The first nuclear explosive devices, cumbersome and inefficient, provided the basic design building blocks of all future weapons. Pictured is the Gadget device being prepared for the first nuclear test, Trinity.
Stanisław Ulam
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Stanisław Ulam
Diagram of a gun-type fission weapon
The Scottish Café's building now houses the Universal Bank in Lviv, Ukraine.
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Ulam's ID badge photo from Los Alamos
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Stan Ulam holding the FERMIAC
Flash X-Ray images of the converging shock waves formed during a test of the high explosive lens system.
Ivy Mike, the first full test of the Teller–Ulam design (a staged fusion bomb), with a yield of 10.4 megatons on 1 November 1952
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The Sausage device of Mike nuclear test (yield 10.4 Mt) on Enewetak Atoll. The test was part of the Operation Ivy. The Sausage was the first true H-Bomb ever tested, meaning the first thermonuclear device built upon the Teller-Ulam principles of staged radiation implosion.
An artist's conception of the NASA reference design for the Project Orion spacecraft powered by nuclear propulsion
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When the positive integers are arrayed along the Ulam spiral, prime numbers, represented by dots, tend to collect along diagonal lines.
Ivy Mike, the first two-stage thermonuclear detonation, 10.4 megatons, November 1, 1952.
An animation demonstrating the lucky number sieve. The numbers in red are lucky numbers
Bassoon, the prototype for a 9.3-megaton clean bomb or a 25-megaton dirty bomb. Dirty version shown here, before its 1956 test. The two attachments on the left are light pipes; see below for elaboration.
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Subsidence Craters at Yucca Flat, Nevada Test Site.
A diagram of the Green Grass warhead's steel ball safety device, shown left, filled (safe) and right, empty (live). The steel balls were emptied into a hopper underneath the aircraft before flight, and could be re-inserted using a funnel by rotating the bomb on its trolley and raising the hopper.

There, he worked on the hydrodynamic calculations to predict the behavior of the explosive lenses that were needed by an implosion-type weapon.

- Stanislaw Ulam

The design breakthrough came in January 1951, when Edward Teller and Stanislaw Ulam invented radiation implosion – for nearly three decades known publicly only as the Teller-Ulam H-bomb secret.

- Nuclear weapon design
The first nuclear explosive devices, cumbersome and inefficient, provided the basic design building blocks of all future weapons. Pictured is the Gadget device being prepared for the first nuclear test, Trinity.

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Edward Teller

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Hungarian-American theoretical physicist who is known colloquially as "the father of the hydrogen bomb" (see the Teller–Ulam design), although he did not care for the title, considering it to be in poor taste.

Hungarian-American theoretical physicist who is known colloquially as "the father of the hydrogen bomb" (see the Teller–Ulam design), although he did not care for the title, considering it to be in poor taste.

Teller in his youth
Teller's ID badge photo from Los Alamos
Physicists at a Manhattan District-sponsored colloquium at Los Alamos on the Super in April 1946. In the front row are (left to right) Norris Bradbury, John Manley, Enrico Fermi and J. M. B. Kellogg. Robert Oppenheimer, in dark coat, is behind Manley; to Oppenheimer's left is Richard Feynman. The Army officer on the left is Colonel Oliver Haywood.
The Teller–Ulam design kept the fission and fusion fuel physically separated from one another, and used X-rays from the primary device "reflected" off the surrounding casing to compress the secondary.
The 10.4 Mt "Ivy Mike" shot of 1952 appeared to vindicate Teller's long-time advocacy for the hydrogen bomb.
Teller testified about J. Robert Oppenheimer in 1954.
One of the Chariot schemes involved chaining five thermonuclear devices to create the artificial harbor.
Teller became a major lobbying force of the Strategic Defense Initiative to President Ronald Reagan in the 1980s.
Edward Teller in his later years
Appearing on British television discussion After Dark in 1987

A special group was established under Teller in March 1944 to investigate the mathematics of an implosion-type nuclear weapon.

It included Stanislaw Ulam, Jane Roberg, Geoffrey Chew, Harold and Mary Argo, and Maria Goeppert-Mayer.

The Trinity test of the Manhattan Project on 16 July 1945 was the first detonation of a nuclear weapon.

Manhattan Project

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Research and development undertaking during World War II that produced the first nuclear weapons.

Research and development undertaking during World War II that produced the first nuclear weapons.

