Fat Man

A post-war "Fat Man" model.

"Fat Man" was the codename of the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945. It was the second of the two nuclear weapons to be used in . The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core.

It was detonated at an altitude of about 1,800 feet (550 m) over the city. It was dropped from the B-29 bomber Bockscar, piloted by Major Charles Sweeney. The bomb had a yield of about 21 kilotons of TNT, or 8.78×10¹³ joules = 88 TJ (terajoules).[1] Because of Nagasaki's hilly terrain, the damage was somewhat less extensive than that in relatively flat Hiroshima. An estimated 40,000 people were killed outright by the bombing at Nagasaki, and about 25,000 were injured.[2] Many thousands more would die later from related injuries, and radiation sickness from nuclear fallout.

Technology

Illustration of the implosion concept.

The weapon was 7 feet 8 inches (2.34 m) long, five feet (1.52 m) in diameter, and weighed 10,200 pounds (4,630 kg). In accordance with the name, it was more than twice as wide as Little Boy, dropped on Hiroshima three days earlier; however, the mass was only 10% more.

"Fat Man" was an implosion-type weapon using plutonium. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive. Numerous detonators located on the surface of the high explosive were fired simultaneously to produce a powerful inward pressure on the core, squeezing it and increasing its density, resulting in a supercritical condition and a nuclear explosion.

The difficulty in the design lay primarily in properly compressing the plutonium core into a near-perfect sphere; if the compression was not symmetrical it would cause the plutonium to be simply ejected from the weapon, making it an unefficient and un-impressive "dirty bomb". In order to accomplish the compression, the high-explosive system had to be carefully designed as a series of explosive lenses which used alternating fast- and slow-burning explosives to shape the explosive shockwave into the desired spherical shape. An early idea of this sort had been raised by physicist Richard Tolman during early discussions of possible bomb designed, though Seth Neddermeyer is credited for developing the idea further. The idea of using shaped charges came from mathematician John von Neumann, and both he and George Kistiakowsky eventually ended up being the principle architects behind the lens system.

The "Trinity" device was similar to the "Fat Man" bomb.

At first it was thought that two pieces of subcritical plutonium (Pu-239) could simply be shot into one another to create a nuclear explosion, and a plutonium gun-type design of this sort (known as the "Thin Man" bomb) was worked on for some time during the Manhattan Project. However in April 1944 it was discovered that plutonium created for the bomb in the nuclear reactors at Hanford, Washingtonâ€"even though it was supergrade weapon plutonium containing only about 0.9% Pu-240â€"was not as pure as the initial samples of plutonium developed at the cyclotrons at Ernest O. Lawrence's Radiation Laboratory in Berkeley, California. Because of the presence of the isotope Pu-240, reactor-bred plutonium had a much higher rate of spontaneous neutron emission than was previously thought, and if a gun-type device was used it would most likely pre-detonate and result in a messy and costly "fizzle." The spontaneous fission rate of Pu-240 is 415,000 fissions/(s·kg), and the amount was 0.9% of 6.2 kg, is 56 g. Thus there were 23,000 spontaneous fissions per second. This means that the last few centimeters would have to be travelled in preferably much less than 40 microseconds.

It is theoretically possible to build a plutonium gun-type device, but it would need to be 19 feet long in order to allow the subcritical masses to be fused into a critical mass before a fizzle occurs. The mass of a plutonium gun-type device would have been beyond the payload of the B-29.

After this problem was realized, the entire Los Alamos laboratory re-organized around the problem of the implosion bomb, the "Fat Man" starting in June 1944.

The gun-type method could still be used for highly enriched uranium, though, and was employed in the "Little Boy" weapon, which was used against Hiroshima, Japan. For a variety of reasons, the implosion method is both more efficient than the gun-type method, and also far safer, as a perfect synchronization of the explosion lenses is required for the core to properly detonate, greatly reducing the chances of an accidental nuclear detonation.

