Los Alamos National Labs with logo 2021

Beyond Trinity: 75 years of weapons advances

In addition to creating the world’s first nuclear device, Los Alamos has made nuclear weapons more effective, safe, and specific to military needs.
March 1, 2019
A black and white photo of the initial explosion caused by the Trinity test.

At 5:30 a.m. on July 16, 1945, Trinity, the world’s first atomic device, was detonated in southern New Mexico. “It looked like a giant magnesium flare which kept on for what seemed a whole minute but was actually one or two seconds,” said physicist Hans Bethe. “The white ball grew and after a few seconds became clouded with dust whipped up by the explosion from the ground and rose and left behind a black trail of dust particles.”


“It was a damn good thing that the bomb was developed, that it was recognized as something important and new, and that it would have an effect on the course of history. In that world, in that war, it was the only thing to do.”- Robert Oppenheimer

In 1942, the U.S. government selected Los Alamos for Project Y of the top-secret Manhattan Project. Civilian and military men and women came to Los Alamos to solve a seemingly unsolvable problem: design and build an atomic fission bomb to end World War II.

On the morning of July 16, 1945, Manhattan Project scientists conducted a test that proved the feasibility of weaponizing energy from the atom. Trinity, as the test was known, was the detonation of the “Gadget”—the world’s first atomic device—near Alamogordo, New Mexico.

After the success of the Trinity test, two atomic bombs were sent to the Pacific for use in the war: Little Boy was dropped on Hiroshima on August 6, 1945, and Fat Man was dropped on Nagasaki three days later. World War II officially ended on September 2.

“It was a damn good thing that the bomb was developed, that it was recognized as something important and new, and that it would have an effect on the course of history,” former Laboratory Director Robert Oppenheimer told The New York Times Magazine in 1965. “In that world, in that war, it was the only thing to do.”

In the decades since, Los Alamos has continued to pioneer weapons technology that is just as significant but perhaps not as well known as those first fission bombs. During the Cold War in particular, deterrence theory—the idea that nuclear weapons deter attacks—became the dominant military strategy and drove the Laboratory to design and deliver increasingly more powerful and compact nuclear weapons for ever-improving delivery systems. With the development of these weapons came the responsibility to make them safer. Innovative science and engineering were—and still are—necessary in both the development and safety of these complex weapons.

Here’s a look at 10 Los Alamos–developed weapons advances that continue to impact America’s national security.

A mushroom cloud rises into a cloudy sky.

On May 8, 1951, George was the first experiment that produced thermonuclear fusion.

1. Thermonuclear fusion and boosting

Fission bombs are the prerequisite for developing other types of nuclear weapons. Not long after the success of the Gadget, Fat Man, and Little Boy, scientists began learning to make even more powerful bombs using fission to initiate thermonuclear fusion, which occurs when atoms combine at high temperatures to produce tremendous amounts of energy—aka nuclear yield.

In April 1951, Los Alamos began the Greenhouse nuclear test series at Enewetok Atoll in the Pacific Ocean. The series consisted of four explosive experiments (called shots): Dog, Easy, George, and Item. George was the first experiment that produced thermonuclear fusion. With a 225-kiloton yield (the equivalent of 225,000 tons of TNT), George was the largest nuclear explosion up to that time.

“George was an important way station on the path to development of thermonuclear devices,” according to a 1951 report by the Defense Nuclear Agency. The report explained that George proved that a fission reaction could be used to start a sustained thermonuclear reaction, leading to the first test of a thermonuclear device in 1952.

On May 24, 1951, two weeks after George, the 45.5-kiloton test called Item was the first test of the principle of fusion “boosting”: the use of a thermonuclear fusion reaction to increase the rate of a fission reaction in order to boost efficiency and therefore, yield.

In 1956, the concept of hollow-boosted primary (first stage) designs was proved in Los Alamos’ Operation Teapot experiments. Hollow boosting used neutrons from the fusion of a gas mixture blown into a hollow core made of fissile materials, just before detonation to accelerate the chain reaction.

A wide mushroom cloud rising into the sky.

In January 1950, President Harry Truman directed the Atomic Energy Commission to continue its work on atomic weapons, including the development of a hydrogen bomb. “It is part of my responsibility as commander in chief of the armed forces to see to it that our country is able to defend itself against any possible aggressor,” he said.

2. The hydrogen bomb

Thermonuclear weapons are colloquially called hydrogen bombs, or H-bombs, because they use the fusion of different forms (isotopes) of hydrogen. Using these isotopes—specifically deuterium and tritium—allows for yields in the megaton range. (A 1-megaton yield is the equivalent of 1 million tons of TNT.)

The world’s first megaton-class thermonuclear test was Mike, a Los Alamos–designed test in the Ivy series at Enewetok Atoll on October 31, 1952. Mike was a two-stage test device that weighed 82 tons and used a fission bomb as the first, or primary, stage to initiate a thermonuclear-fueled secondary stage (the “Teller-Ulam” concept). The resulting 10.4-megaton test was, at that time, the highest-yield device ever exploded and created a crater 6,240 feet across and 164 feet deep.

In 1954, the Operation Castle series successfully proved the U.S. could make a deliverable thermonuclear weapon. “[The] Castle results can be described as sensational,” wrote Los Alamos weapons designer John Richter in his book, Risk Versus Threat. The two-stage designs tested in the Ivy and Castle series, together with the hollow-boosted primary designs, set the template for the subsequent U.S. stockpile. 

A truck with a missile positioned on top of it.

Honest John battalions were deployed in Europe in early 1954.

3. Battlefield nuclear weapons

Also called tactical or theater weapons, battlefield nuclear weapons are compact weapons designed to be used on the battlefield (to destroy 100 tanks, for example). In addition to designing nuclear bombs (dropped from planes) in the 1950s, Los Alamos designed nuclear missile warheads for the Army’s Honest John and Corporal short-range missiles and the Air Force’s Matador cruise missile. It also developed the Army’s 11-inch artillery-fired atomic projectile.

Battlefield nuclear weapons were a substantial part of the peak nuclear weapons stockpile levels during the Cold War. They “changed the tactical calculus between the Soviet Union and the U.S. on the Eastern European front,” says Jeremy Best of the Lab’s Office of Nuclear and Military Affairs. “Battlefield nuclear weapons were how we balanced the manpower and conventional force difference between us and the Soviet Union.” Most of America’s battlefield nuclear weapons were either retired or dismantled after the Cold War. 

Three WWII aircraft in flight.

One-point safety made transporting nuclear weapons—on planes such as this B-52 Stratofortress—safer because no nuclear yield would occur in the event of an accident.

4. One-point safety

The concept of one-point safety was developed in the mid-1950s after physicist Harold Agnew, who would become the Laboratory’s third director, visited an Air Force base. There, he saw how casually the airmen handled nuclear weapons and became alarmed about the possibility of an accident. He wondered if, for example, inadvertently dropping a weapon on the tarmac could produce a detonation with nuclear yield and, if so, how such a disaster could be prevented.

“It thus became a major design objective to assure that even when fissile and high-explosive components were fully assembled, there would be no nuclear yield if an accident resulted in detonation of the high explosive,” wrote Los Alamos weapons designers Robert Thorn and Donald Westervelt in a 1987 report. “Since such a detonation might start at any single point on or in the explosive components, this design objective came to be known as ‘one-point safety.’”

In the mid-to-late 1950s, multiple tests were devoted to studying one-point safety. The first of these was the Project 56 series in Nevada, followed quickly by the Project 58 series. After these two series, the one-point safety tests were integrated into many of the test series conducted at Nevada. One-point safety was a game-changer because it made the stockpile safer.

Three sailors inspect a weapon on the deck of a ship.

When a B-52 crashed at Palomares, Spain, in 1966, the conventional explosives of a nuclear weapon detonated, but no nuclear yield occurred—due to modifications that were made as a result of the hydronuclear program.

5. Hydronuclear tests

When a nuclear testing moratorium went into effect on October 31, 1958, designers “took advantage of the resulting opportunity to study in more detail the somewhat puzzling results of recent one-point safety tests,” according to Thorn and Westervelt. “The safety behavior of a given design seemed to depend critically on the particular point at which detonation of the high explosive was initiated.” Because these designs could not be tested during the moratorium, Los Alamos developed a hydronuclear test program.

Hydronuclear tests are small-scale underground tests that use nuclear material and create fission but are engineered to generate no nuclear yield. These tests involve a combination of high explosive and fissile material (enriched uranium and/or plutonium) in quantities too low to generate a nuclear explosion.

Once the Atomic Energy Commission (the earliest predecessor of the Department of Energy) and President Eisenhower approved the hydronuclear test program, tests were conducted at Los Alamos rather than at the Nevada Test Site. Supposedly, this was so Los Alamos designers would not be tempted to create nuclear yield (because if they did, they risked contaminating the town in addition to violating the test moratorium).

The Laboratory’s first hydronuclear test was conducted on January 12, 1960, and after several series, “by April 1 the most urgent safety questions had been answered,” according to Thorn and Westervelt. “The hydronuclear experiments…made it possible to identify, and in some cases to resolve, otherwise crippling safety issues.”

Ultimately, the tests generated the data needed to further study one-point safety while still fitting within the bounds of the test moratorium. They advanced the development of one-point safety design and engineering and improved the understanding of how much fissile material could be used in a pit and remain safe in almost any conceivable accident scenario. Located inside a weapon, the pit triggers nuclear fission when compressed by high explosives.

“The speed at which our scientists designed, built, and diagnosed the hydronuclear experiments was remarkable,” says Mark Chadwick, chief scientist and chief operating officer of the Laboratory’s Weapons Physics Directorate. “These experiments allowed them to quickly identify and resolve safety concerns.”

6. Plastic-bonded conventional high explosives

High explosives trigger nuclear weapons that use an implosion process. By the early 1950s, Los Alamos had developed the plastic-bonding process for conventional high explosives. In a plastic-bonded explosive (PBX), explosive powder is bound together using a synthetic polymer, which allows the explosive to be formed into specific shapes.

