黑洞社区

News item, 23 August 1946

THE ATOMIC BOMBS THEIR EFFECTS AT HIROSHIMA AND NAGASAKI

Just over a year ago, on August 6, 1945, an American aeroplane, flying at 30,000 ft., dropped a single atomic bomb over the Japanese mercantile city of Hiroshima. The bomb exploded over the centre of the city. Three days later another aeroplane, flying at the same height, dropped a second atomic bomb over the factory area of the industrial city of Nagasaki. From the effects of these attacks it was evident that the destructive potential of bombing from the air had assumed a character and scale beyond anything previously experienced by man.

In November of the same year a British mission, comprising scientists, engineers and medical men visited Japan, and what they saw and learned three months after the bombing is summarised in a report recently published.

The report opens with brief general descriptions of the two Japanese cities before and after the respective attacks, and goes on to deal in some detail with the blast effects, the damage to commercial and industrial buildings, houses and to public services. It is from this latter section of the report that we take the following extracts:-

Mass Distortion. - It is usual for a bomb to damage only part of a large building, which may then collapse further under the action of gravity. The blast wave from the atomic bomb, however, was so large that it engulfed whole buildings, pushing them askew. The effect, which occurred with all types of buildings, resembles damage done by wind, and operates somewhat in the same way.

Infrequency of Blast Suction.-  After the blast pressure has fallen from its peak back to zero, there always follows a period of suction (unrelated to the wind suction just mentioned). Although this suction is weaker than the original pressure, it lasts several times as long, and therefore normally does much damage to objects which had no time to fail under the usually brief initial pressure. Pressures from the atomic bomb, however, lasted long enough to give windows, doors, walls, and even chimneys and telegraph poles time to fail. As a result, effects which could be ascribed to blast suction were unusually scarce, although a few were observed in Hiroshima.

Downward Thrust. - Because the explosion was high in the air, much of the damage was due to downward pressure. Most characteristic was the 鈥渄ishing鈥� of the flat roof slabs of reinforced concrete buildings, some of which assumed a saucer shape. For the same reason, telegraph and other poles remained upright immediately below the explosion, but were overturned or tilted at greater distances from the centre of damage. Trees below the explosion remained upright, but had their branches torn downward. 

Screening from blast by large features, and similar effects, were not unusual; for example, almost the whole of the smaller valley in Nagasaki was screened by the intervening mountain ridge. The reflection of blast, which adds considerably to its force, was unusually marked, but important only in special cases, among them some bridges. Finally, there may be remarked the absence of the carriage of any heavy debris over large distances. Small debris such as tiles and battens appeared to have been carried considerable distances, and was found on tall buildings. But larger pieces of debris were always found close to their point of origin, and where massive slabs such as bridge decking had been shifted, the movement, although on occasion critical, was small.

Damage to Houses.

The bulk of the damage in both cities, naturally, was to Japanese houses. These houses are constructed on a frame of 4 in. or 6 in. square timbers. The roofs are not trussed in the orthodox manner and are a source of weakness, particularly since their covering of pantiles bedded in mud on 0.5 in. boarding is disproportionately heavy. The walls are of bamboo covered with 3 in. of mud, which is sometimes protected by 0.25 in. boarding; but much of the wall space is occupied by paper-covered screens. Complete collapse of these buildings from blast extended to 1.25 miles from the centre of damage in Hiroshima, and to an average of 1.5 miles in Nagasaki. Fire completed the destruction almost to the same distance, except in one congested area of Nagasaki, where it exceeded it. Beyond the region of complete collapse damage decreased rapidly, the further zone in which houses had been damaged beyond repair being little more than a quarter of a mile wide. Minor damage extended to large distances, 3 miles or more from the centre of damage.

