Impacts and Consequences

Nuclear explosions cause a variety of devastating effects

The blast effect creates fumes and the shock wave produced from an explosion

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The blast effect of an atomic explosion is similar to that of traditional explosives, but is on a much greater scale in terms of destruction.  

"The force from which the sun draws its power has been loosed against those who brought war..."  

Harry S. Truman
The enormous blast of atomic bombs creates thermal radiation.  This effect is caused by extremely high temperatures, and starts fires miles from the epicenter of the explosion.  Also, the explosion also creates radiation effects.  This radiation causes burns to any exposed skin (severity depending on distance from the epicenter.)  The radiation effect is the result of the gamma rays created by the fission of atoms.  Radiation causes death and injury through damage to living tissue.  Survivors of the bombs were injured mostly by the thermal, and blast effects because radiation doesn't travel as far as the other two effects the larger the explosion gets.  Although, radiation did effect all persons within a mile of the epicenter. 
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Radiation effects on exposed skin
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These bombs brought and unprecedented level of death and destruction never before seen from the efforts of a single bomber.

Hiroshima cities original population of 400,000 was immediately reduced by at least a fourth when the bomb exploded.  Official statistics state that as of September 1st, 1945 70,000 had died from the bomb, not counting the missing or incinerated bodies, 130,000 people were wounded, and 43,000 critically wounded.  Nagasaki, being less densely populated (200,000)  reported 70,000 dead by January following the bomb, which would double in the next 5 years, adding up to approximately 75% of the cities population.  All persons and animals within a kilometer of the epicenter were instantly killed, and those within 250 meters were completely incinerated.  These bombs destroyed all of the living in entire cities, and is therefore seen as Japanese genocide because of the devastation to the existing communities and environment.  
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Incredible levels of suffering and injury induced by the bomb made the survivors unlucky.  Those that did not die from the explosion found themselves instantly naked because the thermal radiation instantly incinerated their clothing.  Those not under cover suffered immediate burns to the back and extremities, which survivors held out from their bodies to avoid the painful friction as they lumbered to the hospitals and care centers that were not burned to the ground.  About 1/2 of the of the survivors saved by the rescue parties died within the next few days.  
The blast and thermal radiation effects not only took many lives, but also destroyed any and all infrastructure in its path, leaving these cities with nothing.   The pressure from the blast shattered all of the windows and doors of buildings within several kilometers.  Virtually all buildings in a kilometer radius of the epicenter were set on fire due to the thermal radiation.  The fires quickly spread, as the explosion left no water for the victims to drink, let alone put out the fires. As if this wasn't bad enough, a the blast created a gust of wind, effectively spreading the fire throughout the city.  The bombing of Nagasaki, for example, destroyed 14,000 homes, and seriously damaged 5,400, leaving only 12% unaffected.  
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City of Hiroshima in ruins
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International Effects of WWII and the Bombing

The death toll of WWII range from 40 to 60 million people, and it is estimated that just as many civilians died as combatants due to bombings.  At least 2 million Japanese died from the war, and about 300,000 from the atomic bomb.  Since the first use of atomic weapons many organizations have sprang up in opposition of their use.  A movement against the nuclear weapons called the "Stockholm Appeal" was started in 1950 with the threat of their use in the Korean War.  Since then, it is estimated by the UN General Secretary that there were between 40,000 to 50,000 nuclear weapons stockpiled around the world as soon as 1980.