Report of the British Mission to Japan

FLASHBURN

48. It has been explained in paragraph 20 that some of the energy which is created in the process of atomic fission is radiated as heat, on a scale immensely larger than the heat flash released by a conventional explosion. A wide range of materials was affected by the high temperature and heat flow which resulted from this heat radiation in Hiroshima and Nagasaki. These effects on any material subject to scorching are called flashburn,

49. Photograph 21 shows the manner in which heat has roughened the surface of polished granite, which retains its polish only where it has been shielded from the radiation (which travels in straight lines from the explosion), for example by the man who was evidently sitting on the stone at the instant of the explosion. This roughening is caused by the unequal expansion of the constituent crystals of the stone ; for the quartz crystals this becomes critical somewhat below 600° C. Therefore the depth of roughening and ultimate flaking helps to determine average ground temperatures in the instants following the explosion. Such roughening extended about 1½ times as far in Nagasaki as in Hiroshima.

50. The Japanese commonly employed two kinds of roof tiles, on both of which the heat flash raised characteristic bubbles. These were largest where the radiation was most intense, that is, either where the tile was closest to the explosion or, as in photograph 24, where it faced the radiation squarely. This effect will serve to give independent estimates of ground temperatures. It extended nearly 1½ times as far in Nagasaki as in Hiroshima.

51. Among miscellaneous materials affected by heat radiation were the bronze of the ceremonial animals guarding a Shinto temple, immediately below the explosion in Hiroshima ; the asphalt surface of roads, which retained the •" shadows " of those who had walked there at the instant of the explosion, objects of macabre interest and pilgrimage for visitors ; and concrete and mortar renderings of buildings, which had flushed to various shades of pink.

52. As photograph 22 shows, unpainted wood displayed the most striking flashburns. These ranged from a just perceptible deepening of the natural colour, which could be observed on telegraph poles at nearly two miles from the centre of damage, through progressive scorching to complete superficial charring at small distances, where isolated timbers had sometimes been fired. Painted wood was also scorched, with no discernible selection of colours. In all timbers, the soft portions of the annual rings were most deeply charred.

53. Some paints were also directly affected, the most sensitive being the black coating on gas holders, where, as photograph 23 shows, exposure to heat radiation gave a polished appearance to the original dull and sooty finish.

54. In all these cases, " shadows " were cast by intervening objects where they shielded a part of the otherwise exposed surface from the direct heat radiation. In the shadow, the surface retained its original state ; outside it, across a more or less sharply defined boundary, it was scorched. When neither the shielding object nor the scorched surface had moved, as in photograph 23, it was therefore possible to determine the direction from which the radiation had come. A number of such determinations then served to fix the position of the explosion. This procedure had already been used by the Japanese and others, and there was reasonable agreement between the determinations.

55. Where shadows were susceptible to precise measurement, for example the shadows of vertical or horizontal bars, they were usually found to be narrower than the shielding obstruction. In particular, there could be observed the striking phenomenon of the complete disappearance of the shadows of narrow objects at sufficient distances from the scorched surface. For example, on a bridge in Hiroshima the lowest of three equal and parallel sets of bars had cast a well-defined shadow, and the middle a perceptible one, but the highest none. The main cause of this effect was probably the finite size of the radiating fire ball.

56. There were cases where a dump of grass or the leaf of a tree had cast a sharp shadow on otherwise scorched wood. Therefore the most intense flash from the ball of fire had ended in a time less than that required to shrivel vegetation. On the other hand, since direct injuries to the eye-ball were not common, the heat radiation may be presumed to have required a perceptible time to build up to its maximum intensity, during which some people had closed their eyes.

57. A phenomenon more striking than important, but which attracted wide attention, was the charring of darker patches on fabrics at distances from the explosion at which lighter patches were no longer charred. Among the materials which the Mission examined in Hiroshima, by the courtesy of the Medical Section of the Joint Commission for the Investigation of the Effects of the Atomic Bomb, were:—

• A white cotton blouse the pale pink sleeves of which were patterned with small sprays, each about 3/16 in. in diameter, of green leaves and red flowers. This blouse had been worn in the open well over a mile from the centre of damage, and was unscorched; except that over an area on the left shoulder the sprays alone had been burnt out and had left holes. Over a larger surrounding area the sprays had begun to burn, and here the red flowers had smouldered markedly earlier and more extensively than the green leaves.
• A white dress, unscorched except for its blue polka dots, which had been burnt out over a large area. On the edges of this area the dots had begun to smoulder, each from its centre outward.
• A shirt of alternate dark and light grey stripes, each about ½ in. wide, over an area of which the dark stripes were completely burnt out but the light stripes remained.
• A kimono patterned with white lozenges on a blue ground, large areas of which were burnt and had had to be beaten out. This garment like many others was said to have been fired directly by heat radiation. On the edges of its burnt area, the white lozenges had survived and only the blue ground had been burnt. Considering this in the light of the preceding exhibits, the story of its flash ignition appears reasonable.
• A piece of Japanese paper exposed nearly 1½ miles from the centre of damage, on which characters written in black ink had been neatly burnt out.

These effects are in line with known experimental facts: for example, that black cotton begins to char at temperatures perhaps 30° C. lower than does white cotton. They may be the result of differences in colour, in colouring matter, or both. These effects are interesting but not important, because they can occur only where the heat radiation is falling just below the intensity which would fire the fabric as a whole.

