Hiroshima: The Atomic Bomb

(International Schools Cyber Fair 1998 Project)

Index

	Development of the Atomic Bomb
	Diagram of Little Boy & Fat Man
	The Mechanism of the Bomb

Development of the Atomic Bomb

As Researched by: Dorothy Van Duyne, Grade 7

Many events led up to the Atomic Bomb. There were about 6 years of research, experimentation, and building before the bomb was used on Hiroshima in the autumn of 1945. Nearly 2 billion dollars were spent, and 14 men had migraines through the whole thing. And surprisingly, even though the Americans were the ones who first unleashed the Atom's power, they were not the ones who thought of it. It was a man named Leo Szilard, a Hungarian, who first thought that the Atomic Bomb could be made possible. And it didn't happen in the way you would think. It's not like he had a vision and suddenly thought of and Atomic Bomb, or Atomic power in general. It all has to do with a fantasy, a stubborn man, and, well, two geniuses.

Leo Szilard was a student of Albert Einstein, who spoke very highly of him. They were the two geniuses. Szilard liked Science Fiction writing, and had read a book named The World Set Free and written by H.G. Wells, the man who began Science Fiction writing. In this book Wells described a process of "atomic disintegration" that unleashed "limitless power" which led to global nuclear war. This book was written in 1913. This book was the fantasy. Szilard promptly went to Ernest Rutherford, who had previously called Atomic Energy "moonshine," and wanted to convince him he was wrong. This didn't turn out to well, because Szilard got kicked out of Rutherford's office. But this only made him the more determined. He is also the stubborn man.

The most complicated problem to be solved was the production of sufficient amounts of improved uranium to cause a chain reaction. Then, U-235 was very hard to extract. In fact, the ratio of conversion from Uranium ore to Uranium metal is 500:1. An additional disadvantage is that the 1 part of Uranium that is refined from the ore consists of over 99% Uranium-238, which cannot be used to make an atomic bomb. To make it even more difficult, U-235 and U-238 are precisely similar in their chemical makeup. No ordinary chemical extraction could separate the two isotopes. Only mechanical methods could separate U-235 from U-238. Several scientists at Columbia University succeeded in solving this dilemma by magnetically separating the two isotopes.

From 1939 to 1945, more than 2 billion dollars were spent on the Manhattan Project. The formulas for refining Uranium and putting together a working bomb were created and completed by some of the greatest minds of our time.

Finally the day came when all at Los Alamos would find out whether or not the atomic bomb was either going to be the major dud of the century or maybe end the war. It all came down to a morning in the midsummer of 1945.

At 5:29:45 on July 16th, 1945, in a white blaze the first atomic bomb ever tested lead in the Atomic Age. The light of the explosion turned orange as the fireball shot up at 360 feet per second, reddening and pulsing as it cooled. The mushroom cloud of radioactive vapor materialized at 30,000 feet. Beneath the cloud, all that remained of the soil at the blast site were fragments of jade green radioactive glass. All of this caused by the heat of the bomb.

Several participants, shortly after viewing the results, signed petitions against loosing the monster they had created, but their protests were not heard. As it turned out, New Mexico was not the last place to experience an atomic explosion.

Atomic bombs have been used only twice in warfare. The first blast site of the atomic bomb was Hiroshima. A Uranium bomb that weighed about 4 and 1/2 tons, and nicknamed "Little Boy" was dropped on Hiroshima August 6th, 1945. The Aioi Bridge was the aiming point of the bomb. Ground Zero was set at 1,980 feet. At 8:15, the bomb was dropped from the Enola Gay. It missed by only 800 feet. At 8:16, in a split-second, 66,000 people were killed and 69,000 people were injured.

The point of total vaporization from the blast measured one half of a mile in diameter. Total destruction ranged at one mile in diameter. Severe blast damage carried as far as two miles in diameter. At two and a half miles, everything flammable in the area burned. The remaining area of the blast zone was riddled with serious blazes that stretched out to the final edge at a little over three miles in diameter.

While just the explosion from an atomic bomb is deadly, its destructive ability doesn't stop there. Atomic fallout is another hazard. The rain that follows any atomic bomb is full of radioactive particles. Many survivors of the Hiroshima and Nagasaki blasts had radiation poisoning due to this occurrence.

When a U-235 atom splits, it gives off energy in the form of heat and Gamma radiation, which is the most powerful form of radioactivity and the most lethal. When this reaction occurs, the split atom will also give off two or three of its `spare' neutrons, which are not needed. These spare neutrons fly out with sufficient force to split other atoms they come in contact with. In theory, it is necessary to split only one U-235 atom, and the neutrons from this will split other atoms, which will split more...etc. This will all happen within a millionth of a second.

The minimum amount to start a chain reaction as described above is known as supercritical mass. The mass needed to cause this chain reaction depends upon the purity of the material, but for pure U-235, it is 50 kilograms, but no Uranium is really pure, so in reality more will be needed.

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Diagram of Little Boy & Fat Man

Go to Diagram of Little Boy

Little Boy		Little Boy & Fat Man

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The Mechanism of the Bomb

Altimeter

As the bomb begins to descend, the altimeter transmitter sends out a pulse starting at 4200 MHz. When that pulse returns, the altimeter transmitter emits a higher frequency. The difference depends on how long the pulse has taken to return to the altimeter. When these two frequencies are mixed electronically, a new frequency is measured by microchips that are built into the bomb. This value gives the actual height. The altimeter errs within five feet, which is not a major concern because the normal air burst setting for an atomic bomb is 1,980 feet.

Air Pressure Detonator

At high altitudes, the air is of lesser pressure. As the altitude drops, the pressure increases. A piece of very thin magnetized metal can be used for an air pressure detonator. The strip of metal must only have a bubble of very thin metal in the center and have it placed directly underneath the electrical contact which will trigger the conventional explosive detonation.The bubble must be pushed so that it is inverted. When the air pressure has reached the correct level, the bubble will snap back into place, hitting the contact, completing the circuit and setting off the explosive.

Detonating Head

The detonating head simply serves as a catalyst to make a bigger explosion. The calibration of this is extremely important. A detonating head that is too small will only cause the bomb to fail. It will be even more dangerous because someone will have to disarm and refit the bomb with another head. The detonating head will receive an electrical charge from either the air pressure detonator or the altimeter.

Conventional Explosive Charge

This is used to weld the greater amount of Uranium to the lesser within the bomb's housing.

Neutron Deflector

The neutron deflector is made out of U-238 because it is non-fissionable and has the power to reflect neutrons back to their original source. In the Uranium bomb, the neutron deflector is a safeguard to keep accidental supercritical mass from occurring. In the Plutonium bomb it helps the wedges of Plutonium retain their neutrons by reflecting them back to the center.

Lead Shield

The lead shield's purpose is to prevent the radioactivity of the bomb's payload from interfering with the rest of the mechanisms of the bomb. Without this, premature detonation is likely to happen.

Fuses

The fuses serve as another safeguard against accidental detonation. They are placed near the surface of the nose of the bomb so they can be installed easily during flight. They are installed only shortly before the bomb is launched, or a disaster is asked for.

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