The Trinity Test: The World’s First Nuclear Explosion

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Podcast Transcript

On July 16, 1945, at 5:29 am, 35 miles southwest of Socorro, New Mexico, the world’s first nuclear bomb was detonated. 

This was the culmination of the Manhattan Project, one of the largest, and most expensive programs in world history. 

Yet, just before the event, the scientists and engineers who worked on the project weren’t entirely sure it would work, and if it did, just what the results would be. 

Learn more about the Trinity Test, the world’s first atomic bomb detonation, on this episode of Everything Everywhere Daily.


The story of the Manhattan Project is not one that can easily be covered in a single episode of this podcast. The cost of the project was staggering at a time when a billion dollars was a lot of money. The number of people who worked on the project was likewise staggering, especially considering that the United States was in the middle of a globe-spanning two-front war. 

While the theoretical science behind making a bomb was known, it was a far cry between knowing it could theoretically be done, and actually doing it. 

There were several debates amongst the scientists in the Manhattan project as to how a bomb could and should be built. There were differing theories as to what course of action should be taken. 

It wasn’t just a matter of what could work, but also what would be the easiest to build and the most effective.

There were two competing designs that were debated in the program.


Before I get into what those designs were, I need to explain the concept of critical mass and supercriticality.

For a nuclear reaction, you need to have an isotope of some radioactive element which is considered fissile. A fissile isotope is one that will split after capturing a neutron. In the process of splitting, it will then give off more than two neutrons on average. Those neutrons will split more atoms, ejecting more neutrons, causing a chain reaction. 

However, the ability of that reaction to sustain itself requires a certain amount of that isotope, and in particular a certain density of that isotope. If the neutrons which are expelled after an atom is split aren’t hitting more fissile atoms, the chain reaction will not continue. 

The amount you need to have a self-perpetuating chain reaction is known as its critical mass. 

A self-perpetuating chain reaction is great if you are running a nuclear reactor, but that isn’t helpful if you want to make a bomb. In that case, you need an even greater amount to achieve supercriticality. 

At supercriticality, the growth of the reaction grows exponentially and it is a runaway process. 

On the one hand, this can be pretty easy to do. Just get a sufficient amount of a fissile isotope all in one place, and you can hit supercriticality. On the other than, that is really dangerous. You want to be able to create this runaway chain reaction on demand. 

So, the designs for the first atomic bomb were different means to achieve supercriticality. 

When Robert Oppenheimer and his team of scientists first began to debate their options, the first and most popular choice was a design known as a “gun-type” fission weapon.

The way it works is very straightforward. You have two different parts, one is called the target and the other is the projectile. The projectile would consist of 9 rings made up mostly of Uranium-235.  The projectile would be forcefully expelled by conventional explosives down a barrel towards a cylinder of Uranium-235. 

When the projectile crashed into the target, its total mass would achieve supercriticality, and you have yourself an atomic explosion. 

This gun-type bomb was was the design used for the Little Boy bomb which was dropped on Hiroshima. From an engineering standpoint, it was rather straightforward. In fact, this bomb design was never tested once before it was actually used in combat. 

The scientists and engineers were confident that it would work because, as they said, it was just math. 

However, the gun-type design was not the only one that was proposed. Richard Tolman, a physicist at Caltech, proposed another design called an implosion-type device. In this device, you would have a conventional explosion in the shape of a sphere. Inside it, you’d have something called a seed which was made up of fissile material. The explosion would compress the seed, and the compression would increase its density, allowing it to achieve supercriticality. 

The implosion device didn’t initially get much support because it was considered to be too complex. 

The scientists weren’t just looking at U-235 as a fissile material. They were also looking at Plutonium-239. 

The Plutonium-239 version of the Little Boy bomb was known as The Thin Man, mostly because it was very long and thin. However, a plutonium gun-type bomb wasn’t very practical. The Thin Man needed to be at least 17 feet or 5.2 meters long. There weren’t any planes that could carry it without modification, and even then the bomb wouldn’t be very aerodynamic when dropped. 

Also, there was a much greater risk of an accidental detonation with a plutonium gun-type bomb. 

This resulted in the reconsideration of Tolman’s implosion idea. 

On paper, implosion worked really well. Far better than the gun-type bombs. It results in a faster reaction, which results in more energy release for the same amount of fissile material. 

The problem was the engineering of actually building something that could implode properly. 

Nothing like this had ever been built before and the mathematics behind it wasn’t that well understood. 

They went ahead with the development of an implosion device.

Here I should note that the real challenge in the Manhattan Project, and the thing where most of the money was spent, was on the enrichment and creation of Uranium-235 and Plutonium-239. This was 80% of the budget and where the vast majority of the manpower was put. The Uranium-235 was created in Oak Ridge, Tennessee, and the Plutonium was created at the Hanford, Washington. 

If you remember back to my episode on if the Nazis came close to making an atomic bomb, Werner Heisenberg at first didn’t believe the news when he heard it because he knew the cost of enriching that much uranium would have been prohibitively expensive.

Because it was so difficult and costly to make, enriched Uranium-235 and Plutonium-239 were the most expensive substances on Earth in the 1940s.

In 1944, the idea was floating of testing one of the implosion devices, because the engineering was so tricky. Oppenheimer and the other scientists wanted to do a test, but the head of the Manhattan Project, General Leslie Groves, was concerned about the cost. He didn’t want to waste valuable plutonium. 