58. Witnesses say that people who were directly under the explosion in the open had their exposed skin burnt so severely that it was immediately charred dark brown or black : these people died within minutes or at most hours. Both in Hiroshima and in Nagasaki, burns on exposed skin were very severe up to about 1,500 yards from the centre of damage. At this distance, some escaped with less severe bums, although very severe burns were occasionally reported at nearly 1½ miles from the centre of damage. Mild burns extended to distances of 2½ miles and more. Stories that white people were unharmed in Hiroshima where the darker skinned Japanese received fatal bums were not substantiated.

59. In general, even thin clothing protected from flashburn. There were a few exceptions, when the skin was burnt through uncharred fabric where the latter was stretched tightly, say over the point of the shoulder. On other occasions, equally rare, clothing caught fire without burning the skin. But in general the correspondence between charred clothing and skin was close ; for example, the girl wearing the flowered blouse described in paragraph 57 had burns, some of them severe, only where the sprays had burnt.

60. Buildings and walls gave complete protection from flashburn. For example, of the men in the prison grounds of Hiroshima, roughly 1½ miles from the centre of damage, all suffered bums except those in or screened by buildings. Of a group of 580 workers marching across a bridge in Hiroshima, nearly 11 miles from the centre of damage, all were burnt except three at the rear of the column, whom subsequent investigation showed to have been screened by the eaves of a building. There were some very severe burns in this group, and nine deaths.

61. The Japanese reported that crops and other vegetation above ground had been completely burnt at 1,000 yards from the centre of damage in Nagasaki. In Hiroshima, some trees had been fired, apparently by flashburn, at greater distances ; flashburn marks on trees in Nagasaki extended to 1½ miles. It was noticeable in both cities; that where trees which had been stripped of foliage were putting out new leaves, the fresh shoots sprang not from the branches but from the trunk.

THE PROBLEM OF FIRE

62. Attention has been drawn to the great extent of fire damage in both cities ; and that fire was not confined to wooden Japanese houses, but raged fiercely in many concrete buildings, in some machine shops, and in other buildings not normally subject to fire. The Mission therefore attached importance to the determination of the causes of fire, particularly in industrial, commercial, and public buildings, in order to assess the risk of fire in similar conditions in western cities. The aim was to determine the relative importance of two possible direct causes of fire—heat radiation, and convection by heated air ; and two indirect causes—damaged appliances, and the spread of fire from adjacent buildings.

63. The evidence presented throughout this chapter, particularly in paragraphs 52, 57 and 61, and similar evidence, leaves little doubt that heat radiation is a cause of fires in unscreened buildings, probably up to distances of 1 mile from the centre of damage. The Mission was most impressed by the accumulation of indirect evidence from the many reinforced concrete buildings the basements, the stairs, or some floors of which had been screened and contained no fire, when floors of the same buildings with windows exposed to the flash were gutted by fire. A number of reinforced concrete buildings in Hiroshima with shuttered windows escaped fire, apparently because the heat radiation, travelling at the speed of light, had arrived and died away before the blast, travelling only at a few thousand feet per second, blew out the shutters to expose the interior. The dense surrounding fires did not spread to these buildings.

64. No evidence could be found of direct fires caused by convected heat alone, that is, by heat transmitted by the air. It is difficult to think how this cause could have been isolated, or that it can be important in the presence of such intense radiated heat.

65. Indirect causes undoubtedly produced many fires. Braziers, widely used in industry as well as in the home, must have started some; some will have begun at gas leaks; and the primitive electric wiring no doubt started others, however rapid was the circuit-breaking system. It was not important to press this part of the enquiry, since experience has shown that the danger of indirect fires always exists in large-scale bombing, in every part of the world.

66. It is certain that firespread did occur in both cities; but more striking is the evidence for vast numbers of separate points of fire, which made fire-fighting among these combustible buildings hopeless from the outset. In incombustible buildings, particularly in the industrial buildings of Nagasaki, the incidence of fire in neighbouring bays was markedly erratic, and fires had not spread.

67. In summary, both direct and indirect fires must be regarded as active dangers from atomic bombs. Indeed, whether radiated heat is or is not an important cause of fire, the high temperatures produced by it plainly create conditions exceptionally favourable to the emergence and continuance of serious fires however caused. For example, the debris of demolished Japanese houses beyond the fire zone which was examined in Hiroshima would rarely have supported fire at ordinary temperatures. Yet it must have been debris in this state which burnt there for days, presumably as a result of the initial drying and scorching by the bomb.

68. Combustible materials in fire-resistant buildings and heavy timber sections in others were burnt with unusual completeness. This is evidence of the long duration of fires even of medium intensity, which were allowed to bum virtually unchecked. It must be appreciated that the conditions of disorganization and the multitude of individual incidents created by the atomic bomb will throw an unequalled strain on any fire services. In addition, the Japanese had provided fuel for the fires by introducing a mass of wooden detail into otherwise fireproof buildings. Photograph 20 shows the interior of one of the reinforced concrete buildings of the hospital in Nagasaki ½ mile from the centre of damage. Having resisted the blast, these buildings and their services were denied to the city at a critical time because they were filled with such material as that shown in the photograph: a false lath and plaster ceiling hung on heavy timbers, a wooden floor raised on wooden beams, and plaster walls on battens and laths. As a result, about half the occupants were killed or were trapped and died in the fires which broke out nearly everywhere among this material. It is a very plain lesson that a fireproof building should not be converted into a major fire risk and a trap for its tenants by ill-chosen fittings.

---