Oppenheimer: Welcome back to "Science For Everyone", ABP Live's weekly science column. Last week, we discussed the science behind yawning, and the fact that it does not indicate a need for sleep. Since Christopher Nolan's latest film Oppenheimer released in India on July 21, 2023, we discuss this week the Manhattan Project and the science behind the atomic bomb. The nuclear weapons developed by American theoretical physicist Julius Robert Oppenheimer through the Manhattan Project, a US government research programme, helped end World War II. The atomic bomb was designed at the Manhattan Project's laboratory at Los Alamos, New Mexico. Since Oppenheimer was the director of the laboratory, and played the central role in the development of this powerful weapon, he is known as the 'Father of the Atomic Bomb'. World War II was the first and only time such destructive weapons were used in conflict.
In Nolan's Oppenheimer, Cillian Murphy plays the titular character; Emily Blunt stars as Oppenheimer's wife, Kitty Oppenheimer; Robert Downey Jr appears as Lewis Strauss, former chairman of the Atomic Energy Commission (AEC) who denied Oppenheimer's security clearance; and Matt Damon portrays the character of Leslie Groves, who was in charge of all Army activities relating to the Manhattan Project.
Why the Manhattan Project was a huge scientific breakthrough
The Manhattan Project was one of the greatest scientific breakthroughs of the 20th century. Leaders and scientists from all scientific fields and engineering disciplines had come together during the Manhattan Project to build the first atomic bombs. They explored the fields of nuclear physics and chemistry, and used the concepts of practical engineering to process radioactive materials uranium-235 and plutonium-239, and develop atomic bombs. In order to produce the nuclear weapons, the scientists leveraged nuclear fission of uranium-235 and plutonium-239, and the chain reactions that can liberate colossal amounts of destructive heat energy.
Fission is a form of chemical interaction between subatomic particles, and nuclear fission is the process in which an atom is split by bombarding the nucleus with neutrons, a type of subatomic particle carrying no charge. The other subatomic particles in an atom are protons and electrons, which are positively- and negatively-charged particles respectively. An atomic nucleus has both protons and neutrons, but no electrons, because of which the core of an atom is positively-charged. The electrons revolve around the nucleus in shells. It is the collision of atomic particles that leads to different chemical reactions.
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The Manhattan Project was a success due to several factors, including scientific intellect, and communication between scientific minds.
Before delving into the chemistry and engineering aspects of the Manhattan Project, let us try to understand the events that led to this groundbreaking research programme.
In the early 20th century, fission was a newly recognised process, and in order to make the most of it, American scientists, many of whom were refugees from fascist regimes in Europe, took steps in 1939 to organise a project.
In 1939, Nazi Germany invaded Poland, following which GB Pegram of Columbia University arranged a conference between Enrico Fermi and the Navy Department in March that year. After this, in the summer of 1939, physicists Albert Einstein, Leo Szilard, and Eugene Wigner warned the then President Franklin D Roosevelt of the jeopardy entire humanity would be subject to if the Nazis were the first to make a nuclear bomb. They urged Roosevelt to establish a programme to study the potential military use of fission, and find ways to perform an uncontrolled fission chain reaction. In Nolan's Oppenheimer, Danny Deferrari portrays the role of Fermi, Tom Conti plays Einstein, and Máté Haumann appears as Szilard.
Roosevelt agreed, and appropriated $6,000 for the task in February 1940. A committee, headed by Lieutenant General Briggs, director of the National Bureau of Standards, which later became the National Institute of Standards and Technology, was formed to supervise the research.
On December 6, 1941, the project came under the direction of the Office of Scientific Research and Development, led by Vannevar Bush. Matthew Modine plays Bush in Oppenheimer.
After Japan attacked Pearl Harbour on December 7, 1941, the US declared war on Japan. The US became fully involved in World War II when Germany and Italy declared war on the US three days after the country declared war on Japan.
After this, the War Department was given joint responsibility for the project. In May 1942, a decision was made to proceed simultaneously on all methods that would help obtain fissionable materials, including the electromagnetic process developed at the University of California, Berkeley, under Lawrence, and the diffusion process developed under Harold Urey at Columbia University. The US Army Corps of Engineers started the construction of pilot plants, laboratories and manufacturing facilities for the scientists.
Since a significant portion of the early research had been performed at Columbia University in Manhattan, the Manhattan Engineer District, an office of engineers in New York City, was assigned management of the construction work. In September 1942, Army Officer Groves was given the responsibility of supervising all Army activities relating to the project. The research work went on to be known by its code name "Manhattan Project".
He chose three key sites for the Manhattan Project: Oak Ridge in Tennessee, Los Alamos in New Mexico, and Hanford in Washington.
Groves recognised Oppenheimer's exceptional scientific brilliance, and selected him to direct weapons development.