As a result of espionage information procured by Klaus Fuchs, the first Soviet bomb, "RDS–1" (above) closely resembled, even in its external shape, the "Fat Man" bomb.

Because of its complicated firing mechanism, and the need for previously untested synchronization of explosives and precision design, it was felt that a full test of the concept was needed before the scientists and military representatives could be confident it would perform correctly under combat conditions. On July 16, 1945, a device utilizing a similar mechanism (called the "gadget" for security reasons) detonated in a test explosion at a remote site in New Mexico, known as the "Trinity" test. In the end, it gave somewhere around 20 kt (80 TJ), 2 to 4 times the expected yield.

The Soviet Union's first nuclear weapon detonated at Operation First Lightning (known as "Joe 1" in the West) was more or less a purposeful copy of the "Fat Man" device, on which they had obtained detailed information from the spy Klaus Fuchs.

The names for all three projects ("Fat Man", "Thin Man", and "Little Boy") were created by Robert Serber, a former student of Los Alamos director Robert Oppenheimer's who worked on the project. According to his later memoirs, he chose them based on their design shapes; the "Thin Man" would be a very long device, and the name came from the Dashiell Hammett detective novel and series of movies by the same name; the "Fat Man" bomb would be round and fat and was named after Sidney Greenstreet's character in The Maltese Falcon. "Little Boy" would come last and be named only to contrast to the "Thin Man" bomb.Robert Serber, Peace & War: Reminiscences of a Life on the Frontiers of Science (New York: Columbia University Press, 1998): 104.

The bombing of Nagasaki

Main article: Atomic bombings of Hiroshima and Nagasaki

Final touches being put on the "Fat Man" bomb before its mission against Japan.

By August 1, two nuclear weapons had been produced and a third nearly completed. The U.S. Army Air Forces had trained and deployed to Tinian a unit to drop the bombs. Components of the uranium bomb "Little Boy" were delivered by the USS Indianapolis along with its U-235 projectile, and the target rings for "Little Boy" and all components of the "Fat Man" bomb were delivered by air. On August 6, 1945, "Little Boy" was dropped on Hiroshima.

On the morning of August 9 1945, the U.S. B-29 Superfortress Bockscar, flown by the crew of 393rd Squadron commander Major Charles W. Sweeney, carried the "Fat Man" bomb, with Kokura as the primary target and Nagasaki the secondary target. The mission plan for the second attack was nearly identical to that of the Hiroshima mission, with two B-29's flying an hour ahead as weather scouts and two additional B-29's in Sweeney's flight for instrumentation and photographic support of the mission. Sweeney also took off with his weapon already armed but with the electrical safety plugs still engaged.

Observers aboard the weather planes reported both targets clear. When Sweeny's aircraft arrived at the assembly point for his flight off the coast of Japan, the third plane (flown by the group's Operations Officer, Lt.Col. James I. Hopkins, Jr.) failed to make the rendezvous. Bockscar and the instrumentation plane circled for forty minutes without locating Hopkins. Already thirty minutes behind schedule, Sweeney decided to fly on without Hopkins.

The "Fat Man" bomb being prepared for the mission against Japan on Tinian.

By the time they reached Kokura a half hour later, a 7/10 cloud cover had obscured the city, prohibiting the visual attack required by orders. After three runs over the city, and with fuel running low because a transfer pump on a reserve tank had failed before take-off, they headed for their secondary target, Nagasaki. Fuel consumption calculations made en route indicated that Bockscar would not have sufficient fuel to reach Iwo Jima and they would be forced to divert to Okinawa. Aftering initially deciding that if Nagasaki were obscured on their arrival they would carry the bomb to Okinawa and dispose of it in the ocean if necessary, the weaponeer Navy Commander Frederick Ashworth decided that a radar approach would be used if the target was obscured.

