The Nuclear Trinity

How the Gadget, the Little Boy and the Fat Man shaped the atomic age – and how science took centre stage for the nuclear arms race

Nuclear war has never been closer than in October 1962 when confrontation between the Soviet Union, Cuba and the United States of America reached crisis point but the nuclear arms war began 23 years earlier in 1939, when a letter signed by Albert Einstein landed on the desk of American President Franklin Roosevelt.

Einstein was one of many who believed Nazi Germany was developing nuclear weapons, and his letter encouraged Roosevelt to order a research program – the Uranium Committee – to investigate the possibility of using nuclear fission for wartime purposes.

Six years later, ‘Little Boy’ and ‘Fat Man’ were dropped over Japan. Within minutes – seconds even – two Japanese cities were destroyed by uranium and plutonium bombs: but it could have been sooner.

The British atomic bomb project, the MAUD Committee – who first met in April 1940 – made a major breakthrough by discovering the fissile properties of uranium-235. They concluded an atomic bomb would lead to decisive results in the war and suggested work should continue at the highest priority to develop a weapon in the shortest time possible. These reports were ignored by Uranium Committee chairman Lyman Briggs.

Frustrated by the lack of progress, MAUD member Mark Oliphant met with the whole Uranium Committee in August 1941 to prompt them into taking action. As a result, the Office of Scientific Research and Development (OSRD) was created in December, with scientist and engineer Vannevar Bush as its director. The project – now named the Manhattan Project – was transferred to the OSRD and began to pick up pace.

American physicist and Nobel laureate Arthur Compton began to study plutonium and fission piles (primitive nuclear reactors) at the University of Chicago Metallurgical Laboratory. J Robert Oppenheimer took over research on fast neutron calculations, neutron diffusion – how neutrons move in the chain reaction – and hydrodynamics, how the explosion produced by the chain reaction might behave.

The measurement of the interactions of fast neutrons with materials from the bomb were essential as scientists needed to know the number of neutrons produced in the fission of uranium and plutonium. The substance surrounding the nuclear material needed to be able to reflect or scatter neutrons back into the chain reaction before it was blown apart in order to increase the energy produced. The properties of both materials were still relatively unknown.
A meeting at the University of California, Berkeley in June 1942 reviewed Oppenheimer’s work and the general theory of fission reactions: it was quickly confirmed that a fission bomb was feasible.

It was determined that although there were many possible ways of arranging the fissile material into a critical mass, the simplest was the gun-type method. This involved shooting a cylindrical plug into a sphere of active material with a ‘tamper’ – a dense material that would focus neutrons inward and keep the reacting mass together to increase efficiency.

In the summer of 1942, the project was assigned to the US Army under Col. Leslie Groves who appointed Oppenheimer – a theoretical physicist – as the scientific director of the Manhattan Project, a position that would lead him to be dubbed ‘the father of the atomic bomb’. By December, the first self-sustaining nuclear reaction in an experimental nuclear reactor took place in at the University of Chicago, headed by Enrico Fermi.

Three years later, the first test of an atomic bomb took place – it was codenamed Trinity. On 16th July 1945, ‘The Gadget’ – a plutonium device with the explosive power equivalent to 20 kilotons of TNT – was lifted to the top of a 100ft tower to give scientists a better idea of how the bomb might behave when dropped from a plane. Predictions ranged from a dud, to 18 kilotons to destruction of nearby New Mexico or ignition of the atmosphere and incineration of the planet.

The bomb was primarily made from plutonium-239 – a synthetic element created when neutrons released by the fission of uranium-235 are absorbed by uranium-238 to give uranium-239; this rapidly decays to neptiunium-239, which undergoes further radioactive decay to plutonium-239 in just over two days.

Development had been directed towards a gun-type fission weapon using plutonium – codenamed Thin Man – but the risk of spontaneous fission was too great for this to be feasible, so scientists switched to an implosion type weapon with a plutonium core.

They settled on a subcritical core of plutonium in the centre of a hollow sphere of high explosives, 32 pairs of detonators on the surface of the explosive were fired simultaneously to produce a powerful inward pressure on the core.  The core had to be a near-perfect sphere to ensure the bomb was clean – meaning it would leave relatively little radioactive contamination – so a carefully designed system used a series of explosive lenses and alternating fast and slow burning explosives to shape a spherical shock wave. This focussed the dangerous force inwards with enough force to physically compress the plutonium core to several times its original size, leading to a supercritical conditions and nuclear initiation.

The Gadget exploded at 5.29 local time, and left a crater of radioactive glass – now known as trinitite – 3m deep and 330 feet wide. The surrounding mountains lit up and the now infamous mushroom cloud reached 12km in height. The shockwave took 40 seconds to reach the observers 10 miles away and the heat wave was described as being as hot as an oven.

In the official Trinity test report, General Thomas Farrell, deputy commanding general and chief of field operations wrote: "The lighting effects beggared description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, gray, and blue. It lighted every peak, crevasse and ridge of the nearby mountain range with a clarity and beauty that cannot be described but must be seen to be imagined...1"

Fat Man used the same conceptual design as The Gadget, and was dropped by B-29 bomber Bockscar over Nagasaki on 9th August 1945. It was detonated at 550m with a yield equivalent to 21 kilotons of TNT. The bomb missed its target – perhaps due to the hilly terrain – but killed 39,000 and injured another 25,000.

Just three days earlier, Little Boy – a gun-type fission weapon – had been dropped on Hiroshima by Enola Gay. Little Boy was the first uranium atomic bomb and used the rare isotope uranium-235. This isotope had to be physically separated from uranium-238 using gaseous diffusion. This method – developed by Franz Simon and Nicholas Kurti – involved forcing uranium hexafluoride through a semi-permeable membrane so that there was a slight separation of uranium-235 and -238.

The core of Little Boy contained 50kg of uranium enriched to 89%, and 14kg at 50%, giving an average of 80% and 2.5 critical masses. It was designed to detonate around 580m above ground for the largest destructive effect.

A uranium-235 bullet was fired down a conventional gun barrel into another mass of uranium-235, rapidly creating the critical mass of the element and resulting in an explosion. The design was so sure to work that it wasn’t tested in advance. The bomb exploded at 8.15 Japanese time with an energy equivalent to between 13 and 18 kilotons of TNT. It killed 140,000.

However, it wasn’t just the British and Americans who were developing nuclear weapons: a similar effort was undertaken in the USSR in September 1941, headed by nuclear physicist Igor Kurchatov. In 1949 they tested ‘First Lightning’ – the Soviet Union’s first plutonium-based nuclear device. It was very similar to Fat Man – an implosion type device with a plutonium core – and its explosion yielded an equivalent of 22 kilotons of TNT2.

Twelve years later in 1961, the Soviet Union tested the Tsar Bomba – a thermonuclear weapon. It was the only hydrogen bomb of its kind to be tested – the effects were devastating; buildings up to 170 km were destroyed and the peak of its mushroom cloud peaked inside the mesosphere3.

A year later, America and the USSR were locked in intense negotiations to avoid nuclear war. On 14th October 1962, the US had photographic evidence that the USSR was building bases for medium range ballistic missiles and IRBMs capable of striking the US in Cuba. This was in response to the American deployment of Thor intermediate range ballistic missile (IRBM) with nuclear warheads in the UK, and Jupiter IRBMs in Italy and Turkey in 1958. Both were capable of hitting Moscow.

What followed were 14 intense days of negotiations between John F Kennedy and Nikita Khrushchev – the fallout from Hiroshima and Nagasaki just 17 years earlier was still raw and both knew they couldn’t risk nuclear war. On 28th October Kennedy secretly agreed to remove all missiles in southern Italy and Turkey, and Khrushchev agreed to remove all missiles in Cuba.

Nuclear war had been avoided, but a new and menacing military legacy had been left – scorched into the political landscape like the infamous nuclear shadows left by the bombs themselves. As for the scientific legacy of the nuclear arms race, that is altogether a different story and is based on a greater understanding of the subatomic world. Particle accelerators, synchrotrons, nuclear power (for all its pitfalls, still a remarkable scientific feat), MRI scanners, materials engineering, even radiocarbon dating – all spawned or enhanced in some way by the rapid focussing of expertise and research that the Manhattan Project demanded. From the shadow of war, great science can flourish – and in this case the most destructive weapon even created by man has led to some truly staggering scientific advances.   

    
References:
1. NuclearFiles.org http://www.nuclearfiles.org/menu/key-issues/nuclear-weapons/history/pre-cold-war/hiroshima-nagasaki/decision-drop-bomb-chronology.htm
2. First Lightning http://en.wikipedia.org/wiki/First_Lightning
3. Tsar Bomba http://en.wikipedia.org/wiki/Tsar_Bomba

Author:
Kerry Taylor Smith is Laboratory News's staff writer. She has a degree in Natural Sciences.