By Jim Eckles
About 800 yards south of ground zero at Trinity Site, construction engineers built a heavy-duty 20-foot wooden platform. It had to be sturdy because they then stacked 108 tons of TNT on top of it. Then, on May 7, 1945, well before dawn, the neatly stacked boxes of TNT were detonated.
This little-known event was the dress rehearsal for the test of the first atomic bomb at Trinity Site – when the world was ushered into the atomic age on July 16, 1945. It also provided the terminology we have used for the past 70 years in trying to grasp how much energy is released in a nuclear explosion.
This 100-ton event was Dr. Ken Bainbridge’s idea. He was the test director at Trinity Site and he thought it made sense to do a dry run to make sure they had it right before trying the real deal. Oppenheimer apparently disagreed but eventually relented.
First of all, scientists planned to explode the atomic bomb at Trinity Site atop a 100-foot tower to simulate the aerial burst that was planned for its combat use over Japan. Also, elevating the detonation point, gave them an excellent view of the explosion during the first few fractions of a second. Scaling that down for the 100-ton explosion, the platform was built 20 feet off the ground.
During the 100-ton shot, the project ran the critical timing and data collection systems. These systems turned on measuring and photography equipment at the correct times and triggered the explosion itself. Since the Fatman test would be a one-and-done test, it wouldn’t do to have cameras and radiation detectors come on a second too late or so early the film ran out before the explosion even began.
To simulate a radioactive crater after the explosion, scientists made a slurry of radioactive material from the reactor in Hanford, Washington and ran it into plastic tubes buried in the stack of boxes. Afterward, the scientists measured the dispersion of this material in and around the small crater. The crater was five feet deep and 30 feet across.
After the test, they drove one of the lead-lined tanks into the crater. They were rehearsing the collection of soil samples after the main event when radiation levels would be intense at ground zero.
Finally, the event was a way to calibrate many of the instruments measuring blast effects by using an exact quantity of explosives. It is no coincidence that since the Trinity test 70 years ago that the yield of nuclear weapons has been communicated in tons of TNT.
One problem with communicating that yield in tons of TNT is that most people have nothing in their everyday lives to compare it to. Just a few tons of anything is hard to visualize. Changing the units to pounds puts it in the realm of everyday experience – everyone knows what five pounds feels like – but the numbers then get too large to grasp. For instance, using pounds, the TNT shipped to Trinity weighed 200,000 pounds. How many train cars did that take?
We don’t know how many train cars it took, but we know the TNT came in wooden boxes from the Volunteer Army Ammunition Plant at Chattanooga, Tennessee. The plant was built at the beginning of World War II to make trinitrotoluene or TNT as it is more commonly called. The plant also operated during the Korean and Vietnam Wars. It is now closed down and has been transformed into an industrial park and nature park.
The boxes were shipped to Trinity Site by rail and were unloaded from the boxcars at Pope siding. In looking at several of the Los Alamos photos of the men stacking boxes of TNT at Trinity Site, it is easy to count the number of layers and approximately how many boxes are in each layer – a total of more than 3,200 boxes.
In the end, military briefers, reporters and others in the know, will say that the Trinity Site Fatman explosion was equal to “20KT of TNT.” The eyes of most Trinity Site visitors glaze over then this number is unwrapped and put on display for their benefit. More than likely it just means “big” for most folks.
Of course, the number is shorthand for 20 kilotons of TNT which, in turn, is shorthand for 20,000 tons of TNT. Again, this is where the numbers are just too large to clearly comprehend. When you compare the figures, the atomic explosion was 200 times larger than the dry run TNT blast, coming in at 40,000,000 pounds of TNT.
By the way, there are no reports of people living in the area hearing the 100-ton test on May 7.
After the Trinity test, all attention shifted to the island of Tinian in the Pacific. There the Enola Gay, a B-29 piloted by Colonel Paul Tibbets, was loaded with the untested Little Boy bomb. It was dropped on Hiroshima on August 6. Three days later, a Fatman bomb, pretty much identical to the one tested at Trinity Site, was dropped on Nagasaki from Bockscar, flown by Major Charles Sweeney. Japan surrendered six days later.
TRIVIA NOTE: When most people hear the name of the second plane they assume the person is saying “boxcar.” It makes sense. Actually, the B-29 was named after its pilot Fred Bock and sometimes the name appears as Bock’s Car. Major Sweeney was scheduled to fly the Nagasaki mission but his bomber, The Great Artiste, was not ready to handle the Fatman bulk. He and Bock traded planes for the mission and the bomb was delivered by Bockscar. The Great Artiste served as an observation platform for the mission.