The Trinity test of the Manhattan Project on 16 July 1945 was the first detonation of a nuclear weapon.
Enrico Fermi, John R. Dunning, and Dana P. Mitchell in front of the cyclotron in the basement of Pupin Hall at Columbia University
March 1940 meeting at Berkeley, California: Ernest O. Lawrence, Arthur H. Compton, Vannevar Bush, James B. Conant, Karl T. Compton, and Alfred L. Loomis
Different fission bomb assembly methods explored during the July 1942 conference
Manhattan Project Organization Chart, 1 May 1946
Oppenheimer and Groves at the remains of the Trinity test in September 1945, two months after the test blast and just after the end of World War II. The white overshoes prevented fallout from sticking to the soles of their shoes.
Groves confers with James Chadwick, the head of the British Mission.
Shift change at the Y-12 uranium enrichment facility at the Clinton Engineer Works in Oak Ridge, Tennessee, on 11 August 1945. By May 1945, 82,000 people were employed at the Clinton Engineer Works. Photograph by the Manhattan District photographer Ed Westcott.
Physicists at a Manhattan District-sponsored colloquium at the Los Alamos Laboratory on the Super in April 1946. In the front row are Norris Bradbury, John Manley, Enrico Fermi and J. (Jerome) M. B. Kellogg (1905-1981). Robert Oppenheimer, in dark coat, is behind Manley; to Oppenheimer's left is Richard Feynman. The Army officer on the left is Colonel Oliver Haywood.
Map of Los Alamos site, New Mexico, 1943–45
Hanford workers collect their paychecks at the Western Union office.
The majority of the uranium used in the Manhattan Project came from the Shinkolobwe mine in Belgian Congo.
Oak Ridge hosted several uranium separation technologies. The Y-12 electromagnetic separation plant is in the upper right. The K-25 and K-27 gaseous diffusion plants are in the lower left, near the S-50 thermal diffusion plant. The X-10 was for plutonium production.
Alpha I racetrack at Y-12
Calutron Girls were young women who monitored calutron control panels at Y-12. Gladys Owens, seated in the foreground, was unaware of what she had been involved in.
Oak Ridge K-25 plant
The S-50 plant is the dark building to the upper left behind the Oak Ridge powerhouse (with smoke stacks).
Workers load uranium slugs into the X-10 Graphite Reactor.
Aerial view of Hanford B-Reactor site, June 1944
Map of the Hanford Site. Railroads flank the plants to the north and south. Reactors are the three northernmost red squares, along the Columbia River. The separation plants are the lower two red squares from the grouping south of the reactors. The bottom red square is the 300 area.
A row of Thin Man casings. Fat Man casings are visible in the background.
An implosion-type nuclear bomb
Remote handling of a kilocurie source of radiolanthanum for a RaLa Experiment at Los Alamos
The explosives of "the gadget" were raised to the top of the tower for the final assembly.
The Trinity test of the Manhattan Project was the first detonation of a nuclear weapon.
Major General Leslie R. Groves, Jr., speaks to service personnel Oak Ridge Tennessee in August 1945.
A billboard encouraging secrecy among Oak Ridge workers
Security poster, warning office workers to close drawers and put documents in safes when not being used
Allied soldiers dismantle the German experimental nuclear reactor at Haigerloch.
Silverplate B-29 Straight Flush. The tail code of the 444th Bombardment Group is painted on for security reasons.
Little Boy explodes over Hiroshima, Japan, 6 August 1945 (left);
Fat Man explodes over Nagasaki, Japan, 9 August 1945 (right).
Presentation of the Army–Navy "E" Award at Los Alamos on 16 October 1945. Standing, left to right: J. Robert Oppenheimer, unidentified, unidentified, Kenneth Nichols, Leslie Groves, Robert Gordon Sproul, William Sterling Parsons.
President Harry S. Truman signs the Atomic Energy Act of 1946, establishing the United States Atomic Energy Commission.
The Lake Ontario Ordnance Works (LOOW) near Niagara Falls became a principal repository for Manhattan Project waste for the Eastern United States. All of the radioactive materials stored at the LOOW site—including thorium, uranium, and the world's largest concentration of radium-226—were buried in an "Interim Waste Containment Structure" (in the foreground) in 1991.
A "bomb" (pressure vessel) containing uranium halide and sacrificial metal, probably magnesium, being lowered into a furnace
After the reaction, the interior of a bomb coated with remnant slag
A uranium metal "biscuit" from the reduction reaction

The Project let to the development of two types of atomic bombs, both developed concurrently, during the war: a relatively simple gun-type fission weapon and a more complex implosion-type nuclear weapon.

At the University of Wisconsin–Madison, Stanislaw Ulam gave one of his students, Joan Hinton, an exam early, so she could leave to do war work.

