Why did Fat Man have dark paint markings?

You asked a terrific question about the black paint on Fat Man's protruding parts. Fat Man was delivered to Tinian Island in parts and assembled on the island. The implosion design had been tested in New Mexico, but had never been exposed to the kind of humidity it experienced in the Pacific. After assembly, but before loading it onto the bomber Bock's Car,  all of Fat man's seams were sprayed with a sealer. I think it was asphaltum or something similar. Many of the protruding parts were bolted on, so their seams were included in the treatment.

Are the Little Boy and Fat Man models at the BSM full scale?

Our replicas are full scale. Actually around 2005 ago we replaced our original models with two even better replicas that are more accurate in representing the appearances of the two weapons.  The two new versions, constructed of steel and fiber glass are actually ballasted with concrete to represent the actual weights of the original weapons. Not that we ever heft them.

Little Boy Replica Question

We were asked why we replaced our Little Boy artifact.

Our current Little Boy model is actually a very faithful fiberglass and metal replica of the bomb that was dropped on Hiroshima. It is even ballasted with concrete to simulate the weight of the original. As such it is a more faithful model than the one it replaced.

Our original Little Boy was built after WWII for the US' nuclear weapons program, but it never carried any active materials, nuclear or high explosive. It was a second generation gun type weapon, and in some ways different from Little Boy. I am told that after 9-11, the National Nuclear Security Administration felt that the several of these objects that were on display (including one at the American History Museum at the Smithsonian Institution) contained more information inside them than the NNSA was willing to share in case of a theft. Some of the weapons they pulled have had their guts removed and are back on display, but the BSM chose instead to have a more faithful appearing reproduction fabricated.

Mushroom Cloud Question

Any explosion above a pretty small scale is likely to result in a mushroom cloud. An explosion occurs when a dense substance, usually solid or liquid, turns into a gas. The result is an enormous and fast increase in its volume, or size. Most explosions we are familiar with involve the production of heat. An atomic explosion is no exception. The hot gas, being less dense than the air around it, rises.

As the gas rises, it also continues to expand. The expansion and contact with cool air above the rising column causes the gas to cool. Hot gas from below is still trying to rise, and the cooling gas above it is in the way. This makes the cloud of smoke and dust spread out like the head of a mushroom. This structure is a vortex, like a smoke ring (Do people even make smoke rings any more?) with a torus, or doughnut, shape, the gas on the inside traveling up, out across the top, down the sides, and back up through the center.

Volcanoes can make mushroom clouds if they explode with enough force. The volcanic explosions that formed the Valles Caldera behind Los Alamos must have made spectacular mushroom clouds! According to Wikipedia, the largest mushroom clouds ever seen were on Jupiter during the impacts of Comet Shoemaker-Levy.

Manhattan Project questions from a high school student.

          I was tempted to send you off to do your own research, but after poking around a little, I realize that your questions fall into some difficult holes in the published record.

1.  How many employees worked in Los Alamos and what did most do?

There were several thousand people living in Los Alamos at the end of the war. Most of them were support staff; technicians, clerical workers, the army, etc. I haven't come across an actual figure yet. According to Wikipedia.org, there were 130,000 people working in the entire Manhattan Engineering District across the country by that time.

2.  How much later after the bomb was dropped did the world find out about Los Alamos?

Little Boy was dropped on Hiroshima on Aug 6, 1945. Chapter 12 of the Smyth Report (Atomicarchive.com), released to the public on August 12, 1945, describes Los Alamos' work in developing the atomic bombs. OTOH, the city and lab were still highly secret for many years. I have a USGS map of Bandelier National Monument  in my office dated 1953 that shows White Rock, NM. but not a trace of Los Alamos.

3.  Where was most of the uranium found for the bomb?

Much of the Manhattan Project uranium came from pitchblende mined in the Belgian Congo (the Democratic Republic of Congo today.) I believe most of the uranium used at that time came from Africa. The uranium mining industry in America came later.

Good luck with your project!

Why did they build two different kinds of bombs?

The short answer: At the beginning of the Manhattan Project, enough of the science of fission was uncertain, and General Leslie Groves was given so much power, that he decided to try to build a bomb any way they possibly could. There were two possible materials to use, uranium (U) and plutonium (Pu), and they went after both materials.

The longer answer: The Manhattan Project started with a basic design, known as the Little Boy, or gun-type weapon. They didn’t know if they would have enough either U or Pu nuclear fuel for a weapon, so they chose to pursue both types of fuel. Also, while they knew quite a lot about uranium, plutonium was a relative stranger. (The Pu gun weapon was nick-named “Thin Man.”)

Uranium is a natural element that is mined (At the time much of ours came from Africa, but the Germans had set off alarms in the US scientific community when they invaded what was then Czechoslovakia, where there was another known supply.) and occurs as mostly a slightly heavier isotope, U238, and a small amount of slightly lighter, but fissionable, U235.* The two isotopes need to be separated to increase the proportion of U235. This is very difficult because the two isotopes are the same element and behave the same chemically. The only handle they had for sorting them is the fact that U238 atoms are about 1% heavier than U235 atoms. That means they move ever so slightly more slowly through filters, or when spun, they drift ever so slightly more to the outside. The Manhattan Project scientists tried a number of ways to enrich their uranium and ended up using a combination of techniques, but it was difficult, expensive, and slow.

Plutonium does not occur in nature in any considerable amount. It is made in special nuclear reactors by bombarding other elements with neutrons. While the reactors were huge and expensive, separating the Pu is comparatively simpler, because it is chemically different from the other by-products.

The bad news came half way into the Project, when chemists studying Pu discovered that it was too reactive to use in the gun-type bomb design. This had been a possibility all along, so early research had explored the idea of the implosion, or Fat Man type of weapon. By this time Pu production was strong enough that they decided not to drop the Pu program, but to take it in the new direction. The U235 supply was by then good enough that they decided to keep going with that design as well. In the end, they created two types of weapon. They were confident enough in the U gun weapon that they didn’t test it before dropping it on Hiroshima. The uncertainties about the Pu implosion weapon led them to make the Trinity device test at White Sands before they dropped Fat Man on Nagasaki.

*Little physics lesson: An element is determined by the number of protons (atomic number) in its atoms’ nuclei. An isotope of an element has the same number of protons, but can have different numbers of neutrons. The atomic weight (the 235 or 238 after the U) is the sum of neutrons and protons. A neutron or proton has a value of 1.