The PBX 9501 formulation, introduced in the 1960s, improved safety in handling and transportation scenarios, while maintaining performance and facilitating compact warhead designs. It allowed the Lab to retire the earlier, more sensitive (and less safe) PBX 9404 explosive.

7. Insensitive high explosives

Los Alamos began researching insensitive high explosives in the 1950s, with the goal that no nuclear weapons would ever be unintentionally detonated. The Laboratory developed manufacturing and formulation methods for the explosive TATB (triaminotrinitrobenzene), a molecule that was first synthesized at Harvard University in the 19th century. TATB burns but does not explode when it’s heated and does not react even when struck by bullets or shrapnel. Deliberately detonating this unique material requires a well-engineered initiation system.

The Laboratory played a key role in refining TATB and patenting its manufacturing process. Los Alamos also became the first national lab to use a TATB composition in stockpile nuclear weapons, specifically in the B61 bomb, the W80 cruise missile, and the W85 Pershing II missile.

TATB is typically the main component of insensitive plastic-bonded explosives, including PBX 9502, an insensitive high explosive that the Lab produced for use in nuclear warheads. “PBX 9502 is uniquely suited for nuclear bombs,” says Laboratory high-explosives expert Cary Skidmore.

A missile is launched into the night sky.

An unarmed Minuteman III ICBM is test-launched from Vandenburg Air Force Base in California. Photo: U.S. Air Force.

8. ICBM and SLBM warheads

In the 1970s and ’80s, Los Alamos designed the W76, W78, and W88 warheads to be used on intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs). A ballistic missile is rocket powered and is guided into outer space in a high, arching trajectory. It falls, unguided, with gravity until it reenters the Earth’s atmosphere and descends to its target.

These powerful, small, accurate warheads can go anywhere in the world and are an essential component in deterring our adversaries. The warheads can be carried inside a multiple independently targetable reentry vehicle (MIRV) and can each be programmed to hit a different target.

These warheads remain the cornerstone of the U.S. deterrent—its stockpile of nuclear weapons. Since 1979, Minuteman ICBMs have been armed with the W78 nuclear warhead. Ohio-class submarines can carry up to 24 Trident II D5 SLBM missiles, with each missile being capable of delivering up to 12 independently targetable W76 or eight W88 warheads with a range beyond 4,600 miles.

A man uses a glovebox.

A worker uses a glovebox to handle plutonium.

9. Plutonium R&D and pit production

Early in its history, Los Alamos measured the critical mass of plutonium—the smallest amount of plutonium needed for a sustained nuclear chain reaction—and to this day, the Laboratory leads experimental and simulation work in nuclear criticality and criticality safety. Since 1943, Los Alamos has designed a variety of plutonium alloys and pit types to meet specific weapons requirements. The Laboratory has the nation’s only facility capable of handling large quantities of plutonium for manufacturing pits and power sources and for conducting basic R&D.

Plutonium pits are critical components of every nuclear warhead, but nearly all the pits in the current U.S. stockpile were produced from 1978 to 1989 at Colorado’s Rocky Flats facility, before it was shut down. In the 2000s, Los Alamos demonstrated an ability to build war-reserve pits. A war-reserve pit is one that meets the engineering and physics standards for use in deployed nuclear weapons. In May 2018, the NNSA reconfirmed that Los Alamos will establish a safe, secure, reliable, and efficient capability to manufacture at least 30 war-reserve plutonium pits per year by 2026 (the Savannah River Site in South Carolina will develop the capability to manufacture 50 war-reserve pits per year by 2030).

“Make no mistake, Los Alamos is—and will remain—the nation’s plutonium center of excellence,” NNSA Administrator Lisa Gordon-Hagerty said during an April 2018 visit to Los Alamos. “The work that is done here is critical to our nation’s nuclear security and central to our stockpile stewardship mission.”

A crane lifts a piece of equipment off the ground.

Science-based stockpile stewardship combines scientific and experimental capabilities (such as those at DARHT, pictured), with high-performance supercomputing simulations.

10. The U.S. nuclear stockpile today

Historically, of the 63 types of nuclear weapons entered into the U.S. stockpile, 46 were designed at Los Alamos. Of today’s seven types of nuclear weapons, five are Los Alamos–designed: the B61 gravity bomb; the W80 cruise missile warhead; and the W76, W78, and W88 ballistic missile warheads.

Los Alamos continues to maintain the stockpiled variants of the B61, W76, W78, and W88 and is actively modernizing the W76, the W88, and the B61 to ensure that these nuclear weapons remain safe, secure, and reliable. Los Alamos is integral to ensuring a continued effective deterrent in the coming decades.

“What you do here is the most important work in the country,” General John Hyten, commander of U.S. Strategic Command (STRATCOM), told Laboratory employees during a visit last year. “Deterrence starts and ends with nuclear weapons.”


This article was inspired by a list compiled by Mark Chadwick and Michael Bernardin. Chadwick and Bernardin are the chief scientist/chief operating officer and the associate Laboratory director, respectively, of the Laboratory’s Weapons Physics Directorate.