In fact, they initially tried to find ways to recover plutonium which wasn’t spent, and also considered doing a much smaller explosion.

However, they eventually realized that if the test was to be meaningful, they would have to do a test on a par with what the bomb would be eventually asked to do. 

In March 1944, Groves gave approval for a full-scale test, with his biggest concern being that of explaining to a congressional committee how he blew a billion dollars worth of plutonium if the test failed.

Approval for the test occurred well before they actually had anything ready to detonate. However, that time was necessary just to find a place to conduct the test and build the necessary facilities. 

The site they selected in September 1944 was in the Alamogordo Bombing Range in New Mexico. It was a federally owned property that was flat, with little danger of fallout landing on civilians.  


The site was dubbed the Trinity Site by Robert Oppenheimer, after a poem from the 17the century English poet John Donne. 

An entire base was built for conducting the test with an ever-growing number of people working at it through late 1944 into 1945. 

They actually built an enormous steel shell that the bomb was initially going to be put inside. It was nicknamed Jumbo, and its purpose was in the event of a failure, it would contain all the plutonium inside, so it could be resued. 

In the end, however, it was never used, because it would have interfered with the blast data from the detonation, and it probably wouldn’t have worked anyhow. It weighed over 200 tons and at the time, it was the largest object ever transported by rail.

The camp also created special bunkers, viewing areas, and special high-speed cameras were developed to capture the explosion. 

Because of all the unique devices required to record and observe the test, they actually had a rehearsal explosion before the actual explosion. 

On May 7, over 100 tons of conventional explosives were detonated on a 20-foot high wooden platform. 

It turned out that the rehearsal was necessary as it was accidentally detonated 0.2 of a second before it was supposed to. That might not sound like a lot, but when you have cameras filming thousands of framers per second, that’s really important. 

Likewise, they found many problems with the communications and observation platforms they had set up. 


As the test site was being readied, the bomb, which was known as “the gadget” was being assembled. 

The gadget was to go on the top of a 100 foot, or 30 meter, steel tower. When the gadget was hoisted to the top of the tower, they literally had a truck full of mattresses below it in case it was dropped. 

The final arming of the device took place at 10 pm, on July 15. 

There were about 450 people, including VIPs, scientists, and soldiers who were observers for the test. 

Here I should note that there was still a great deal of debate as to what would happen. Up until now, everything was still just theory. Even some very esteemed nuclear physicists like Enrico Fermi and Edward Teller thought that there was a very small, but real chance, that the device would cause some sort of chain reaction that could destroy the atmosphere of the entire Earth, or maybe just the state of New Mexico.

People observing the test set up a betting pool for the actual size of the blast in equivalents of tons of TNT.  The bet ranged from zero, a total dud, to 45 kilotons, by Edward Teller.  Oppenheimer, himself, thought it would only be 0.3 kilotons. 

There were 50 different cameras set up, both still and moving pictures to capture the explosion. One camera was capable of taking 10,000 images per second. 

They actually wanted better weather, which was forecast for July 18 through 21, but politics got in the way. Harry Truman was scheduled to attend the Potsdam Conference with Stalin and Churchill starting on July 17th, so July 16th was the date that was chosen. 

The detonation was scheduled for 4 am, but light rain postponed it for an hour and a half. 

At 5:29 and 21 seconds, the world’s first atomic bomb was detonated. 

The explosion was larger than most of the scientists had predicted. In 2021, it was remeasured at 25 kilotons of TNT using the original data. It was the largest explosion in human history at that time, over 10x greater than the power of the Halifax Explosion.

Observers who later record their recollections of the blast all commented on the brightness of the light and the power and heat of the blast. 

One of the unforeseen consequences of the blast was the sand underneath the bomb had been turned into a glass. The glassy substance was subsequently called Trinitite. Trinitite was actually collected by people for years who wandered out to the test site until it was eventually made illegal by the US government. 

Trinitite which was collected before the ban is still legal, and you can actually purchase small samples of it online today. 

The gadget which was detonated at the Trinity Test site was the same type of bomb which was dropped on the city of Nagasaki. The bomb dropped on Hiroshima was a uranium-based gun-type device, which was never used again, because the implosion devices proved so much more effective. 

The Trinity Test was eventually made public after the bombing of Hiroshima, but oddly enough, scientists at the Kodak corporation had figured out there must have been a detonation on US soil before it was announced. 

Customers began complaining about spots appearing on sensitive film made for x-ray machines just a month after the Trinity test. It had to do with the paper used in the cardboard boxes which came from contaminated trees in Indiana, which had trace amounts of fallout.

Today, the Trinity Test site is a national historic site, and there is a monument that was erected in 1965 at ground zero. 

You can visit the site, but you can only do so on two days each year. It is open on the first Saturdays in April and October. The reason why access is restricted is that it is within the boundary of the  White Sands Missle Testing Range, and there are active missile tests that go on, on a regular basis. 

Once the Trinity Test took place, it basically marked the end of the Manhattan Project. Their mission had been accomplished. 

The initial joy of a successful test quickly turned somber when they realized the implications of what they had done. 

The mood was perhaps summarized best by Robert Oppenheimer himself, who years later wrote of Trinity Test:

We knew the world would not be the same. A few people laughed, a few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad Gita; Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his multi-armed form and says, ‘Now I am become Death, the destroyer of worlds.’ I suppose we all thought that, one way or another.