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Little Boy’s Shadow - Japan


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Little Boy’s Shadow

A bad day in wartime history, countless innocents melted away when we dropped the first atomic bomb on Hiroshima. A name like “Little Boy” is disingenuous given the massive, immediate and long-term destruction that was wrought (many of those exposed to the radiation are still suffering the horrifying side-effects). As if deteriorating flesh and structural disarray wasn’t enough of a scar on the city, shadows are permanently fixed all about, burnt-in imprints left behind like tanning bed tattoos, taking the shape of etched flowers on telephone poles and outlined guardrails on the streets. Some memories simply don’t wash away.



"Little Boy" was the codename for the atomic bomb dropped on Hiroshima on August 6, 1945 by the Boeing B-29 Superfortress Enola Gay, piloted by Colonel Paul Tibbets of the 393rd Bombardment Squadron, Heavy, of the United States Army Air Forces. It was the first atomic bomb to be used as a weapon. The second, the "Fat Man", was dropped three days later on Nagasaki.
The weapon was developed by the Manhattan Project during World War II. It derived its explosive power from the nuclear fission of uranium 235. The Hiroshima bombing was the second artificial nuclear explosion in history, after the Trinity test, and the first uranium-based detonation. Approximately 600 to 860 milligrams of matter in the bomb was converted into the active energy of heat and radiation (see mass-energy equivalence for detail). It exploded with an energy between 13 and 18 kilotons of TNT (54 and 75 TJ) (estimates vary). It has been estimated that 130,000 to 150,000 persons had died by the end of December 1945. Its design was not tested in advance, unlike the more complex plutonium bomb (Fat Man). The available supply of enriched uranium was very small at that time, and it was felt that the simple design of a uranium "gun" type bomb was so sure to work that there was no need to test it at full scale.



Physical effects of the bomb

After being selected in April 1945, Hiroshima was spared conventional bombing to serve as a pristine target, where the effects of a nuclear bomb on an undamaged city could be observed. While damage could be studied later, the energy yield of the untested Little Boy design could be determined only at the moment of detonation, using instruments dropped by parachute from a plane flying in formation with the one that dropped the bomb. Radio-transmitted data from these instruments indicated a yield of about 15 kilotons.
Comparing this yield to the observed damage produced a rule of thumb called the 5 psi lethal area rule. The number of immediate fatalities will approximately equal the number of people inside the lethal area.
The damage came from three main effects: blast, fire, and radiation.

Blast

The blast from a nuclear bomb is the result of X-ray-heated air (the fireball) sending a shock/pressure wave in all directions at a velocity greater than the speed of sound (aka, the "Mach-Stem"), analogous to thunder generated by lightning. Most knowledge about nuclear weapon urban blast destruction originates from studies of Little Boy at Hiroshima. Data from the explosion at Nagasaki offers less insight, since hilly terrain deflected the blast and generated a more complicated pattern of destruction.
At Hiroshima, severe structural damage to buildings extended about 1 mile (1.6 km) in radius from ground zero, making a circle of destruction 2 miles (3.2 km) in diameter. The blast sent out a hyper-intensified shock wave which travelled at (slightly above) the speed of sound, turning buildings into shrapnel. There was little or no structural damage outside of this one-mile (1.6 km) radius. At one mile (1.6 km), the force of the blast wave was 5 psi, with enough duration to implode houses and reduce them to kindling.
Later test explosions of nuclear weapons with houses and other test structures nearby confirmed that 5 psi (34,000 Pa) is an important threshold. Ordinary urban buildings experiencing it will be crushed, toppled, or gutted by the force of air pressure. The picture at right shows the effects of a nuclear-bomb-generated 5 psi pressure wave on a test structure in Nevada in 1953.
The most important effect of this kind of structural damage was that it created fuel for a firestorm. For this reason, the 5 psi contour defines the lethal area for blast and fire.

