Official Report on the Hiroshima and Nagasaki Atomic Bombings Issued

Official Report on the Hiroshima and Nagasaki Atomic Bombings Issued

The United States dropped two atomic bombs on Japan towards the end of World War II, one on the city of Hiroshima on August 6, 1945 and one on the city of Nagasaki on August 9, 1945. During the American occupation of Japan, the Manhattan Engineer District (relating to the Manhattan Project that developed the atomic bomb) under the command of General Leslie Groves undertook a thorough investigation. The goal was to get a better idea of the potential of this new weapon and the consequences of its use. On June 29, 1946, their report was issued.

One particularly interesting section is the portion that describes the power of an atomic explosion. This sobering passage is set forth below in virtually its entirety, and it should be kept in mind that the weapons used at Hiroshima and Nagasaki were mild by modern standards. Both of the atomic bombs had an explosive yield of roughly 20 kilotons (20,000 tons of TNT), and today many countries including the United States have weapons with explosive yields in the megatons (millions of tons of TNT).

The most striking difference between the explosion of an atomic bomb and that of an ordinary TNT bomb is of course in magnitude; as the President announced after the Hiroshima attack, the explosive energy of each of the atomic bombs was equivalent to about 20,000 tons of TNT. But in addition to its vastly greater power, an atomic explosion has several other very special characteristics. Ordinary explosion is a chemical reaction in which energy is released by the rearrangement of the atoms of the explosive material. In an atomic explosion the identity of the atoms, not simply their arrangement, is changed. A considerable fraction of the mass of the explosive charge, which may be Uranium 235 or Plutonium, is transformed into energy. Einstein's equation, E=mc², shows that matter that is transformed into energy may yield a total energy equivalent to the mass multiplied by the square of the velocity of light. The significance of the equation is easily seen when one recalls that the velocity of light is 186,000 miles per second. The energy released when a pound of TNT explodes would, if converted entirely into heat, raise the temperature of 36 lbs. of water from freezing temperature (32° F) to boiling temperature (212° F). The nuclear fission of a pound of uranium would produce an equal temperature rise in over 200 million pounds of water.

The explosive effect of an ordinary material such as TNT is derived from the rapid conversion of solid TNT to gas, which occupies initially the same volume as the solid; it exerts intense pressures on the surrounding air and expands rapidly to a volume many times larger than the initial volume. A wave of high pressure thus rapidly moves outward from the center of the explosion and is the major cause of damage from ordinary high explosives. An atomic bomb also generates a wave of high pressure which is in fact of much higher pressure than that from ordinary explosions; and this wave is again the major cause of damage to buildings and other structures. It differs from the pressure wave of a block buster in the size of the area over which high pressures are generated. It also differs in the duration of the pressure pulse at any given point: the pressure from a blockbuster lasts for a few milliseconds (a millisecond is one thousandth of a second) only, that from the atomic bomb for nearly a second, and was felt by observers both in Japan and in New Mexico [during the test detonation] as a very strong wind going by.

The next greatest difference between the atomic bomb and the TNT explosion is the fact that the atomic bomb gives off greater amounts of radiation. Most of this radiation is “light” of some wavelength ranging from the so-called heat radiations of very long wavelength to the so-called gamma rays which have wavelengths even shorter than the X-rays used in medicine. All of these radiations travel at the same speed; this, the speed of light, is 186,000 miles per second. The radiations are intense enough to kill people within an appreciable distance from the explosion, and are in fact the major cause of deaths and injuries apart from mechanical injuries. The greatest number of radiation injuries was probably due to the ultraviolet rays which have a wave length slightly shorter than visible light and which caused flash burn comparable to severe sunburn. After these, the gamma rays of ultra short wave length are most important; these cause injuries similar to those from overdoses of X-rays.

The origin of the gamma rays is different from that of the bulk of the radiation: the latter is caused by the extremely high temperatures in the bomb, in the same way as light is emitted from the hot surface of the sun or from the wires in an incandescent lamp. The gamma rays on the other hand are emitted by the atomic nuclei themselves when they are transformed in the fission process. The gamma rays are therefore specific to the atomic bomb and are completely absent in TNT explosions. The light of longer wave length (visible and ultraviolet) is also emitted by a TNT explosion, but with much smaller intensity than by an atomic bomb, which makes it insignificant as far as damage is concerned.

