in diameter and 258m deep although of course this would vary depending on the nature of the ground at that point.

Amazingly it has been estimated that out of the present world population of 4800 million people about 1000 million would survive a global nuclear war to start all over again. However there is absolutely no doubt that civilisation as we know it would have disappeared for ever.

Those of you who have stood on a hillside and seen the sunrise in the dawn will understand how precious our world is. My uncle described this well in these three lines from a poem that he wrote:

This is the morning I would not forget
For then we stood in awe
And saw the world created in a day

It is up to us to be sure that we do not see it destroyed - it is likely that there would be no second chance.

© Keith Gibbs