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Visit to Trinity site - the site of the first nuclear explosion

Doug Kerr

Well-known member
At dawn on July 16, 1945, the world's first nuclear explosion took place at Trinity Site, only about 60 miles (as the crow files) from our current home in Alamogordo, New Mexico. It was an event that changed the world.

The site is open to the general public only two days each year, the first Saturdays of April and November. The reason for the restriction is that the site is buried deep in the northern end of the White Sands Missile Range. It is in a stretch of desert traditionally known as Jornado del Muerto (journey of the dead man).

The most practical way to reach the site on one of these two "Open House" days is by joining a caravan of civilian vehicles that forms up at the high school in the charming small town of Tularosa, N.M, about 10 miles north of our home. The caravan is led by a military police escort onto the Range and over (mostly) well-paved roads through the desert to the site, a total distance of 75 miles.

We did just that this past Saturday (2012.10.06).

The visit was actually an event of Carla's chapter of The Red Hat Society, a social order for mature women. But only two of the other members were able to participate.

Here we see the caravan forming up:

Douglas A. Kerr: Trinity Site caravan forming up

There were over 150 vehicles in the caravan.

The device (not yet a "bomb" in that configuration, and in any case, that word was never used—the nickname of this particular device was "The Gadget") was detonated at the top of a 100-foot tall four-legged steel tower.

Here we see The Gadget just before it was hauled up the tower, from a historical U.S. Government photo:

The Gadget ready to go up
U. S. Government photo.​

Here we see the tower from a historical U.S. Government photo:

The tower
U. S. Government photo.​

Ground zero, the point on the ground directly below the device, is today marked by a simple monument in the form of an obelisk, actually constructed of lava fragments from the nearly Carrizozo Malpais lava field (the result of a lava flow only about 1000 years ago).

Here we see Carla along with her fellow Red Hatters Penny and Monica standing as close to Ground Zero as the monument allowed:

Douglas A. Kerr: New Mexico Roadrunner Red Hatters Penny, Carla, and Monica at Ground Zero

Today, parts of two of the four concrete footings for the tower legs are still with us. We see one here:

Carla C. Kerr: North leg footing of main tower

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Doug Kerr

Well-known member
[Part 2]

Many years ago, that same footing was examined by a party including J. Robert Oppenheimer, scientific director of The Manhattan Project, which developed the nuclear device, and Major General Leslie R. Groves, its military and administrative head:

VIP party examines the remains of the north footing of the main tower
U. S. Government photo.​

As the planning for the Trinity test progressed, ranchers who operated on land leased from the government were forced to leave (I think they were compensated for their private property such as their homes).

The simple ranch home of rancher George McDonald was chosen to be converted into a makeshift assembly laboratory, where the core of The Gadget would be assembled. Here we see the home as it is today (a National Historic Landmark in fact):

Douglas A. Kerr: McDonald Ranch House from the rear

Here we see the house from the front, with Carla:

Douglas A. Kerr: Carla and the McDonald Ranch House from the front

The window behind Carla (and the door just to its left) lead to the room where the core was assembled.

In this historical government photograph, we see the core components being taken from the 1942 Plymouth sedan in the back seat of which they were transported from Los Alamos to Trinity site:

The Gadget core arrives at trinity site
U. S. Government photo.​

In this historical government photograph, we see an Army sergeant carrying the core components into the assembly room at the McDonald Ranch house.

The Gadget core brought into the assembly room
U. S. Government photo.​

The value of the plutonium in the core was said to be $300M (1945 dollars, of course).


Doug Kerr

Well-known member
[Part 3]

A fascinating thread in the Trinity saga is the story of Jumbo.

The Gadget was an implosion device. In such a device, the core (or "pit") is a sphere of plutonium not large enough to constitute a critical mass (such that nuclear fission could proceed).

In fact in The Gadget, the pit was about 4.5" in diameter (about the size of a moderate-sized grapefruit) and comprised about 13.6 pounds of plutonium 239.​

But if we could for example double the density of the pit, then that same mass would constitute a critical mass. How can we do that? Even with immense pressure, we can't do it just by mechanical compression—after all, metals are almost incompressible?