A large fraction of the gamma rays is emitted in the first few microseconds (millionths of a second) of the atomic explosion, together with neutrons which are also produced in the nuclear fission. The neutrons have much less damage effect than the gamma rays because they have a smaller intensity and also because they are strongly absorbed in air and therefore can penetrate only to relatively small distances from the explosion: at a thousand yards the neutron intensity is negligible. After the nuclear emission, strong gamma radiation continues to come from the exploded bomb. This generates from the fission products and continues for about one minute until all of the explosion products have risen to such a height that the intensity received on the ground is negligible. A large number of beta rays are also emitted during this time, but they are unimportant because their range is not very great, only a few feet. The range of alpha particles from the unused active material and fissionable material of the bomb is even smaller.

Apart from the gamma radiation ordinary light is emitted, some of which is visible and some of which is the ultraviolet rays mainly responsible for flash burns. The emission of light starts a few milliseconds after the nuclear explosion when the energy from the explosion reaches the air surrounding the bomb. The observer sees then a ball of fire which rapidly grows in size. During most of the early time, the ball of fire extends as far as the wave of high pressure. As the ball of fire grows its temperature and brightness decrease. Several milliseconds after the initiation of the explosion, the brightness of the ball of fire goes through a minimum, then it gets somewhat brighter and remains at the order of a few times the brightness of the sun for a period of 10 to 15 seconds for an observer at six miles distance. Most of the radiation is given off after this point of maximum brightness. Also after this maximum, the pressure waves run ahead of the ball of fire. The ball of fire rapidly expands from the size of the bomb to a radius of several hundred feet at one second after the explosion. After this the most striking feature is the rise of the ball of fire at the rate of about 30 yards per second. Meanwhile it also continues to expand by mixing with the cooler air surrounding it. At the end of the first minute the ball has expanded to a radius of several hundred yards and risen to a height of about one mile. The shock wave has by now reached a radius of 15 miles and its pressure dropped to less than 1/10 of a pound per square inch. The ball now loses its brilliance and appears as a great cloud of smoke: the pulverized material of the bomb. This cloud continues to rise vertically and finally mushrooms out at an altitude of about 25,000 feet depending upon meteorological conditions. The cloud reaches a maximum height of between 50,000 and 70,000 feet in a time of over 30 minutes.

To summarize, radiation comes in two bursts: an extremely intense one lasting only about 3 milliseconds and a less intense one of much longer duration lasting several seconds. The second burst contains by far the larger fraction of the total light energy, more than 90 percent. But the first flash is especially large in ultraviolet radiation which is biologically more effective. Moreover, because the heat in this flash comes in such a short time, there is no time for any cooling to take place, and the temperature of a person's skin can be raised 50 degrees centigrade by the flash of visible and ultraviolet rays in the first millisecond at a distance of 4,000 yards. People may be injured by flash burns at even larger distances. Gamma radiation danger does not extend nearly so far and neutron radiation danger is still more limited. The high skin temperatures result from the first flash of high intensity radiation and are probably as significant for injuries as the total dosages which come mainly from the second more sustained burst of radiation. The combination of skin temperature increase plus large ultraviolet flux inside 4,000 yards is injurious in all cases to exposed personnel. Beyond this point there may be cases of injury, depending upon the individual sensitivity. The infrared dosage is probably less important because of its smaller intensity.

On August 6, 1945, the atomic bomb that was dropped on Hiroshima killed an estimated eighty thousand people immediately. Up to another sixty thousand died later from radiation sickness. Approximately sixty-nine percent of the city's buildings were completely destroyed. Three days later, a second bomb was dropped on Nagasaki, killing seventy-thousand people by the end of that year and destroying forty percent of the city. The US military prepared more atomic bombs for use against Japan, but they were not launched because Japan surrendered on August 15, ending World War II. The use of atomic bombs in Japan may have saved up to one million US and Japanese lives. However, the use of weapons of mass destruction has since come under scrutiny. The residual radiation caused genetic mutations, stillbirths, and cancers in the surviving population of Hiroshima and Nagasaki; however, the children of survivors supposedly do not carry genetic mutations from their parents' exposure. The cities have since been rebuilt and are thriving.

In March 2011, a tsunami hit the coast of Japan, triggered by an earthquake, setting off a chain reaction at the Fukushima Daiichi nuclear plant. It was the second-largest release of radioactive materials since the 1986 Chernobyl accident. The area around Fukushima was evacuated, with three hundred thousand people moving temporarily, and there were no immediate deaths. However, radiation exposure of those living close to the plant is expected to cause a slight increase in some cancers. The radiation also seriously affected the Japanese food supply, including fruit, grains, and meat produced in the area, negatively affecting residents' health. The area was still deemed unsafe for habitation in 2014. Radioactive water continued to leak from the nuclear plant's storage tanks well after the disaster. The amount of radiation released by the Fukushima disaster was estimated to be equivalent of up to one thousand Hiroshima nuclear bombs.