Manhattan Project

I

Introduction

Manhattan Project, name given to the American programme to construct the first atomic bombs during World War II. It was the biggest single military construction project of the war, costing US$2 billion, and also the biggest and best-kept secret. Despite its size and the number of people involved, the world at large knew nothing about it until the first atomic bomb exploded over Hiroshima on August 6, 1945. It was a scientific and industrial achievement unique in its scale and its speed.

II

The Race for the Bomb

The theoretical possibility that an explosion could be brought about by atomic fission became known in 1939, the year that war broke out in Europe. Scientists discovered then that uranium atoms can fission when struck by neutrons to split other atoms in a chain reaction, releasing large amounts of energy.

Two Hungarian physicists who had recently emigrated to the United States, Leo Szilard and Eugene Wigner, alerted the US government to the possibility of an atomic bomb. Along with Albert Einstein, they wrote a letter to President Roosevelt warning that Nazi Germany might also be working towards a uranium bomb; many of the important discoveries in atomic physics had been made at German universities. Roosevelt responded by setting up an advisory committee on uranium in October 1939.

Under the aegis of this committee, American scientists at several centres examined the problem. The uranium that fissions is an isotope, a variation of an element that is chemically indistinguishable but different in its atomic structure. It is uranium-235, which constitutes only 0.7 per cent of uranium. Scientists questioned whether significant quantities could ever be separated.

Much of the initial work was done at Columbia University in New York, and the military direction was from an office in Manhattan. This was located in the Manhattan Engineering District, and the whole programme became the Manhattan Project, under the command of Major General Leslie Groves.

Meanwhile, in Britain, two immigrant physicists, the Austrian Otto Frisch and the German Rudolf Peierls, decided that an atomic explosion using uranium-235 was a practical possibility. They alerted the British authorities in early 1940 with a memorandum showing how such a bomb could be produced. After more studies, the British government set up a project to build an atomic bomb.

The United States and Britain exchanged information on weapons developments. British progress towards an atomic bomb convinced American scientists in the summer of 1941 that it could be done, and spurred on their work. In November 1941 a committee of scientists recommended that the United States embark on an all-out research programme, and President Roosevelt gave the go-ahead the following January. In the intervening weeks the Japanese attack on Pearl Harbor had brought the United States into the war, and provided an added emergency. American scientists were still worried that Germany might be working on an atomic bomb.

Scientific work was concentrated at first at the University of Chicago; scientists there working under the Italian Enrico Fermi created the world's first atomic chain reaction in an atomic reactor in December 1942.

III

Creation of the Bombs

The heads of the programme set out to create two kinds of atomic bomb in one parallel: one using uranium-235 as the material for atomic fission, the other plutonium.

Plutonium is a man-made element discovered by Glenn Seaborg in 1940, created by the fission of uranium. The plutonium then has to be separated chemically from other products of nuclear fission by a painstaking chemical process, and at that time only a few micrograms had ever been produced.

Because of the need for speed, the scientists had to concentrate on using two different methods of separating uranium-235 at the same time. One method was gaseous diffusion: the uranium is turned into a gas, uranium hexafluoride, and then passed through thousands of membranes again and again, so that the lighter uranium-235 passes through and some of the uranium-238 is left behind. The other method is electromagnetic separation, virtually atom by atom, a process developed at Berkeley, California, by the physicist Ernest O. Lawrence. Both systems were very expensive and involved difficult engineering, tackling entirely new problems and dealing with new materials.

The gaseous diffusion process was devised in New York by a team of American and British scientists. It involved creating membranes finer than any that had ever been seen, and resistant to the corrosive effect of uranium hexafluoride. New kinds of pumps had to be devised.

