Saturday, March 17, 2012

17 September 2010 – Atomic History


If you enter the phrase “bomb core arrives” into Google’s image search engine, the first hit that pops up is this:

It’s a faded, sepia-toned black and white photo of a small group of men carrying what looks like a heavy animal cage.  It’s not clear what direction they’re walking, but they’re between what looks like a scaffold, to the right; and to the left, a 1942 Plymouth.  In this photo, you can’t tell that the Plymouth is olive drab.

The story behind the photo is more interesting than any of the internet-generated nonsense about its origins. 

Los Alamos National Laboratory (LANL) in New Mexico - one of the homes of the US nuclear weapons programme - publishes a web-based magazine called “Nuclear Weapons Journal”, and in their March-April 2003 edition, the editors put the mystery of the men, the box, and the Plymouth to rest.

In the summer of 1945, the Manhattan Project was reaching its logical conclusion, with all technological paths converging on the Trinity Site in the New Mexico desert (at what is now the White Sands Missile Range), and the live test of the first nuclear explosive, scheduled for mid-July of that year.  The device - which was only a bomb in the sense that it was something that was designed to explode; it could not have been carried by an aircraft - weighed more than ten thousand pounds, and was nicknamed “the Gadget”.  It was an implosion-type weapon with a plutonium core, the forerunner of the design that would later be used against Nagasaki under the code-name “Fat Man”.  The Project managers - principally US Army General Leslie Groves, and LANL Director Robert Oppenheimer - decided to test the implosion design first, as implosion-driven detonation was a much more challenging technical problem than the simpler gun-type, enriched-uranium-based “Little Boy”, which was expected to work on the first try.

(Another reason was the slow production by gaseous diffusion and centrifuging of highly-enriched uranium, and the vast difference in the relative amounts of fissile material required - about 6 kg of Pu-239 for an implosion bomb, whereas the gun design required more than 60 kg of highly-enriched U-235).

The plutonium core for The Gadget was produced at LANL.  This meant that it had to be driven 300 miles to the Trinity site, on a route passing through both Santa Fe and Albuquerque.  The core made the journey in the small ventilated box that you see in the picture.  The vents were necessary because plutonium has a very high radioactive decay rate, producing a significant amount of heat.  The core would have been warm to the touch.  Plutonium, which in its pure metallic form is hard and brittle, also has a number of odd and potentially dangerous physical properties.  Unlike most metals, it is a poor conductor of heat and electricity.  It oxidizes when exposed to air, and the oxides tend to flake, and are pyrophoric, meaning that they can spontaneously combust.  Plutonium in its metallic form is an alpha emitter, and as such is not terribly dangerous; but combustion of plutonium oxides produces micrometer-sized particles than can be inhaled and ingested, and that can settle into the lungs and bone marrow, causing potentially fatal radiation poisoning.  Finally, plutonium has six allotropic forms; it goes through phase transitions as it changes temperature, and changes density with each transition.  Because criticality is a function of atoms per unit volume, a density change can cause sub-critical masses to suddenly become critical.  The metal is usually stabilized by being alloyed with other metals, like aluminum, cerium or gallium.  While this means that more material must be used to make a weapon core, it also improves the safety of the core, and makes the material easier to weld or machine.

And also easier to transport, as we can see in the original photo, which NWJ helpfully published (see page 22).

(Source: Nuclear Weapons Journal, March-April 2003, p. 21 – Back Cover)

To the right of the scaffold in the picture, you’ll see a low rock wall.  “Nuclear Weapons Journal” helped explain that, too.  As the next picture shows, the wall surrounded the MacDonald Ranch, where the final components of the Gadget were assembled (in the master bedroom, which had been converted into a dust-free “clean room”) before being driven the final few miles to Ground Zero at the Trinity test site.  According to the Journal article, the scaffolding was not part of the original house; it had been constructed for the purpose of loading and unloading heavier components.  The article also notes that the ranch had two water storage tanks, and that the Manhattan Project personnel used one of them as a swimming pool.

(Source: Nuclear Weapons Journal, March-April 2003, p. 21 – Back Cover)

At 05:29:45 local time (Mountain War Time) on 16 July 1945, The Gadget was detonated atop a 100-foot-tall tower.  The explosion generated 90 TJ of energy, roughly equivalent to 20,000 tons of TNT (20 kt), created a glass-covered crater 10 feet deep and 1000 feet in diameter, and was felt over 100 miles away.  The mushroom cloud reached 7.5 miles.  Everybody remembers that LANL Director Robert J. Oppenheimer, quoting the Hindu goddess Shiva from the Bhagavad Gita, said, “Now I am become Death, the destroyer of worlds.”  Nobody seems to remember that he was responding to the test director, Ken Bainbridge, who, upon witnessing the explosion, had said, “Now we are all sons of bitches.”

Sergeant Herbert Lehr, carrying the plutonium core for the Gadget. 

The two guys carrying the box in the original photo - Sgt. Herbert Lehr, left, and Harry Daghlian, right - don’t look like ‘sons of bitches’, do they?  They look like regular guys.  You wouldn’t know from the photos that 44 years later, Daghlian would be used as the basis for a fictional character called Michael Merriman, who was played by John Cusack in “Fat Man and Little Boy”.  You probably remember what happened to Cusack’s character in the movie, though; well, the same thing really happened to Daghlian.  You see, about a month after he and Sgt. Lehr delivered the box containing the core for the Trinity test (you can see the handling case in Lehr’s hand, above), Harry Daghlian suffered a lethal dose of radiation when he accidentally dropped a tungsten carbide brick onto another plutonium core during a criticality experiment. 

Harry Daghlian

He died 26 days later, 13 days after Japan’s surrender, on September 15th 1945, from acute radiation poisoning.  He was 24.

History is people.



Atomic Archive photo site:
Harry Daghlian: