The Atomic Bombings of Hiroshima and Nagasaki: The Nature of An Atomic Explosion Summary

Shortly after the US Army detonated the first atomic weapons over the Japanese cities of Hiroshima and Nagasaki in August 1945, the Army assembled a special Manhattan Project Bomb Investigating Group to assess the sites of the atomic bomb detonations. This military report summarizes the strategy, execution, and aftermath of the bombings. In this section, the report explains the scientific nature of an atomic explosion and its impact by describing the force, heat, and radiation generated in comparison to the more traditional explosive TNT. It emphasizes the great deal of energy generated and released as heat at the moment of detonation, capable of raising temperatures to high levels so rapidly as to cause instant incineration. The report also discusses the radioactive forces unleashed by nuclear weapons, including gamma and beta rays. Along with the physical forces of heat and pressure, radiation is acknowledged as one of the most damaging effects of the atomic bomb and the most unique compared to traditional explosives.

During the early twentieth century, new theories by innovative physicists such as Albert Einstein greatly advanced human understanding of energy. As early as 1939, research was already underway in Europe to harness the energy released by the division of atoms. Mastering this force would enable the development of weaponry with immense physical power.

In the spring of 1945, Harry S. Truman became president upon the death of Franklin D. Roosevelt. Truman had been vice president only a short time before Roosevelt's death and had little knowledge of the secret military strategies of World War II. It was with great surprise, therefore, that he learned after taking office of an ongoing military program to develop a powerful new kind of weapon based on the principles of nuclear physics. Even more surprising was that such a weapon was nearly ready to be used. That July, scientists in New Mexico successfully detonated the first atomic bomb. Soon thereafter, Truman and other Allied countries fighting Japan issued the Potsdam Declaration, which demanded that the Japanese agree to an unconditional surrender or face “prompt and utter destruction.”

No immediate surrender was forthcoming. Truman faced a choice between either mounting a traditional military invasion of the Japanese islands or authorizing the use of an atomic weapon on a combined civilian and military Japanese target. An invasion involving ground, sea, and air attacks was likely to require many more months of fighting and tens of thousands of American lives as well as hundreds of thousands of Japanese civilian casualties. An atomic attack, by contrast, was expected to kill many thousands of Japanese without any US human costs. It also had the benefit of making a very public demonstration of the incredible new destructive power controlled, at that time, solely by the United States. Some advisers, including a group of scientists who had worked on the bomb and fully grasped its potential for devastation, opposed a nuclear attack, however.

Truman decided to proceed with the nuclear bombing in the hopes that it would force a Japanese surrender and avoid the need for a large-scale invasion. On August 6, 1945, a US bomber dropped the first atomic bomb on the city of Hiroshima. The devastation was immense, but the Japanese government did not immediately agree to a surrender. Three days later, the Soviet Union declared war on Japan and the United States used a second nuclear weapon against the city of Nagasaki. Facing certain defeat, the Japanese surrendered. The atomic age, however, was just beginning, as US scientists and military experts embarked on a period of continued nuclear research and consideration of further applications of their newly proven destructive force.

The US Army Corps of Engineers officially oversaw the development and implementation of the atomic bomb through its management of a top-secret research program known as the Manhattan Project, taken from the name of the administering Manhattan Engineer District. Under the leadership of Major General Leslie Groves, Manhattan Project teams across the country worked to solve the numerous technological challenges required to create the uncontrollable nuclear reaction needed for a successful weapon. In conjunction with high-ranking political officials, they also considered how, where, and whether the weapons they constructed should be used.

After the attacks on Hiroshima and Nagasaki, some of these same military and scientific experts contributed to the US Army's summative report on the matter. These experts included Hans Bethe—a nuclear physicist whose ideas are specifically referenced in this section of the report—along with other physicists active in the Manhattan Project.

The most striking difference between the explosion of an atomic bomb and that of an ordinary T.N.T. 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 T.N.T.

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 = mc^2, 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 T.N.T. explodes would, if converted entirely into heat, raise the temperature of 36 lbs. of water from freezing temperature (32 deg F) to boiling temperature (212 deg 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 T.N.T. is derived from the rapid conversion of solid T.N.T. 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 as a very strong wind going by.