A whole new town was created for the uranium separation plants—Oak Ridge, on the Clinch River in eastern Tennessee. Both separation methods were used in tandem. The gaseous diffusion plant did most of the separation, reducing the proportion of the uranium-238, so that what remained was mostly uranium-235. This plant was huge, in a building almost 1 km (about 0.5 mi) long and covering 17.8 hectares (44 acres), containing hundreds of kilometres of piping. The uranium coming out of this plant was fed into the electromagnetic separation plant, where giant magnets, constructed with more than 2,032 tonnes of silver in the wiring, refined it still further so that it was almost pure uranium-235. Oak Ridge had its own power plant, as the separation process alone used more electricity than the city of Boston.

Another town was created at Hanford, in Washington State, where 45,000 construction workers excavated 19 million cu m (25 million cu yd) of earth and built 617 km (386 mi) of road to create the site for the nuclear reactor where the plutonium was created, and the plant that separated it from the reactor's other products. Work was being done at several other centres also.

Many of America's biggest industrial corporations were recruited for the project. Stone and Webster did much of the construction work, the Kellog Corporation created a separate corporation to build the gaseous diffusion plant, General Electric and Westinghouse built much of the equipment, and Du Pont built the plutonium separation plant.

In the summer of 1943 another town was rapidly created at Los Alamos atop the mesas of northern New Mexico, 48 km (30 mi) from Santa Fe. Los Alamos was the hub of the Manhattan Project; here the bombs were to be designed and assembled. The physicist J. Robert Oppenheimer, who was made director of the Los Alamos laboratory, chose it for its remoteness and many of the greatest figures in physics were brought by him to work there, including Szilard, Wigner, Niels Bohr, Hans Bethe, Edward Teller, and the young Richard Feynman, many of them refugees from Nazi-occupied Europe. Among Oppenheimer's great achievements as the organizer of what Groves called “the greatest collection of eggheads ever” was to maintain the free exchange of scientific ideas at Los Alamos, and to bridge the gap between the scientists and their military employers. The barracks town of Los Alamos, where the scientists worked in total secrecy, had no street names; mail was received at a post office box number in Santa Fe.

IV

How the Bombs Worked

A chain reaction starts in fissionable material once enough of it has been brought together to form what is called a “critical mass” (see Nuclear Weapons). The mechanism of an atomic bomb brings together enough fissile material to form a critical mass, and holds it together for a few millionths of a second while the chain reaction takes place.

The first method devised was the so-called gun method. A piece of fissionable material is fired into another, so that at the moment of impact the two together make up a critical mass.

This worked for uranium-235 but not for plutonium. For plutonium, an even faster method of assembly had to be found. Scientists devised what they called implosion. The plutonium is shaped into a sphere, and high explosives are packed all around it, and detonated in such a way that the sphere is compressed into a critical mass. A neutron initiator is incorporated into this to guarantee that a chain reaction occurs immediately: this was made of beryllium and plutonium. The shock waves from the explosion that compress the material are made to arrive at the same time by distorting them through specially constructed lenses consisting of high explosive.

The plutonium bomb was tested in the early hours of July 16, 1945, near Alamogordo, in the New Mexico desert, some 320 km (200 mi) south of Los Alamos, in the world's first nuclear explosion. Code-named Trinity, the test bomb, erected on a metal tower 33m (110 ft) high, exploded with a power that was equivalent to 20,000 tonnes of TNT and sent a mushroom cloud as high as 9,000 m (30,000 ft) into the sky. At that time the first uranium-235 bomb was already on its way to Tinian, the Pacific Island where the B-29 crews that were to drop the bombs were training for their mission.

On August 6 the uranium-235 bomb was dropped over Hiroshima, and the Manhattan Project was made public for the first time. The plutonium bomb was dropped on Nagasaki three days later. The following day the Japanese Government announced that it was prepared to surrender. The first atomic bomb killed 129,558 people out of a population of 343,698; the second killed or injured some 66,000 people in the port of Nagasaki.

See also Nuclear Energy; Arms Control (International); Radiation Effects, Biological; Radiation Sickness.