Fire

The first effect of the explosion was blinding light, accompanied by radiant heat from the fireball. The Hiroshima fireball was 1,200 feet (370 m) in diameter, with a temperature of 7,200 °F (3,980 °C). Near ground zero, everything flammable burst into flame, glass products and sand melted into molten glass. Contrary to popular opinion, nobody would have been vapourised by the burst - the detonation occurred at over 500 m altitude and the fireball did not reach the ground. The thermal pulse, while being hot enough to ignite flammable material at distance, was short in duration and these fires went out immediately after the flash or were blown out by the blast wave. The ensuing firestorm was caused by things like damaged fuel or gas pipes and tanks, overturned stoves, destroyed furnaces etc. One famous, anonymous Hiroshima victim left only a "shadow", permanently etched into stone steps near a bank building. Again, converse to popular knowledge, the "shadow" is not the remains or ash of the victim, but in fact it was the area around the shadow, not obscured by the person's silhouette that was lightened by X-rays from the burst, leaving a dark "negative" shadow.
The Hiroshima firestorm was roughly two miles (3.2 km) in diameter, corresponding closely to the severe blast damage zone. (See the USSBS map, right.) Blast-damaged buildings provided fuel for the fire. Structural lumber and furniture were splintered and scattered about. Debris-choked roads obstructed fire fighters. Broken gas pipes fueled the fire, and broken water pipes rendered hydrants useless.
As the map shows, the firestorm jumped natural firebreaks (river channels), as well as prepared firebreaks. The spread of fire stopped only when it reached the edge of the blast-damaged area, encountering less available fuel.
Accurate casualty figures are impossible to determine, because many victims were cremated by the firestorm. For the same reason, the proportion of firestorm victims who survived the blast and died of fire can never be known. Casualty figures are based on the estimated population inside the lethal area when the bomb detonated.

Radiation

Local fallout is dust and ash from a bomb crater, contaminated with radioactive fission products. It falls to earth downwind of the crater and can produce, with radiation alone, a lethal area much larger than that from blast and fire. With an air burst, the fission products rise into the stratosphere, where they dissipate and become part of the global environment. Because Little Boy was an air burst 1,900 feet (580 m) above the ground, there was no bomb crater and no local radioactive fallout.
Intense neutron and gamma radiation came directly from the fireball. Most people close enough to receive lethal doses of direct radiation died in the firestorm before their radiation injuries would have become apparent. Survivors on the edge of the lethal area and beyond suffered injuries from radiation.
Some who initially survived died soon afterward due to acute radiation sickness, but most of the radiation effects are evident only statistically, as increases in the incidence rates of cancer, leukemia and certain non-cancer diseases over the lifetimes of the survivors and their children who were exposed in utero. To date, no radiation-related evidence of heritable diseases have been observed among the survivors' children.

The bombing of Hiroshima

The bomb was armed in flight 31,000 feet (9,400 m) above the city, then dropped at approximately 08:15  (JST) August 6, 1945. After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism. The detonation happened at an altitude of 1,968 feet (600 m). With a yield of 13 to 18 kilotons, it was less powerful than "Fat Man", which was dropped on Nagasaki (21–23 kt). The official yield estimate of "Little Boy" was about 16 kilotons of TNT equivalent in explosive force, i.e. 6.3 × 1013 joules = 63 TJ (tera-joules). However, the damage and the number of victims at Hiroshima were much higher, as Hiroshima was on flat terrain, while thehypocenter of Nagasaki lay in a small valley.

According to figures published in 1945, 66,000 people were killed as a direct result of the Hiroshima blast, and 69,000 were injured to varying degrees.

The U.S. Department of Energy gives this account of the death toll of the bombing of Hiroshima:

"By the end of 1945, because of the lingering effects of radioactive fallout and other after effects, the Hiroshima death toll was probably over 100,000. The five-year death total may have reached or even exceeded 200,000, as cancer and other long-term effects took hold."

The success of the bombing was reported with great enthusiasm in the United States in the days following the attacks. See Atomic bombings of Hiroshima and Nagasaki for discussion of contemporary support vs. opposition to the bombings, on both moral and military grounds.

 

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