Edward Teller in 1958

Thermonuclear weapon

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[[File:Teller-ulam-multilang.svg|right|thumb|200px|A basic diagram of a thermonuclear weapon.Note: some designs use spherical secondaries.

[[File:Teller-ulam-multilang.svg|right|thumb|200px|A basic diagram of a thermonuclear weapon.Note: some designs use spherical secondaries.

Edward Teller in 1958
One possible version of the Teller–Ulam configuration
Operation Castle thermonuclear test, Castle Romeo shot
Operation Grapple on Christmas Island was the first British hydrogen bomb test.
One of France's Triomphant-class nuclear-armed submarines, the Téméraire (S617)
Shakti-1
Photographs of warhead casings, such as this one of the W80 nuclear warhead, allow for some speculation as to the relative size and shapes of the primaries and secondaries in U.S. thermonuclear weapons.

A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design.

The design of all modern thermonuclear weapons in the United States is known as the Teller–Ulam configuration for its two chief contributors, Edward Teller and Stanislaw Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of physicist John von Neumann.

J. Robert Oppenheimer, c. 1944

J. Robert Oppenheimer

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American theoretical physicist.

American theoretical physicist.

J. Robert Oppenheimer, c. 1944
Heike Kamerlingh Onnes' Laboratory in Leiden, Netherlands, 1926. Oppenheimer is in the middle row, second from the left.
The University of California, Berkeley, where Oppenheimer taught from 1929 to 1943
Physicists Albert Einstein and Oppenheimer conferring circa 1950
Oppenheimer's ID badge from the Los Alamos Laboratory
Presentation of the Army-Navy "E" Award at Los Alamos on October 16, 1945. Oppenheimer (left) gave his farewell speech as director on this occasion. Robert Gordon Sproul right, in suit, accepted the award on behalf of the University of California from Leslie Groves (center).
A group of physicists at the 1946 Los Alamos colloquium on the Super. In the front row are Norris Bradbury, John Manley, Enrico Fermi and J.M.B. Kellogg. Behind Manley is Oppenheimer (wearing jacket and tie), and to his left is Richard Feynman. The army colonel on the far left is Oliver Haywood. In the third row between Haywood and Oppenheimer is Edward Teller.
The Trinity test of the Manhattan Project was the first detonation of a nuclear device.
Oppenheimer's Van Gogh, Enclosed Field with Rising Sun (1889).
Institute for Advanced Study in Princeton, New Jersey
Oppenheimer in 1946 with his trademark cigarette
President Dwight D. Eisenhower receives a report from Lewis L. Strauss, Chairman of the Atomic Energy Commission, on the Operation Castle hydrogen bomb tests in the Pacific, March 30, 1954. Strauss pressed for Oppenheimer's security clearance to be revoked.
Oppenheimer's former colleague, physicist Edward Teller, testified on behalf of the government at Oppenheimer's security hearing in 1954.
Oppenheimer Beach, in Saint John, U.S. Virgin Islands
Award of honorary degrees at Harvard to Oppenheimer (left), George C. Marshall (third from left) and Omar N. Bradley (fifth from left). The President of Harvard University, James B. Conant, sits between Marshall and Bradley. June 5, 1947
Oppenheimer and Leslie Groves in September 1945 at the remains of the Trinity test in New Mexico. The white canvas overshoes prevented fallout from sticking to the soles of their shoes.
J. Robert Oppenheimer giving a speech during a 1966 visit to Israel

In July 1944, Oppenheimer abandoned the gun design in favor of an implosion-type weapon.

In 1951, Edward Teller and mathematician Stanislaw Ulam developed what became known as the Teller-Ulam design for a hydrogen bomb.

Robert Oppenheimer (left), Leslie Groves (center) and Robert Sproul (right) at the ceremony to present the Los Alamos Laboratory with the Army-Navy "E" Award at the Fuller Lodge on 16 October 1945

Project Y

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Secret laboratory established by the Manhattan Project and operated by the University of California during World War II.

Secret laboratory established by the Manhattan Project and operated by the University of California during World War II.

Robert Oppenheimer (left), Leslie Groves (center) and Robert Sproul (right) at the ceremony to present the Los Alamos Laboratory with the Army-Navy "E" Award at the Fuller Lodge on 16 October 1945
In nuclear fission, the atomic nucleus of a heavy element splits into two or more light ones when a neutron is captured. If more neutrons are emitted, a nuclear chain reaction becomes possible.
In nuclear fusion, the nuclei of light elements are fused to create a heavier element.
Map of Los Alamos site, New Mexico, 1943–45
Four-family apartment units at Los Alamos
The Technical Area at Los Alamos. There was a perimeter fence around the entire site, but also an inner fence shown here around the Technical Area.
The main gate at Los Alamos
Passage between buildings A and B in the Technical Area
Isidor Isaac Rabi, Dorothy McKibbin, Robert Oppenheimer and Victor Weisskopf at Oppenheimer's home in Los Alamos in 1944
A row of Thin Man casings. Fat Man casings are visible in the background. The tow truck was used by the 216th Army Air Forces Base Unit to move them.
A ring of electrorefined plutonium. It has a purity of 99.96%, weighs 5.3 kg, and is about 11 cm in diameter. It is enough plutonium for one bomb core. The ring shape helps with criticality safety.
Plutonium has six allotropes at ambient pressure: alpha (α), beta (β), gamma (γ), delta (δ), delta prime (δ'), & epsilon (ε)
Explosive lenses are used to compress a fissile core inside an implosion-type nuclear weapon.
An implosion-type nuclear bomb. In the center is the neutron initiator (red). It is surrounded by the plutonium hemispheres. There is a small air gap (white, not in the original Fat Man design) and then the uranium tamper. Around that is the aluminium pusher (purple). This is encased in the explosive lenses (gold). Colors are the same as in the diagram opposite.
Norris Bradbury, group leader for bomb assembly, stands next to the partially assembled Gadget atop the Trinity test tower. Later, he became the director of Los Alamos vice Oppenheimer.
A Little Boy unit on Tinian connected to test equipment, possibly to test or charge components within the device
Water Boiler
The April 1946 colloquium on the Super. In the front row are (left to right) Norris Bradbury, John Manley, Enrico Fermi and J. M. B. Kellogg. Robert Oppenheimer, in dark coat, is behind Manley; to Oppenheimer's left is Richard Feynman. The Army officer on the left is Colonel Oliver Haywood.
Herbert Lehr and Harry Daghlian loading the assembled tamper plug containing the plutonium pit and initiator into a sedan for transport from the McDonald Ranch House to the Trinity shot tower
The explosives of "the gadget" were raised to the top of the tower for the final assembly.
Deak Parsons (right) supervises loading the Little Boy bomb into the B-29 Enola Gay. Norman Ramsey is on his left, with his back to the camera.
Fat Man bomb, with liquid asphalt sealant sprayed on the casing's seams, is readied on Tinian.
Remote handling of a kilocurie source of radiolanthanum for a RaLa Experiment at Los Alamos
Bradbury (left) examines plans for new laboratory sites and permanent housing with Leslie Groves of the Armed Forces Special Weapons Project (center) and Eric Jette (right) in April 1947; Colonel Lyle E. Seeman stands behind Bradbury, second from the left.

Oppenheimer then reorganized the laboratory and orchestrated an all-out and ultimately successful effort on an alternative design proposed by John von Neumann, an implosion-type nuclear weapon, which was called Fat Man.

Nonetheless, in February 1944, Teller added Stanislaw Ulam, Jane Roberg, Geoffrey Chew, and Harold and Mary Argo to his T-1 Group.

A re-creation of the 1945 criticality accident using the Demon core: a plutonium pit is surrounded by blocks of neutron-reflective tungsten carbide. The original experiment was designed to measure the radiation produced when an extra block was added. The mass went supercritical when the block was placed improperly by being dropped.

Critical mass

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Smallest amount of fissile material needed for a sustained nuclear chain reaction.

Smallest amount of fissile material needed for a sustained nuclear chain reaction.

A re-creation of the 1945 criticality accident using the Demon core: a plutonium pit is surrounded by blocks of neutron-reflective tungsten carbide. The original experiment was designed to measure the radiation produced when an extra block was added. The mass went supercritical when the block was placed improperly by being dropped.
Top: A sphere of fissile material is too small to allow the chain reaction to become self-sustaining as neutrons generated by fissions can too easily escape. Middle: By increasing the mass of the sphere to a critical mass, the reaction can become self-sustaining. Bottom: Surrounding the original sphere with a neutron reflector increases the efficiency of the reactions and also allows the reaction to become self-sustaining.
If two pieces of subcritical material are not brought together fast enough, nuclear predetonation (fizzle) can occur, whereby a very small explosion will blow the bulk of the material apart.

The concept is important in nuclear weapon design.

This latter problem provided significant motivation for the development of the Monte Carlo method in computational physics by Nicholas Metropolis and Stanislaw Ulam.