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Physical Effects of the Atomic Bomb

BACKGROUND

1945, Japan: Two atomic bombs were dropped on Hiroshima and Nagasaki. The first bomb, “Little Boy,” was dropped on Hiroshima. The bomb was 3 meters long and weighed over 4000 kilograms. The second bomb, “Fat Man,” was 3 meters long and weighed about 4500 kilograms.  The death rate in both cities was about the same.  Although most of the deaths occurred within 24 hours of the bombing, many people died from the effects months and years afterwards. The total number of people killed by both bombs is probably greater than a quarter million people.

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Noon, August 10. Near the hypocenter. Photo: Yamahata Yosuke

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INITIAL EFFECTS

Blast: The thermal radiation and blast were extremely lethal. About half the energy given off by the atomic bomb was the blast. The blast was a shock wave that spread outward at about the speed of sound. Following this wave was a wind with the force of a hurricane.  This wind stopped for an instant because of a change in pressure, and then a wind blew in the other direction. Most buildings close to the hypocenter were damaged beyond repair.

Fire: A third of the energy from the bomb was given off as heat. The fireballs produced reached the temperature of the sun. The thermal radiation at Nagasaki was more intense than Hiroshima. Even as far as 4 kilometers from the hypocenter, the heat could burn skin. People in the first 1.2 kilometers were either burned to death or vaporized.  Due to ash and moisture in the air, a heavy “black rain” fell. The liquid was radioactive and oily. Two thirds of Hiroshima’s buildings were destroyed and about a quarter of Nagasaki’s were destroyed. Even more were damaged.

 

 
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LATE EFFECTS

Ionizing radiation: Fifteen percent of the radiation given of by the bomb was ionizing radiation. Some of the radiation was released initially while the remainder was residual, and emitted gradually. People who received about 450 rads (unit used to measure level of ionizing radiation) died in a month. All who receive 700 rads die quickly. Those within 1 kilometer of the hypocenter got a dose of 1000 rads and died immediately. The effects of radiation included vomiting and nausea. Later people vomited blood, cultivated a fever, had diarrhea, and had bleeding from their bowels. They lost their hair and resistance to infection dropped. Blood poisoning was a major cause of death. Although people survived, they later developed disease like eye diseases, blood disorders, malignant tumors, and psychoneurological problems. Leukemia in survivors increased during the first 20 years. The incidence of malignant tumors is also much greater among survivors than the non-exposed. Children born to survivors have an increased congenital malformation although interestingly, there has been an absence of genetic damage among survivors shown by statistical research. This does not mean that there is no genetic damage.

 

 

PERSONAL ACCOUNT OF THE BOMBING OF HIROSHIMA, by Michihiko Hachiya, M.D. (a Japanese physician):

“It was a horrible sight,” said Dr. Tabuchi. “Hundreds of injured people who were trying to escape to the hills passed our house. The sight of them was almost unbearable. Their faces and hands were burnt and swollen; and great sheets of skin had peeled away from their tissues to hang down like rags on a scarecrow. They moved like a line of ants. All through the night, they went past our house, but this morning they had stopped. I found them lying on both sides of the road so thick that it was impossible to pass without stepping on them.”

“And they had not faces! Their eyes, noses and mouths had been burned away, and it looked like their ears had melted off. It was hard to tell front from back. One soldier, whose features had been destroyed and was left with his white teeth sticking out, asked me for some water, but I didn’t have any. I clasped my hands and prayed for him. He didn’t say anything more. His plea for water must have been his last words. The way they were burned, I wonder if they didn’t have their coats off when the bomb exploded.”

 

 

 

 

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Day of Hiroshima Bombing

RADIATION DETAILS

Fallout: All nuclear weapons results in fallout, but the amount of fallout depends on the height of the explosion. Thermal radiation causes a fireball to be created. If the fireball touches the ground, soil, dirt, and debris are taken into the fireball. These particles mix with radioactive materials in the bomb. Then these pieces fall to the ground and constitute the local radioactive fallout. But if the bomb explodes in the air, then the fallout decreases. The fallout is a radiation hazard long after the explosion occurs. The dosage rates from the fallout at various times after the explosion are shown in Table 3.  As time goes on, the dose rate decreases.

Radiation from Neutrons: The initial number of neutrons produced per unit of explosive power as well as the energy distribution is different in fusion and fission bombs. In fusion reactions, the number of neutrons produced is about ten times greater than fission reactions. After the bomb breaks up, the neutrons collide with the atomic nuclei in the materials of the bomb. The energy lost in these collisions is captured by nuclei that emit gamma-rays. A dose of radiation is given when these neutrons reach the human body and are partially absorbed. The dosage of radiation varies with the energy contained in the neutrons. The graph shown (Figure 4) shows neutron dose at the surface tissue of the human body as a function of the slant distance. The slant distance takes into account many factors including the height of the burst. The graph shows that a greater slant distance delivers less rads.

Radiation from Gamma-rays: The initial gamma-ray dose from a fission bomb comes mainly from radioactive capture. Neutrons that collide with nitrogen atoms ultimately release energy in the form of gamma rays. Fission product decay also releases gamma rays over time. Gamma rays from other sources are emitted before the shock wave, unlike the fission bomb which is emitted at a later stage.

Both of these components contribute to the total dose of radiation. At a short distance from the Earth, neutrons contribute more the dosage and at greater distances the gamma rays contribute more.

 

SOURCES

Barnaby, Frank; Joseph Rotblat. “The Effects of Nuclear Weapons” in Nuclear War: The

Aftermath Vol. 11 No. 2/3, 1982, pp. 84-93

Hachiya, Michihiko. Hiroshima Diary. Chapel Hill, NC: The University of North

Carolina Press, 1945: pp. 14-15

Hiroshima-Nagasaki: A Pictorial Record of the Atomic Destruction. Tokyo, Japan:

Hiroshima-Nagasaki Publishing Committee, 1978: pp. 26-27

 

 

 


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