Effects of Strategic Nuclear Weapons

Abstract

On August 6th, 1945, a bomb containing 60 kg of uranium 235 was dropped, without previous warning, on Hiroshima. Three days later, Nagasaki was destroyed by a bomb containing 8 kg of plutonium 239. (Barnaby, 1975.) Here the number of victims was somewhat smaller, as the town is situated in hilly country and had fewer inhabitants. According to the latest Japanese estimates, the number of dead in the two towns taken together may have been 250,000. In Hiroshima alone it was 140,000 out of a population of 360,000: nearly 40%. (New Scientist, 15.4.76.) Uncertainty is great as there had been, in the chaotic conditions in the Japan of 1945, an uncontrolled mass influx of people into towns previously intact.The effect of the uranium and plutonium bombs is mainly due to the fact that in nuclear fission 17 million times more energy, per unit weight, is released than in the explosion of the strongest chemical explosives. One of these is trinitrotoluene (TNT, trotyl), which was widely used in World Wars I and II. Nuclear fissions are induced as a chain reaction that requires less than one millionth of a second (microsecond.) (York, 1962, p. 27.) The reaction is transmitted from nucleus to nucleus by the neutrons released in fission with speeds of some 10,000 km/second. Whereas the heaviest chemical bombs of WW II contained about 10 tons of TNT, the explosive power of the Hiroshima bomb corresponded to that of 14,000 tons (= 14 kilotons, Kt) of TNT; the power of the Nagasaki bomb to 20 Kt Bamaby, 1975). Thus the power exceeded that of the strongest bombs used previously by more than a thousandfold. About twothirds of the energy released appeared as mechanical energy of the pressure wave, onethird as thermal energy of the heat radiation, and 6% as energy of the 'ionizing' radiation, i.e. of radiation as produced by radioactive substances and X-ray tubes. The particles of such radiation, including particles of electromagnetic radiation ('photons'), carry enough energy to 'ionize' atoms, i.e. to deprive them of electrons. The number of dead was distributed over these effects in a ratio of about 20 : 60 : 20. Thus heat radiation was most effective; however, because of overlapping a clear-cut distinction is not possible.In Hiroshima, the explosion was triggered by an automatic mechanism 600 m above the 'hypocentre' ('ground zero'). This height had been chosen to maximise the action of blast and heat. At the centre of the explosion, initial pressure must have amounted to millions of atmospheres. Concrete buildings were destroyed up to I km, solid brick buildings collapsed up to 1.5 km, and glass broke up to 13 km from the hypocentre.During the explosion the material of the bomb reached a temperature of many million degrees, similar to that of the interior of the Sun. The energy of the heat was at first given off mainly as soft (long-wave) X-radiation, i.e. as ionizing photons. The X-rays were absorbed easily by air near the place of the explosion, and the air was thereby heated up to a 'fireball' . The temperature at the surfa...