Atomic Bomb: Hiroshima and Nagasaki

The only Atomic Bombs droppedin anger were on Hiroshima and Nagasaki.
On Monday August 6th, 1945 the United States dropped the first nuclear weapon "Little Boy" on the Japanese city of Hiroshima. This was followed by the dropping of another nuclear bomb "Fat Man" over the city of Nagasaki on August 9th. Further attacks with nuclear weapons were considered but the Japanese government accepted the conditions for surrender demanded by the Allies making the attacks unnecessary.
The two bombs were of fundamentally different designs. The "Little Boy" design used a hollow bullet of Uranium-235 fired into a target cone of Uranium-235 to produce a critical mass that initiated a chain reaction. The design was very simple but so potentially unsafe and prone to accidental detonation that the design was only ever used once in the "Little Boy" bomb. The "Fat Man" bomb used an implosion method to compress a Plutonium core to critical mass.

Manhattan Project

The Manhattan project was the code name given to the project that delivered the first atomic bombs in World War Two.
The project was initiated after Albert Einstein sent a letter in 1939 to President Roosevelt stating that Nazi Germany had started work to develop nuclear weapons. In response Roosevelt set up "The Uranium Committee" to carry out research into the potential for nuclear weapons. Progress was slow until a British team discovered that Uranium-235 had the required properties for producing a nuclear bomb.
In December 1941 the Office of Scientific Research and Development was set up to replace "The Uranium Committee". From 1942 to 1946 the project was under the control of American Lieutenant General Leslie R. Groves and the Scientific director was physicist Robert Oppenheimer. Oppenheimer's team developed methods for using Uranium-235 and the newly discovered Plutonium to create a fission bomb to produce a nuclear explosion.
The project was undertaken at a large number of facilities across America but the main four were: Los Alamos, New Mexico; Oak Ridge, Tennessee; Hanford, Washington and Chalk River, Ontario. Oak Ridge was where the Uranium enrichment process took place, Hanford was where the reactor was located for producing Plutonium. Chalk River was close to industries related to the production of the bomb and Los Alamos was where the bombs were assembled and testing took place.
The idea of using a gun to fire a Uranium or Plutonium pellet into another mass of similar material was well understood but using this method with Plutonium resulted in the critical mass being dispersed and the bomb fizzling out. Development therefore focussed around how to make the Plutonium bomb more efficient.
In the end Oppenheimer decided to adopt a method by which shaped charges produced a perfectly spherical explosive wave that compressed a core of plutonium. This implosion method was highly complex and it was decided that a trial, code named "Trinity", was to be carried out whereby a test device was to be detonated.
The test took place on July 16th, 1945 near the Los Alamos site in New Mexico. This was the first ever man-made Nuclear explosion and proved the design to be feasible.

Little Boy

"Little Boy" was the code name for an atomic bomb developed as part of the Manhattan project during World War Two. The bomb used a gun method to fire one mass of Uranium-235 at another mass of Uranium-235 to produce nuclear fission, which released a vast amount of energy equivalent to around 15 kilotons of TNT.
"Little Boy" was 3m(9.8ft) long and 0.7m (28in) in diameter. It weighted around 4,000 kg (8,900 lbs) of which 64 kg was Uranium 235.
The gun method was very simple and was never tested before being used in anger. There were no safety features in the "Little Boy" design and it was susceptible to accidental detonation for many reasons including crashing, electrical short, fire or water damage. Uranium 235 was in very short supply. For these reasons only one "Little Boy" bomb was ever produced.
The Little Boy a-bomb was a Uranium based bomb
When it was found that Plutonium was unsuitable for a gun type device "Little Boy" was nearly cancelled but went ahead as a back up plan in case the alternative design for the Plutonium bomb was not successful.
The bomb was designed to detonate at an altitude of 580m (1,900 ft), which was calculated to cause the most destruction over the largest area. A timer, activated when an electrical circuit to the aircraft was broken, ensured that detonation would not occur until at least fifteen seconds after being released. At this time a barometric system assumed control that delayed activation of a precision radar altimeter firing system until the bomb was close to the firing altitude. Once the radar altimeters registered the correct detonation altitude they closed a switch that ignited a cordite charge, which launched the Uranium bullet, at around 300m/s, towards the Uranium target at the other end of the bomb. When the two Uranium masses collided the aggregate reached critical mass and set off a chain reaction causing an explosion.