But in fact plutonium can occur in several allotropic forms, with differentiate crystal structures and with dramatically different densities.

In The Gadget, the core was made of the delta form, but great pressure could make it change to the alpha form, with a much greater density. Combined with the actual mechanical compression, this gives the required increase in density.

The amount of pressure required to cause the transition, and the mechanical, compression, is still immense. In The Gadget, this was done with 5300 pounds of high explosive in a complex "explosive lens" configuration.

In fact, in The Gadget, there was a layer of uranium 235 between the explosive lens and the pit. This served three purposes:

• As a "tamper" to make good mechanical contact between the explosive lens and the plutonium pit to assure uniform compression.

• As a neutron reflector, making sure that neutrons generated in the plutonium during the fission process did not leave the battle zone (a lesser amount of fissile material will become critical with such a neutron reflector in place).

• As fissile material itself—the nuclear fission in the uranium tamper/reflector material actually contributed about 20% of the overall energy output of the explosion.

But of course nobody was certain that this would work., One fear was that the compression would not be uniform, or in an case insufficient to attain criticality, and that the non-nuclear explosion would splatter the 13.6 pounds of plutonium (worth $300M!) all over the desert, not only wasting half of the available plutonium (the other half was on Tinian in the Pacific with what turned out to be a date in Nagasaki, Japan) but as well causing a serious contamination problem with the carefully-prepared test site.

So the wonks got the bright idea of building a gigantic steel tank in which the Gadget would be placed for the test. If all worked as planned, the tank would be vaporized, and would not really interfere with the results.

But if the nuclear reaction fizzled, then the plutonium would still be inside the tank, whence it could be recovered.

Test were made of scaled-down tanks to determine how thick the walls of the real tank would have to be (answer: 14-16 inches).

So the tank, dubbed "Jumbo", was made by Babcock & Wilcox of Barberton, Ohio, and transported to Trinity Site. It was 28 feet long, 12'8" in diameter, and weighed 214 tons.

Here, in a historical government photo, we see Jumbo as it arrived at Trinity Site:

Jumbo arrives at Trinity Site
U. S. Government photo.​

But as the date for the test approached, the wonks were very confident that the implosion trigger would work, and that with some of the collateral realities of the Jumbo scheme (such as how immense the main tower would have to be to support it) led to a decision not to use it.

But so it shouldn't be a total loss, they decided to hoist it up into 70-foot tower about 800 yards from Ground Zero to see what the effect of the nuclear explosion would be on such a robust object at that distance.

Here, in a historical government photo, we see Jumbo being hoisted into that tower:

Jumbo hoisted into its target tower
U. S. Government photo.​

The nuclear blast completely destroyed the tower, and Jumbo, intact, fell to the ground erect and embedded itself about one-firth of its length:

Jumbo in repose after the nuclear blast
U. S. Government photo.​

A while after the test, there were of course recriminations of all sorts by various camps, one of which claimed that the whole thing was a gigantic waste of money. General Groves, fearing that these people would make an entertaining example of the Jumbo story (about $12M), ordered army explosives experts to blow it to smithereens. They used eight 500-pound high-explosive bombs.

But all that did was to blow both ends out of it.

General Groves then ordered it buried in the desert, where it remained until the early 1970s.

Now it is on display just outside the outer perimeter fence around the Trinity site (right near where the burrito vender sets up on Open House day—these are by the way superb burritos).

Here we see Jumbo as of this last Saturday:

Douglas A. Kerr: Jumbo in repose 67 years out

Just for a little more human perspective:

Douglas A. Kerr: Red Hatters Monica, Carla, and Penny in Jumbo

Best regards,


Sam Hames

New member
right near where the burrito vender sets up on Open House day—these are by the way superb burritos
Absolutely surreal - I can't figure out what to make of this. Weapons of mass destruction to burritoes - it's amazing how life goes on.

Thanks for posting this - I find it a little unsettling to look through this. Fascinating nonetheless.