Component 4, Part 2
The Atomic Bombs—Hiroshima and Nagasaki

As we learned in Part 1, a sustained nuclear fission (with critical mass) reaction in U-235 can result in a massive explosion if enough uranium material is involved. In the case of the atomic bomb dropped on Hiroshima, U-235 was the fissile material utilized. Within the bomb, U-235 was divided into two sections, both being below critical mass (to keep the bomb from exploding as soon as it was constructed). A detonating device was used to unite the two sections, causing an immediate explosion. This bomb was named “Little Boy” for President Franklin Delano Roosevelt. A different procedure and nuclear material was utilized in the production of the bomb dropped on Nagasaki. Plutonium-239 was the fissile material. It was divided into several subsections within the bomb’s housing and surrounded by gunpowder. Once detonated, the plutonium subsections were squeezed together, thus creating the critical mass necessary for a chain reaction and explosion. This bomb was named “Fat Man,” for England’s Prime Minister, Winston Churchill. Figure 1 provides a comparison of the two different bombs dropped on Hiroshima and Nagasaki (The Nagasaki Peace Declaration, 2006).

Energy Released
Each of the two atomic bomb blasts were thought to have been approximately equivalent to a blast produced from 20,000 tons of TNT (The Manhattan Engineer District, 1946).
Figures 4 and 5 are photos depicting the “mushroom clouds” that formed as the bombs were dropped on Hiroshima and Nagasaki respectively.

(To see recovered video footage of the Hiroshima bombing, click on the following link: Hiroshima bombing)

According to the Atomic Bomb Museum (2006), there were three main forms of energy released as a result of the nuclear bombs dropped over Hiroshima and Nagasaki:

1. Fireball (heat)

2. Shock wave and air blast (accounted for 50% of energy)

3. Radioactivity

Figure 6 is a graphical representation of the energy released from the bomb explosions.

Heat
Directly beneath where the bomb was dropped on the ground (hypocenter) it has been estimated that the temperature reached approximately 7000 degrees F (Atomic Bomb Museum, 2006).

Bomb Blast
The explosions created areas of extremely high pressure which resulted in winds in excess of 980 mph at the hypocenters. The pressures created were approximately 8,600 pounds per square feet. From the hypocenters out to approximately 1/3 of a mile, most substantial concrete buildings were obliterated. Even a mile from the hypocenter, all brick buildings were destroyed as the wind velocity in these areas reached 190 mph and pressure was approximately 1,180 pounds per square feet (Atomic Bomb Museum, 2006).

Radiation
Alpha, beta, gamma, and neutron rays were generated by the nuclear bombs, with the gamma and neutron rays doing the most immediate damage and causing most early raditation deaths. From 1/16 mile out in all directions from the hypocenter, most people died within a few hours. Those located 1/2 mile from the hypocenter died within 30 days (Atomic Bomb Museum, 2006).