Fatman

The Plutonium core was detonated by a spherical charge surrounding it.
"Fat Man" was the code name for the Plutonium-239 based atomic bomb that was developed as part of the Manhattan project during World War Two.
The gun method used for detonating the Uranium based "Little Boy" bomb was unsuitable for use with Plutonium so "Fat Man" used an implosion method to compress a central core of Plutonium to reach critical mass.
"Fat Man" was 3.3m (10ft 8") long, 1.5m (5ft) in diameter and weighed 4,600 kg (10,200lbs). The bomb was much rounder than "Little Boy", being around twice the diameter. This was due to the design of the implosion device.
Thirty two detonators were placed on the surface of a sphere containing shaped explosive charges. These were fired simultaneously to create a perfectly spherical inward shock wave that compressed a core of Plutonium increasing its density to the point that a chain reaction occurred.
The solid core weapon became known as the "Christy Gadget" after Robert Christy the physicist who produced the calculations that indicated the method was possible.
"Fat Man" was much easier to make safe than "Little Boy", electrical safety plugs ensured that no accidental electrical fault would detonate the device. A mechanical plug prevented the insertion of the critical mass. Arming of the bomb, by the removal of safety devices, took place at a time when the weapon was to be used.

Hiroshima

A train left Los Alamos on July 14th, 1945 carrying the non-nuclear components for several "Little Boy" bombs and a single completed Uranium 235 bullet. At the San Francisco Naval Shipyard the items were loaded onto the heavy cruiser USS Indianapolis, which 10 days later on the 26th delivered them to the Island of Tinian, home of the 313th Heavy Bombardment Wing.
On August 6th 1945 the Boeing B29 Superfortress "Enola Gay" piloted by Colonel P. Tibbets of the 393rd Heavy Bombardment Squadron, 509th Composite Group USAAF dropped the "Little Boy" atomic bomb on the Japanese city of Hiroshima. This was the second man-made nuclear explosion after the "Trinity" test. A second B29 simultaneously dropped instruments to measure the effects of the explosion.
The bomb was armed in flight over Hiroshima. At roughly 8:16am "Little Boy" was dropped from a height of 9,400m (31,000ft). Forty three seconds later the firing mechanism started and at the designated height of 580m the bomb detonated. Of the 64kg of Uranium less than 1kg reacted and only 0.6g was transformed into energy yielding the equivalent of around 15 kilotons of TNT. The resulting fireball was nearly 4,000C and over 350m in diameter. It produced a shockwave exceeding the speed of sound in all directions that caused substantial structural damage to buildings in a circular area of 1.6km (1 mile) radius. Fire then spread around the city, which was predominantly made of wood. Survivors at the edge of the blast area were exposed to radiation that caused injuries and deaths for many years after the event.
Hiroshima had been chosen because it was of substantial military significance, being the location of the Second Army Headquarters, a port and a centre of communications. The total number of people killed numbered approximately 130,000.

Nagasaki

The B29 Superfortress "Bockscar", piloted by Major Charles Sweeney of the 393rd Heavy Bombardment Squadron, 509th Composite Group USAAF took off from Tinian during the early hours of August 9th, 1945 carrying the "Fat Man" bomb.
The primary target was intended to be the Japanese city of Kokura and the city of Nagasaki was the secondary target. Sweeney's B29 was preceded by two B29's acting as weather scouts. Two additional B29's accompanied Sweeny's aircraft to monitor the effects of the blast. The weather scouts reported both targets to be clear but because of a delay Sweeny arrived over Kokura late and by this time cloud had obscured the target. Sweeny made the decision to move on to the secondary target Nagasaki.
The city of Nagasaki was an important sea port and industrial centre that imported and produced equipment vital to Japan's war effort.
The Fat Man a-bomb was a plutonium based bomb
An air raid warning was sounded in Nagasaki but the 'all clear' signal was given when it evident that only two bombers were approaching. The Japanese assumed them to be on a reconnaissance mission.
At just after 11-00am Bockscar's bombardier made a visual sighting of the target and released the nuclear weapon. Less than a minute later it detonated 469m (1,540ft) above the city. The weapon was off target by 3km (2 miles) exploding in a valley, the surrounding hills protecting the majority of the city from the blast. The explosion yielded the equivalent of around 21 kilotons of TNT and generated a heat wave of 3,900C. Casualties are estimated at between 40,000 to 80,000 from the immediate effects of the blast and subsequent injuries.
This was the second, and to date the last, time a nuclear weapon has been used. There were plans to drop more bombs but this became unnecessary when, on August 10th, 1945, the Japanese government issued a statement that it was prepared to accept the unconditional surrender demanded by the Allies and at noon on August 15th a recorded message by Emperor Hirohito announcing the surrender of Japan was broadcast to the Japanese nation.