On December 2, 1942, Fermi and Szilard created the first controlled nuclear reactor, marking the beginning of the Atomic Age, an era of unearthing of atomic and subatomic marvels. Fermi and Szilard's model went on to be reconstructed into five different reactor prototypes.
In 1943, Oppenheimer chose the plateaus of Los Alamos, near Santa Fe, New Mexico, for the construction of the Los Alamos Laboratory.
Oppenheimer was searching for a way to separate uranium-235 from natural uranium, and to determine the critical mass of uranium required to make a nuclear bomb. Critical mass refers to the minimum amount of a fissile material required to initiate a self-sustaining fission reaction. The laboratory was tasked with the responsibility of developing methods to reduce fissionable products to pure metal and fabricate the metal to required shapes.
By August 1943, several scientists from Great Britain and Canada had moved to the US to join the Manhattan Project.
Plutonium-239 was the other radioactive material of interest, because the metal is highly fissionable.
The science behind the Manhattan Project
When neutrons collide with the nucleus of another atom, a tremendous amount of energy is released. Since protons as well as the nucleus of an atom are positively-charged, they will repel each other, and hence, protons cannot be used in nuclear bombardment.
Therefore, in nuclear fission, neutrons are used, and are fired towards an atom with the aim of making the neutrons fuse with the nucleus. After this, a less-stable isotope is produced. Since the isotope is less stable than the original element, it is chemically volatile, splits into two stable atoms, releases neutrons, and generates energy in the form of Gamma radiation.
However, this is not the end of the reaction. The newly released neutrons fuse with the nuclei of other atoms, causing them as well to release neutrons. In this way, a chain reaction begins, and not only are neutrons released, but also large amounts of heat radiation. The chain reaction, if left unchecked, can create an atomic bomb.
However, not every atom is fissionable, because if that were the case, there would have been no stability to matter, and chaotic energy transformations would have ensued in the world.
The isotopes of only a few elements can undergo a fissile chain reaction.
German scientists Otto Hahn and Fritz Strassman conducted an experiment in 1939, as part of which they added non-radioactive barium to uranium, and fired neutrons at the latter. At the conclusion of the experiment, they observed some barium atoms had radioactivity. This was surprising because they had used non-radioactive barium in the reaction.
Austrian-Swedish physicist Lise Meitner said that when uranium was bombarded with neutrons, each atom was split into two, forming two elements, one of which was barium. The barium atoms formed as a result of the splitting of uranium were radioactive.
After learning about the theory that uranium is a fissionable element, Danish physicist Neils Bohr told the entire world about the important discovery at an international conference in Washington. In Oppenheimer, Kenneth Branagh plays Neils Bohr.
Uranium occurs in three forms: uranium-234, uranium-235, and uranium-238. Uranium-238 is the most abundant isotope, and is stable compared to the other two forms. However, uranium-235, which easily undergoes nuclear fission, and can sustain a chain reaction, is rare.
Uranium-235 can cause a chain reaction if it is bombarded with neutrons travelling at adequate velocity and at an appropriate angle. The unstable nucleus of uranium-235 absorbs the neutron, becomes uranium-236, which is extremely unstable, and instantly splits into two atoms of different elements, emits two neutrons, and results in a chain reaction, which liberates gargantuan amounts of gamma radiation.
In 1931, John Dunning and Eugene Booth successfully separated uranium-235 from uranium-238, and this is how the element became the most preferred fissionable material. Only a small amount of uranium-235 had massive destructive potential.
The need to create nuclear weapons before the enemy nations created such devastating bombs themselves, and the newfound knowledge of nuclear fission culminated in the Manhattan Project in 1942.
However, two major challenges remained: obtaining sufficient fissionable material, and designing a nuclear bomb that would maintain and maximise a fissile chain reaction. Before the Manhattan Project started, Dunning and Booth had found a method to separate uranium-235 from uranium-238, but their purpose was purely scientific, and would not have served any purpose to a project that was tasked with finding a solution to ending the Second World War.
Also, none of the scientists involved in the project had the experience of developing a huge nuclear reactor that would allow a bomb to be deployed.
The biggest problem was that uranium-235 constitutes only one per cent of natural uranium, which also contains uranium-238.
Lawrence, at the University of California, Berkeley, developed the first technique to isolate a practical amount of uranium-235. Leveraging the electromagnetic properties of atoms, and integrating chemistry and physics, he separated uranium-235 from uranium-238 using a mass spectrometer.
Since Uranium-235 and Uranium-238 had different masses, and according to Newton's law, if the same force is applied on two different masses, the lighter one will be affected more compared to the heavier one, the uranium-235 atoms travelled closer to the electromagnetic arch exerting an electromagnetic force, compared to uranium-238, which is heavier.
