Mark 3 / Fat Man

The Mark 3, more familiarly known as the Fat Man, was the first implosion type atomic weapon. It's prototype was first tested at Alamogordo, New Mexico in 1945, and later a Mk 3 was used operationally against the Japanese city of Nagasaki.

A "Fat Man" bomb was dropped over Nagasaki, Japan, on August 9, 1945 near the end of WWII. Released by the B-29 "Bockscar", the 10,000 pound weapon was detonated at an altitude of about 1,800 feet over the city. The bomb had an expolosive force (yield) of about 20,000 tons of TNT, about the same as the bomb dropped on Hiroshima. But because of Nagasaki's hilly terrain, the damage was somewhat less extensive than at relatively flat Hiroshima.

"Fat Man" was an implosion type weapon using plutonium 239. A subcritical sphere of plutonium was placed in the center of a hollow sphere of high explosive (HE). Numerous detonators located on the surface of the HE were fired simultaneously to produce a powerful inward pressure on the capsule, squeezing it and increasing its density. This resulted in a supercritical condition and a nuclear explosion. A neutron source was placed at the center of a divided sphere of plu-otonium. This was covered by a reflector, with explosives wrapped around theb periphery. When the explosives det-onated, the plutonium in the sphere was hurled to the center, creating more than critical mass. This was an implosion bomb.

A "gun-type" design was designed for uranium. Unable to solve the purification problem, however, bomb designers feared that they would have to turn instead to the relatively unknown implosion method for plutonium. With implosion, symmetrical shockwaves directed inward would compress a subcritical mass of plutonium into a smaller, now-critical sphere. This sphere would be surrounded by a heavy "tamper" that would reflect neutrons back into the active volume and restrain the explosion for a few crucial moments, thereby increasing the efficiency of the blast. An initiator placed at the center of the sphere would ensure that the chain reaction began at precisely the right moment (rather than relying on "background neutrons" caused by spontaneous fission or background radiation).

Because the gun-type bomb design seemed so simple and practical, Deke Parsons had assigned implosion studies a low priority and placed the emphasis on the more familiar artillery method. Consequently, Seth H. Neddermeyer performed his early implosion tests in relative obscurity. Neddermeyer found it difficult to achieve symmetrical implosions at the low velocities he had achieved.

When the Princeton mathematician John von Neumann, a Hungarian refugee, visited Los Alamos late in 1943, he suggested that high-speed assembly and high velocities would prevent predetonation and achieve more symmetrical explosions. A relatively small, subcritical mass could be placed under so much pressure by a symmetrical implosion that an efficient detonation would occur. Less fissionable material would be required, bombs could be ready earlier, and extreme purification of plutonium would be unnecessary.

Von Neumann's theories excited Robert Oppenheimer, who assigned Parsons's deputy, George B. Kistiakowsky, the task of perfecting implosion techniques. (Kistiakowsky would later become President Dwight D. Eisenhower's science adviser.) Because Parsons and Neddermeyer did not get along, it was Kistiakowsky who worked with the scientists on the implosion project.

Oppenheimer acted quickly to maximize the laboratory's efforts to master implosion. Only if the implosion method could be perfected would the plutonium produced at Hanford come into play. Without either a plutonium gun bomb or implosion weapon, the burden would fall entirely on uranium and the less efficient gun method. Oppenheimer directed a major reorganization of Los Alamos in July 1944 that prepared the way for the final development of an implosion bomb. Robert Bacher took over G Division (for "Gadget") to experiment with implosion and design a bomb; Kistiakowsky led X Division (for "explosives") in work on the explosive components; Hans Bethe continued to head up theoretical studies; and Parsons now focused on overall bomb construction and delivery.

The design for an implosion device was approved in March 1945 with a test of the problematic plutonium weapon scheduled for 04 July 1945. Oppenheimer shifted the laboratory into high gear and assigned Samuel Allison, Robert Bacher, and George Kistiakowsky to the Cowpuncher Committee to "ride herd" on the implosion weapon. He placed Kenneth T. Bainbridge (right) in charge of Project Trinity, a new division to oversee the July test firing. "Deke" Parsons headed Project Alberta, known as Project A, which had the responsibility for preparing and delivering weapons for combat.

Operation Crossroads was a series of two nuclear tests, Able and Baker, the first tests conducted at the Pacific Proving Ground, near the Bikini Atoll in the Marshall Islands. These were the fourth and fifth nuclear detonations in history, following the Trinity test in July 1945 and the two weapons dropped on Japan to end World War II. The two bombs used in Crossroads were of the Fat Man configuration, very similar to the implosion-type weapons tested at Trinity and dropped on Nagasaki, Japan. It had a plutonium pit. The only other nuclear weapon available at the time was the uranium gun-type Little Boy, which was dropped on Hiroshima, Japan. The Fat Man was a more complex weapon than Little Boy, thus needing more testing to gain experimental data. On July 24, 1946, the Baker shot was set off in the Bikini lagoon at a depth of about 90 feet with a yield of 21 kilotons. Two million tons of water were contained in the eruption and two million yards of sediment were removed from the lagoon floor. Nine ships were sunk.

Thanks to successful espionage, the Russians tested their first atom bomb in August 1949, just four years after the first American test. Joseph Stalin was determined to have his own bomb. The first Soviet A-bomb test took place in August, 1949. It was a near-exact copy of "Fat Man," the American weapon that destroyed Nagasaki in August 1945. "The Mark III could be in combat ready status for just a short time. The life span of it's lead acid batteries, once charged and installed was only nine days, during which time they had to be recharged twice (the batteries retained their charge for three days at a time). After nine days, the entire bomb had to be disassembled to remove the chemical batteries, lest they begin to corrode. Another problem was core heating: the large amount of plutonium in the weapon radiated so much energy (from alpha decay) that the pit had to be removed after ten days or it's thermal effects would damage the high explosive charges and detonators. Like battery change, pit replacement also required complete disassembley and reassembly of the bomb, a procedure that took forty to fifty men between 56 and 76 hours to complete." (Chuck Hansen "U.S. Nuclear Weapons, The Secret History", page 124).