The major interest outside Japan is the behaviour of unframed brick buildings with load-bearing walls, which make up the bulk of European housing. These buildings are rare in Japan, and those which were found differed in important respects from British housing. The mission had therefore to draw its conclusions from isolated examples. One such building, although somewhat stronger than British houses, had collapsed at 700 yds. from the centre of damage. Interpreting such examples in the light of its European experience, the mission estimated that a bomb of the same power, exploding at the approximate height of those in Hiroshima and Nagasaki, 

(1) would cause the collapse of normal British houses to a distance of 1,000 yds. from the centre of damage; (2) would damage them beyond repair to a distance of 1 mile; (3) would render them uninhabitable without expensive repair, particularly to the roof timbers, to a distance of 1.5 miles; (4) and would render them uninhabitable until first-aid repairs had been carried out to a distance of 2 to 2.5 miles from the centre of damage. 

Unframed masonry construction with load-bearing walls of greater thickness is also widely employed throughout Europe for public buildings and blocks of flats. Such buildings are subject to damage of equal severity, at smaller but considerable distances, as in the case of the monumental Roman Catholic Cathedral of Nagasaki, 600 yds. from the centre of damage. Here the damage was completed by fire and other causes, but effectively the building had already been destroyed by blast.

Air-raid Shelters.

The provision of air-raid shelters throughout Japan was much below European standards. Those along the verges of the wider streets in Hiroshima were comparatively well constructed: they were semi-sunk, about 20 ft. long, had wooden frames, and 1 ft. 6 in. to 2 ft. of earth cover. Exploding so high above them, the bomb damaged none of these shelters.

In Nagasaki there were no communal shelters except small caves dug in the hillsides. Here most householders had made their own backyard shelters, usually slit trenches or bolt holes covered with a foot or so of earth carried on rough poles and bamboos. These crude shelters nevertheless had considerable mass and flexibility, qualities which are valuable in giving protection from blast. Most of these shelters had their roofs forced in immediately below the explosion; but the proportion so damaged had fallen to 50 per cent. at 300 yds. from the centre of damage, and to zero at about half a mile. 

These observations show that the standard British shelters would have performed well against a bomb of the same power exploded at such a height. Anderson shelters, properly erected and covered, would have given protection. Brick or concrete surface shelters with adequate reinforcement would have remained safe from collapse. Morrison shelter is designed only to protect its occupants from the debris load of a house, and this it would have done. Deep shelters such as the refuge provided by the London Underground would have given complete protection.

The Problem of Fire.  

Fire damage was extensive in both cities, and 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.

The evidence presented 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.

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. Indirect causes undoubtedly produced many fires.

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 bomb鈥檚 initial drying and scorching.

Radioactive Effects.

The gamma rays are capable of penetrating considerable thicknesses of building and other material. They thereby pose new problems of protection, the scope of which is not yet fully understood, and is best illustrated by examples.

An unusually strong three-storey reinforced concrete building at Hiroshima, less than 250 yds. from the centre of the damage, contained 23 people whose fate can be traced. The building remained structurally undamaged, and none of these people was killed immediately; nearly all had some lesser injuries from debris and fire, but were able to walk to hospital for treatment. Subsequently, between the sixth and seventeenth day after the explosion, 21 of these people died, probably all from the effects of gamma rays. The two survivors were in the telephone room on the ground floor, where they were shielded by all the floors and possibly by adjacent buildings.

In a five-storey reinforced concrete building 700 yds. from the centre of damage, gamma rays caused many deaths on the fourth and fifth floors and a few on the third, but killed no one below this who was screened by all the higher floors.

A partly underground concrete shelter less than half a mile from the centre of damage had no gamma ray casualties, or indeed any but minor debris injuries.

At distances approaching 1 mile from the centre of damage, less massive buildings began to protect from gamma rays. For example, in the City Hall of Hiroshima, at this distance, there were no deaths from gamma rays and at most six mild cases of radiation sickness.

Only tentative conclusions can yet be drawn from this and similar information. 

The Report Soberly Concludes. 

The overall picture is sombre. Even ignoring changes in attack and in the development of the bomb, the damage figures given 谩re very serious. And these figures are a measure only of the blast effect of the bomb. With them must be kept in mind the grave problem of fire, direct and indirect, which the bomb h谩s created. Both are overshadowed by the scale of casualties produced by the bomb, which make the mere disposal of the dead a major problem.