The bomb

Why a bomb

On 2 August 1939 Albert Einstein signed a letter to President Franklin Roosevelt. The physicist Leo Szilard drafted it with him. It warned that uranium could be used to make extremely powerful bombs of a new type. ^{Sources: 6, 17}

The economist Alexander Sachs, who advised Roosevelt, handed the letter over in person on 11 October 1939. On 21 October 1939 a small Advisory Committee on Uranium, chaired by Lyman Briggs, held its first meeting. ^{Sources: 7, 17, 18}

A British report changed the pace. In July 1941 Britain’s MAUD Committee concluded that a uranium bomb could be built, perhaps in about two years. On 9 October 1941 Roosevelt told Vannevar Bush to find out whether one could be made. ^{Sources: 19, 20, 21}

The Manhattan Project

The Army took charge in 1942. It set up the Manhattan Engineer District on 13 August 1942. Army engineer districts were named after the city where they began, and this one began at 270 Broadway in Manhattan. The name gave nothing away. ^{Sources: 2, 7, 8, 22}

On 17 September 1942 Colonel Leslie Groves was put in charge of the whole project. Six days later he was promoted to brigadier general. ^{Sources: 7, 8}

Three new sites did most of the work. Oak Ridge in Tennessee and Hanford in Washington made the nuclear material, and Los Alamos in New Mexico designed and built the bombs. ^{Sources: 2, 23}

Chicago Pile-1

On 2 December 1942, in a squash court under the west stands of Stagg Field at the University of Chicago, a team led by Enrico Fermi achieved the first man-made, self-sustaining nuclear chain reaction. ^{Sources: 9, 10, 24}

The pile was a stack of about 400 tons of graphite, with about 6 tons of uranium metal and 34 to 40 tons of uranium oxide, built up in 57 layers. ^{Sources: 9, 24, 25}

It went critical at 15:25 local time and ran for 28 minutes at about half a watt. Forty-nine people were there to see it. ^{Sources: 9, 10, 24, 26}

Arthur Compton telephoned James Conant with a phrase he made up on the spot: “The Italian navigator has landed in the New World.” ^{Sources: 9, 26}

Early in 1943 the pile was taken apart and rebuilt, with shielding, outside Chicago as CP-2. ^{Sources: 25}

Los Alamos

The bombs themselves were to be designed at a new, secret laboratory. On 16 November 1942 Groves and J. Robert Oppenheimer chose the Los Alamos Ranch School, a boys’ school on a remote mesa in New Mexico. The Army had authority to take it by 25 November. ^{Sources: 27, 28}

Oppenheimer was named the laboratory’s scientific director in February 1943 and arrived in the middle of March. The University of California signed the contract to run it on 20 April 1943. ^{Sources: 29, 30, 31}

Its mail went to PO Box 1663, Santa Fe. By 1945 about 6,000 people lived on the mesa. ^{Sources: 28, 32}

The plateau was not empty land. Hispano families had homesteaded there since 1887, and the Army took their homesteads in late 1942. They were paid 7 to 23 dollars an acre, while the school received 225 dollars an acre. In 2004 Congress approved a 10 million dollar fund for their heirs. ^{Sources: 33}

Pueblo people, among them the people of San Ildefonso, lost access to ancestral sites on the plateau. ^{Sources: 34}

Oak Ridge: uranium

Little Boy needed uranium rich in uranium-235, the kind that splits easily. It is chemically the same as ordinary uranium, so it has to be separated by its slightly lower weight. ^{Sources: 35}

On 19 September 1942 Groves signed the order to acquire land in east Tennessee for what became Oak Ridge. The Army took about 59,000 acres. About 1,000 families lost their homes, many with only weeks to leave. ^{Sources: 36, 37, 38, 39}

Three plants took the uranium in stages. S-50 used heat, K-25 pumped uranium gas through barriers, and Y-12 separated it with magnets in machines called calutrons. Uranium passed from S-50 to K-25 and then to Y-12, a little richer each time. ^{Sources: 35, 40}

Copper was scarce, so the magnet coils at Y-12 were wound with 14,700 tons of silver borrowed from the US Treasury. Almost all of it went back after the war. ^{Sources: 41}

K-25 was almost half a mile long. When it was built it was the largest building in the world. ^{Sources: 40, 42}

About 10,000 young women from the area watched the dials of the calutrons. They were not told what they were making until 6 August 1945. ^{Sources: 43}

By 1945 about 75,000 people lived in the new town of Oak Ridge. ^{Sources: 38, 39}

Hanford: plutonium

Fat Man used plutonium, an element made in nuclear reactors. On 16 January 1943 Groves approved a site on the Columbia River in Washington State. DuPont designed, built and ran the reactors and the separation plants for a fee of one dollar. ^{Sources: 44, 45, 46}

The site covered about 600 square miles. In March 1943 the people of Hanford, White Bluffs and Richland were told to leave, most within 30 days. Hanford and White Bluffs were cleared, and Richland became the workers’ town. ^{Sources: 47, 48}

The Wanapum, Yakama, Umatilla and Nez Perce peoples lost access to the river lands. ^{Sources: 47, 48}

At the peak, in mid 1944, about 50,000 people lived in the construction camp. ^{Sources: 45, 49}

The B Reactor first went critical on the night of 26 September 1944. Within hours the reaction faded. John Wheeler worked out that xenon-135, made by the reaction itself, was absorbing neutrons. DuPont had built the reactor with more fuel tubes than the design needed, and loading them overcame the problem. ^{Sources: 11, 12, 50, 51}

Chemical separation began on 26 December 1944. The first Hanford plutonium reached Los Alamos on 2 February 1945. ^{Sources: 45, 49, 51}

A change of design

In April 1944 Los Alamos received its first plutonium made in a reactor. Emilio Segrè’s group found that it split on its own far too often, because it contained plutonium-240. In a gun design the two pieces would meet too slowly, and the bomb would fail. ^{Sources: 52, 53}

On 17 July 1944 work on Thin Man, the plutonium gun, stopped. Los Alamos reorganised around implosion, squeezing a ball of plutonium with explosives fired all around it. It was a far harder problem. ^{Sources: 53}

Trinity

At 05:29:45 Mountain War Time on 16 July 1945 the first nuclear explosion lit the Jornada del Muerto desert in New Mexico, on land that is now White Sands Missile Range. The device, called the Gadget, sat on a 100-foot (30 m) steel tower. ^{Sources: 13, 14, 54, 55, 56}

Oppenheimer named the test Trinity. Official histories say the poetry of John Donne inspired the name. ^{Sources: 14, 55}

Oppenheimer watched from the control bunker 10,000 yards south of the tower. Groves was at base camp, about 10 miles to the south-west. ^{Sources: 14, 57}

A steel vessel called Jumbo, weighing 214 tons, had been built to catch the plutonium if the explosives fired but the chain reaction failed. By the time of the test the scientists were confident enough not to use it. ^{Sources: 13, 58}

The heat fused the desert sand into a pale green glass, later named trinitite. ^{Sources: 13, 56}

The yield has been revised as methods improved: 18.6 kilotons in 1945, 21 kilotons in 1963, and about 25 kilotons in a Los Alamos study of 2021. ^{Sources: 59}

Nobody warned the people living nearby. Fallout drifted to the north-east. In 2020 the National Cancer Institute estimated that it may have caused several hundred extra cancers, mostly of the thyroid, while stressing that the figure is very uncertain. ^{Sources: 60, 61}

After decades of campaigning by the Tularosa Basin Downwinders Consortium, Congress extended radiation exposure compensation to people in New Mexico in July 2025. ^{Sources: 60, 62}

Little Boy

Little Boy was a gun-type bomb. A charge fired one piece of uranium down a barrel into another. It was 3.0 m long and 71 cm wide, weighed about 4,400 kg, and held about 64 kg of highly enriched uranium. ^{Sources: 4, 63, 64}

It was never tested before it was used. The scientists were confident in the simple gun design, and a test would have used up all the uranium-235 then made. ^{Sources: 63, 65}

The bomb dropped on Hiroshima was the unit numbered L-11. ^{Sources: 66}

Only about 1.4 per cent of its uranium split. Its yield is put at about 15 kilotons, with an uncertainty of about 20 per cent. ^{Sources: 4, 64, 67}

Fat Man

Fat Man was an implosion bomb built around a plutonium core of about 6.2 kg. It was the same design as the Trinity device. It was 3.25 m long and 1.52 m wide, and weighed about 4,500 to 4,900 kg, depending on the source. ^{Sources: 5, 14, 64, 68, 69, 70}

Its yield is put at about 21 kilotons, with an uncertainty of about 10 per cent. ^{Sources: 5, 67}

The plutonium core went to Tinian by air. Teams had the bomb assembled, loaded and checked by the evening of 8 August 1945, after bad weather ahead brought the mission forward from 11 August to 9 August. ^{Sources: 71}

Project Alberta

Project Alberta, also called Project A, was set up at Los Alamos in March 1945 to prepare the bombs for use in combat. Captain William Parsons of the US Navy led it, with Norman Ramsey as his scientific deputy and Commander Frederick Ashworth in charge of operations. Fifty-one of its members went to Tinian. ^{Sources: 15, 72, 73, 74}

The team has a page of its own: Project Alberta.

The base on Tinian was fully working by 23 July 1945. That day the team dropped its first practice Little Boy, with no nuclear material in it. The first practice Fat Man followed on 1 August. ^{Sources: 75}

The uranium projectile for Little Boy arrived on USS Indianapolis on 26 July 1945. The target pieces came in three transport aircraft on 28 and 29 July, and the bomb was ready by 2 August. The weather was not. ^{Sources: 15, 16}

On the morning of 6 August Parsons loaded the propellant charges into Little Boy in flight, so that a crash on take-off could not set it off. ^{Sources: 15, 76}

A third bomb

On 10 August Groves told the Army Chief of Staff, General George Marshall, that the next implosion bomb could be ready in the first good weather after 17 or 18 August. Marshall wrote that it was not to be used over Japan without the express authority of the President. On 13 August Groves held back its core. ^{Sources: 77, 78}

On 21 August 1945 Harry Daghlian was fatally irradiated at Los Alamos while working with that core. ^{Sources: 79}