The next greatest difference between the atomic bomb and the T.N.T. explosion is the fact that the atomic bomb gives off greater amounts of radiation. Most of this radiation is “light” of some wave-length ranging from the so-called heat radiations of very long wave length to the so-called gamma rays which have wave-lengths 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 ultra-violet 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 over-doses 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 T.N.T. explosions. The light of longer wave length (visible and ultra-violet) is also emitted by a T.N.T. 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 ultra violet 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.

It is of interest to note that Dr. Hans Bethe, then a member of the Manhattan Engineer District on loan from Cornell University, predicted the existence and characteristics of this ball of fire months before the first test was carried out.

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%. But the first flash is especially large in ultra-violet 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 ultra-violet 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 ultra-violet 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 infra-red dosage is probably less important because of its smaller intensity.

This portion of the US Army's report on the bombings of Hiroshima and Nagasaki focuses on the scientific and technical details of the bomb's detonations and effects. To do this, the report explores three key areas: the overall force and unique features of the atomic bomb as compared to that of existing explosive substances, the immediate effects of the bomb's blast, and the short-term impacts of the radioactive energy released as a by-product of the detonations.

Prior to the development of the atomic bomb, bombs typically employed TNT, or trinitrotoluene, as the main explosive force. TNT's relative stability and predictability had long made it the favored explosive for military use and for civilian applications, such as clearing large rocky areas for mining or construction, and its force has become a standard for comparison. The report therefore compares the nuclear explosions to those of TNT in order to give readers a sense of the immense scale of the atomic bomb's power. For example, the report notes that the detonation of one pound of TNT would generate enough energy to heat thirty-six pounds of water “from freezing temperature… to boiling temperature,” whereas the detonation of one pound of uranium, a radioactive element used in atomic weapons, would generate enough energy to cause an equal rise in temperature of two hundred million pounds of water. The scale of energy released by an atomic explosion was therefore almost unimaginably greater than those of the past.

The contrast between the effects of a traditional explosion and a nuclear one was even starker. In a certain understatement, the report notes that “the atomic bomb gives off greater amounts of radiation” than an explosion of TNT. The radiation released by atomic weapons has immediate effects on living creatures, physical structures, and the environment. The report then describes the creation of flash burns from the intense heat and radiation of the detonation and notes that the various forms of radiation emitted from the explosion were “the major cause of deaths and injuries apart from mechanical injuries” caused by the explosions' pressure waves. The report then delves into the impact of the different types of energy waves formed by a nuclear explosion. Gamma rays, the shortest and most powerful of all energy waves in the electromagnetic spectrum, are unique to atomic explosions and can disrupt the structure of atoms, making these waves especially devastating. The report explains that an atomic explosion also emits a large amount of beta and alpha rays, although their effect is less significant due to their limited range. Along with gamma, beta, and alpha rays, the report also notes that an atomic explosion emits high levels of visible and ultraviolet rays, which produce an intense fireball that “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.” The report concludes by summarizing the various bursts of radiation released by an atomic explosion and their effects.

The immense power of nuclear blasts described by this text had significant short- and long-term consequences for the people of Hiroshima and Nagasaki. The heat and pressure created by the atomic explosions were so great that thousands of individuals in the blast zone were incinerated instantly. Flash burns charred human skin, and a high wave of pressure from the explosion and fires from the intense heat damaged structures over a few square miles surrounding each explosion center. Radiation sickness, a condition resulting from exposure to high levels of ionizing radiation that causes cell death throughout the body, caused people to experience symptoms ranging from nausea and vomiting to organ failure and death.

The Radiation Effects Research Foundation has estimated that the effects of the nuclear bomb caused 90,000 to 166,000 acute deaths in Hiroshima and 60,000 to 80,000 acute deaths in Nagasaki within four months of the bombings. In Japan, survivors of the bombings at Hiroshima and Nagasaki became known as the hibakusha, meaning “those who were bombed.” Exposure to intense radiation also made the hibakusha prone to long-term struggles with radiation sickness, higher rates of cancer (particularly leukemia), and biological and psychological problems for decades after the attacks.

• Hersey, John. Hiroshima. New York: Knopf, 1946. Print.
• “‘Hibakusha’: Those Who Survived and How They Survived.” Children of the Atomic Bomb. University of California at Los Angeles, 10 Oct. 2007. Web. 5 Dec. 2014.
• Hogan, Michael J., ed. Hiroshima in History and Memory. New York: Cambridge UP, 1996. Print.
• Kort, Michael. Columbia Guide to Hiroshima and the Bomb. New York: Columbia UP, 2007. Print.