At about 07:50 Japanese time, an air raid alert was sounded in Nagasaki, but the "all clear" signal was given at 08:30. When only two B-29 Superfortresses were sighted at 10:53, the Japanese apparently assumed that the planes were only on reconnaissance and no further alarm was given.

A few minutes later, at 11:00, the support B-29 flown by Captain Frederick C. Bock dropped instruments attached to three parachutes. These instruments also contained messages to Professor Ryokichi Sagane, a nuclear physicist at the University of Tokyo who studied with three of the scientists responsible for the atomic bomb at the University of California, Berkeley, urging him to tell the public about the danger involved with these weapons of mass destruction. The messages were found by military authorities but not turned over to Sagane.Lillian Hoddeson, et al, Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943-1945 (New York: Cambridge University Press, 1993), on 295.

The mushroom cloud rises over Nagasaki after the detonation of the "Fat Man" device.

At 11:01, a last minute break in the clouds over Nagasaki allowed Bockscar's bombardier, Captain Kermit Beahan, to visually sight the target as ordered. The "Fat Man" weapon was dropped over the city's industrial valley. 43 seconds later it exploded 469 meters (1,540 feet) above the ground exactly halfway between the Mitsubishi Steel and Arms Works in the south and the Mitsubishi-Urakami Ordnance Works (Torpedo Works) in the north. This was nearly 3 kilometers (2 mi) northwest of the planned hypocenter; the blast was confined to the Urakami Valley and a major portion of the city was protected by the intervening hills. The resulting explosion had a blast yield equivalent to 21 kilotons of TNT.

According to most estimates, about 70,000 of Nagasaki's 240,000 residents were killed instantly,Ronald Takaki. Hiroshima: Why America Dropped the Bomb. Boston: Little, Brown, and Company, 1995.Rinjiro Sodei. Were We the Enemy?: American Survivors of Hiroshima. Boulder: Westview Press, 1998, ix. and up to 60,000 were injured. The radius of total destruction was about 1.6 km (1 mile), followed by fires across the northern portion of the city to 3.2 km (2 miles) south of the bomb. The total number of residents killed is believed to be as many as 80,000, including those who died from radiation poisoning in the following months.

Immediately after the bombings of Japan, the United States produced a technical history of the Manhattan Project, known as the Smyth Report, that did not disclose the information that the "Fat Man" device was different from the "Little Boy" device, and did not imply that a different method was required for plutonium weapons. The "implosion" design was considered top-secret in the United States until it was declassified and released during the testimony of David Greenglass against Ethel and Julius Rosenberg in 1951. Photographs of the casings of "Little Boy" and "Fat Man" were not released publicly until the 1960s.

The United States produced a small stockpile of "Fat Man" bombs after the war, but they were highly idiosyncratic and extremely delicate. It was eventually reworked in the MK 4 Fat Man bomb, which was similar in principle but was appropriate for long-term stockpiling and use by nonexperts, and used a more efficient implosion system (with a 60-point implosion system, compared to the 32-point weapon used in the war).

Interior of bomb

The original blueprints of the interior of both Fat Man and Little Boy are still classified. However, there is some information about the main parts in the public domain available. Below is a diagram of the main parts of the "Fat Man" bomb itself, followed by a more detailed look at the different materials used in the physics package of the bomb (the part responsible for the nuclear detonation).

center

# AN 219 contact fuse (four)# Archie radar antenna# Plate with batteries (to detonate charge surrounding nuclear components)# X-Unit, a firing set placed near the charge# Hinge fixing the two ellipsoidal parts of the bomb# High explosive pentagonal lens (12 units around the core, made of high and low velocity explosive), together with the next item forming the pattern of a truncated icosahedron # High explosive hexagonal lens (20 units around the core, made of high and low velocity explosive)# California Parachute tail (aluminium)# Dural casing, ~140 cm inner diameter# Cones that contained the whole sphere# Explosive lenses (low and high velocity)# Nuclear material (see figure below for details about the different layers)# Plate with instruments (radars, baroswitches and timers)# Barotube collector