It was important to mobilise the uranium atoms into gaseous particles. This was performed by circulating fluorine gas over a solid uranium core, in order to produce uranium tetrafluoride gas. This gas was made to pass through the arch in order to obtain a purified uranium tetrafluoride.
However, this technique proved to be largely inefficient because the maintenance costs were high, and the process was time-consuming. Millions of dollars were invested in production and repair, but only one gram of uranium-235 was retrieved.
This was when Groves purchased land in Oak Ridge and commissioned scientists to find a new technique of uranium separation. Using the theory of gaseous diffusion of uranium hexafluoride as the basis of the technique, scientists made gaseous particles of uranium pass through a porous membrane, so that according to the laws of thermodynamics, the gaseous particles of uranium-235, which are lighter than uranium-238, pass through the porous member faster than the latter, in a process known as effusion.
Since uranium 235 hexafluoride diffuses faster than uranium 238 hexafluoride, Groves concluded that the technique could be used to obtain uranium-235, according to the University of Pittsburgh.
It was important to carry out the diffusion in completely airtight conditions because the masses of the two isotopes of uranium are similar. Since grease is an organic compound, and uranium hexafluoride reacts quickly with such substances, it could not be used to seal the tubes. Therefore, scientists started fabricating new plastics, and this was how Teflon was invented.
Thousands of consecutive members were used, and diffusion tanks were sealed with Teflon tubing. Due to financial constraints, the plant at Oak Ridge carried out the process only to the point of producing a mixture of the two isotopes. Uranium-235 was subsequently isolated by subjecting the mixture to other separation techniques.
After the discovery of fissionable uranium, Glen Seaborg discovered plutonium, the 94th element in the Periodic Table. In 1942, researchers found that plutonium-239 can also sustain a chain reaction.
Scientists also discovered that if uranium-235 is exposed to alpha particles, or Helium nuclei in a reactor for extended periods of time, the atomic number of uranium-235 increases and plutonium-239 is obtained.
Thus, the scientists working under the Manhattan Project were able to use two fissile materials for developing the atomic bombs.
How the Manhattan Project helped develop the atomic bomb
By the summer of 1945, large-scale production reactors in Hanford Engineer Works produced amounts of plutonium-239 sufficient to generate a nuclear explosion.
A field test was also scheduled to check if the nuclear explosion would be a success or failure. By this time, the budget of the Manhattan Project had risen to $2 billion.
As part of the Manhattan Project, Oppenheimer’s research team bombarded neutrons with plutonium or uranium-23 to initiate a nuclear fission reaction, during which the heavy plutonium or uranium nucleus split into smaller nuclei, and released huge amounts of of energy and other neutrons. These neutrons, in turn, hit other nuclei and caused them to split, releasing more energy and neutrons in the process. This is a chain reaction, which, if left unchecked, can create an atomic bomb.
On May 7, 1945, Nazi Germany surrendered.
On July 16, 1945, the first atomic bomb was exploded at the Alamogordo air base, which Oppenheimer called "Trinity" in reference to one of John Donne's Holy Sonnets. As part of the field test called the "Trinity Test", a plutonium implosion device known as "Gadget" was used.
On August 6, 1945, the untested uranium-235 gun assembly bomb, known as the Little Boy, was dropped on the Japanese city of Hiroshima. This was the first atomic bomb used in war. The bomb instantly killed 70,000 people, and due to the effects of radiation, over 100,000 people subsequently died.
On August 9, 1945, a replica of the plutonium-239 implosion device tested in Trinity, was dropped on Nagasaki. This bomb was called Fat Man.
This bomb instantly killed about 40,000 people, and at least 30,000 more died due to injuries and radiation poisoning by the end of the year.
The next day, the Japanese initiated surrender negotiations. Meanwhile, Groves had informed US President Harry S Truman that another bomb would be ready for delivery within a week. In Oppenheimer, Gary Oldman plays President Truman.
On September 2, 1945, Japan formally announced its surrender on board the USS Missouri.
After the hostilities came to an end, Manhattan Project physicist Philip Morrison travelled to Hiroshima to study the aftermath of the nuclear explosion, but he was so devastated by what he witnessed, that he spent the rest of his life campaigning against nuclear weapons.
The Manhattan Project was followed by Operation Crossroads, as part of which two peacetime atomic weapons tests were performed.
After this, the Manhattan Project came under the jurisdiction of the US Atomic Energy Commission (AEC).
The Manhattan Project marked the true beginning of the Atomic Age. As part of the three-year project, scientists made several important discoveries associated with bomb mechanics, atomic particles, nuclear fission, and also obtained insights into nuclear fusion. Most importantly, it brought together a large number of scientists and leaders together, who put their heart and soul into making the Manhattan Project a quintessence of scientific triumph.