Radation from Hiroshima and Nagasaki bombings
The atomic bombings of Hiroshima and Nagasaki in August 1945 marked a pivotal moment in World War II, resulting in immense devastation and loss of life. The bomb dropped on Hiroshima, known as Little Boy, released energy equivalent to 13 kilotons of TNT, while the subsequent bombing of Nagasaki with Fat Man yielded 22 kilotons. Immediate casualties included around 70,000 deaths in each city, with many more succumbing to injuries and radiation effects in the years that followed.
Radiation exposure occurred in two forms: prompt radiation from the initial explosion and delayed radiation from fallout. Prompt radiation killed many within a kilometer of the blasts, while those farther away experienced varying degrees of exposure. Contrary to earlier fears, the long-term effects of radiation in these cities were less severe than anticipated; radiation levels returned to near normal within a month, allowing for rapid rebuilding and recovery.
Despite the physical restoration of Hiroshima and Nagasaki, many survivors faced ongoing health issues and psychological trauma linked to the bombings. These events continue to evoke discussions about the moral implications of nuclear warfare and its lasting impact on humanity.
Radation from Hiroshima and Nagasaki bombings
THE EVENTS: The dropping of two atomic bombs on cities in Japan during World War II—a uranium bomb on Hiroshima and a plutonium bomb on Nagasaki
DATES: August 6 and 9, 1945
The use of atomic weapons by the United States is credited with ending the war with Japan, but debates continue regarding the necessity and moral justification of the bombings. The bomb blasts and the radiation emitted immediately on detonation were responsible for widespread devastation and the deaths of hundreds of thousands of people, but the effects of radiation on the surrounding areas were less long-lasting than had been expected.
By the summer of 1945, Japan had lost the war against the United States and the other Allied Powers, but the ruling Japanese war cabinet was deadlocked—three for and three against—on a decision on surrender. With the tie vote, the previous policy of war prevailed. The Japanese devised a strategy to end the war that would force the Allies to invade the Japanese homeland, where so many would be killed that Japan would receive more favorable surrender terms to end the war. The Allies did not invade Japan, however; on August 6 and 9, 1945, the United States dropped atomic bombs on the cities of Hiroshima and Nagasaki. Even after the devastation caused by the bombings, the war cabinet remained deadlocked, but Emperor Hirohito survived an attempted coup and gained enough support that he was able to lead Japan’s surrender.
The Bombs
The bomb code-named Little Boy exploded 580 meters (1,900 feet) over Hiroshima. It weighed 4,400 kilograms (9,700 pounds). Its great power came from 64 kilograms (141 pounds) of uranium 235. The uranium was assembled into critical mass (the amount needed to sustain a chain reaction) in 1 millisecond by the firing of a 25-kilogram (55-pound) uranium bullet from a short cannon into 39 kilograms (86 pounds) of uranium shaped into three target rings. Although the bomb was only 1.3 percent efficient, it exploded with the power of 13 kilotons of trinitrotoluene (TNT).
A three-second thermal pulse from the blast burned the exposed skin of people as far away as 3.5 kilometers (2.2 miles) from ground zero (the point directly below the burst) and started fires throughout the city. The blast crushed buildings around ground zero, leaving only skeletons of reinforced concrete or steel. Numerous fires spread, joined, and formed a violent firestorm that consumed everything combustible within 13 square kilometers (5 square miles). It is estimated that some 70,000 people were killed, and another 100,000 died over the next five years from bomb-related injuries. An estimated 60 percent of the deaths were from burns, 30 percent from the blast, and 10 percent from radiation.
Because the Japanese did not surrender immediately, and to demonstrate that the Hiroshima bomb was not a fluke, the United States detonated a plutonium bomb, code-named Fat Man, 595 meters (1,952 feet) over Nagasaki three days later. (The bombs’ code names, Little Boy and Fat Man, referred to US President Harry Truman and British prime ministerWinston Churchill, respectively.) Fat Man weighed 4,900 kilograms (10,800 pounds), half of which was high explosives used to compress the softball-sized plutonium core into critical mass. The 22-kiloton blast and subsequent fire destroyed 6.7 square kilometers (2.6 square miles). The destruction was not greater because part of the city was protected by a hill. Approximately 70,000 people were killed, and another 70,000 died over the next five years from bomb-related injuries. An estimated 77 percent of these deaths were from burns, 16 percent from the blast, and 7 percent from radiation.
Radiation
Prompt radiation consists of the neutrons and gamma rays emitted as a nuclear bomb explodes and during the first minute thereafter. About 85 percent of the people within 1 kilometer (0.6 mile) of ground zero of both blasts were killed outright by blast or heat, or they died within the first year of or injuries. People up to 2 kilometers (1.2 miles) away received significant but nonlethal prompt doses of radiation. A study of thirty women who had been pregnant when they were exposed to radiation within 2 kilometers of ground zero and showed signs of radiation damage found that only sixteen of the children they subsequently bore lived more than one year, and four of those children were developmentally disabled. Deaths from prompt radiation began during the first week and peaked three to four weeks later. Between 1950 and 1990 an estimated 850 people who had been in the vicinities of the Hiroshima and Nagasaki bombings died of radiation-caused cancer, and another 400 to 800 died of radiation-induced noncancer injuries.
Radiation after the first minute of a nuclear bomb blast is called delayed radiation. Near ground zero, neutrons make elements such as sodium, aluminum, and manganese radioactive, but these newborn isotopes have short half-lives and disappear within minutes to hours. The other source of delayed radiation is fallout. It consists of radioactive bomb vapor that has condensed into small, solid particles. Had the Hiroshima and Nagasaki bombs been ground bursts, dirt and rocks would have been sucked up into the fireball and made radioactive by neutrons, which would have made the much worse. Since the bombs were air bursts, this did not happen. Within an hour of each burst, from the fires helped precipitate black rain. In spite of reports to the contrary, the rain was only mildly radioactive. Direct measurements found the highest levels to be in the Nishiyama district of Nagasaki, but even there radiation levels should not have caused lasting harm. People who came into the city right after the explosion and were exposed to the fallout and residual radiation showed fewer symptoms attributable to radiation than some expected.
Although some scientists expected that Hiroshima and Nagasaki would be uninhabitable for generations because of the radiation caused by the atomic weapons, in reality radiation in the cities had returned to near one month after the bombings. Both cities were rebuilt and achieved their prebombing populations within ten years. In the twenty-first century, they are modern and prosperous. Still many survivors dealt with lingering health problems connected to the bombings, as well as lasting psychological trauma.
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