|Themes > Science > Chemistry > Nuclear Chemistry > Nuclear Weapons > The First Nuclear Chain Reaction > The First Nuclear Weapons > Little Boy|
The design of Little Boy was completely different from Gadget/Fat Man. It used the gun assembly method that had originally been proposed for the plutonium bomb. The development of the uranium gun weapon was somewhat erratic. Early design and experimental work directed towards developing a gun system for uranium assembly was conducted during the summer and fall of 1943, after Los Alamos began operating. It was soon discontinued as attention shifted to the technically more demanding plutonium gun. It was felt that once the plutonium gun was successfully developed, the uranium gun would be almost an afterthought since the necessary speed of assembly was much lower.
When the very high neutron emission rate of reactor-produced plutonium was discovered in April-July 1944, the gun method was abandoned for plutonium and serious attention returned to the uranium gun. The uranium gun program (the O-1 group of the Ordnance Division) was lead by A. Francis Birch. He faced an odd combination of considerations in directing the work. The system was straightforward to develop, and sufficient U-235 to build the bomb obviously wouldn't be available until mid 1945, if then. Birch was nonetheless under a great deal of pressure to complete development as quickly as possible so that all of the laboratory's assets could be directed to the risky implosion bomb. Furthermore since the feasibility of the plutonium bomb was now in doubt, he had to make absolutely sure that the uranium bomb would work. Thus although it was a comparatively easy project technically, it still required extraordinary attention to detail.
The design arrived at was a very conservative one, that was as certain to work as any untested device can be. The design was complete by February 1945 (the final version was designated the Model 1850), only preparations for field use were required after that. The actual bomb was ready for combat use by early May, 1945 - except for the U-235 pit.
All of the uranium used in Little Boy had gone through its final stages of enrichment in the Calutron electromagnetic isotope separators at Oak Ridge, Tenn. Other isotope enrichment systems, also at Oak Ridge, contributed as they became available. Most of the uranium went through a three stage enrichment process: the thermal diffusion enriched the feed uranium from the natural concentration (0.72%) to the range of 1-1.5%; gaseous diffusion plant took this as feed and enriched it to increasing concentrations as enrichment stages came on-line.
The pit contained 64.1 kg of highly enriched uranium. By the time Little Boy was assembled, 50 kg of uranium enriched to 89% had been produced by Oak Ridge, and an additional 14 kg of 50% enrichment uranium was on hand. All of it was used in the bomb, giving an average enrichment of 80%, or approximately 2.4 critical masses. This is less than the 5 or so critical masses achieved by Gadget/Fat Man, and is the principal reason for Little Boy's lower efficiency. It is interesting to compare this to the published data on the South African gun-assembly bomb, which used 55 kg of enriched uranium (probably at >90% enrichment) and an inferior reflector, but a superior tamper (tungsten carbide gives a 15% lower critical mass, compared to tungsten metal, but is 25% less dense).
The U-235 mass of Little boy was divided into two pieces: the bullet and the target. The "bullet": a cylindrical stack of U-235 rings about 10 cm wide and 16 cm long, containing 40% of the mass (25.6 kg). It was constructed from six rings, the stack backed by a tungsten carbide disk and a steel backplate, all within a 1/16 inch thick steel can to make the complete projectile. The "target": a hollow cylinder 16 cm long and wide, weighing 38.4 kg, embedded in the tamper assembly. The target was fabricated as two separate rings that were inserted in the bomb separately. Note that even an unreflected sphere of U-235 weighing 64 kg would be supercritical. Almost certainly the bullet was made entirely of 89% enrichment uranium since placing the most fissile material at the center of the core is a basic principle of efficient bomb design.
The bullet was sheathed in a boron "safety sabot" that absorbed neutrons and reduced the chance of a criticality accident. The target also contained a boron safety plug. When the projectile reached the target, the boron sabot would be stripped off, and then the plug would be ejected into a recess in the nose.
The tamper assembly for Little Boy consisted of a thick tungsten carbide tamper/reflector, surrounded by a steel tamper forging about 60 cm wide. The combined tungsten carbide/steel tamper weighed 2300 kg. U-238 is a superior tamper and reflector, but tungsten carbide and steel were used instead due to the spontaneous fission rate of U-238. U-238 undergoes spontaneous fission 100 times more frequently than U-235, and a piece large enough to be useful as a tamper (200 kg) would generate 3400 neutrons a second - too many for gun assembly to be feasible.
A hole was bored into the steel forging, and the carbide tamper was inserted. The target was inserted in the form of several rings. The hole above the target was threaded and the gun barrel was screwed in to attach it securely (otherwise recoil from the bullet's acceleration would pull the target/tamper and barrel apart). At the bottom of the hole one or more beryllium/polonium initiator (different from the implosion initiators; simpler in design, with less polonium) could be mounted.
Although it only took some 0.5 milliseconds for the fissile material in the bullet to traverse the length of the target, the reactivity insertion time for Little Boy was 1.35 milliseconds, indicating that a critical configuration was achieved well before the bullet reached the target. The uranium/steel assembly was designed as a "blind target", one that would stop and hold the bullet upon impact due to expansion of the bullet rings. Even if the neutron initiator failed to work, the bomb would have exploded from spontaneous fission in a fraction of a second. The decision to include initiators in the final weapon wasn't even finalized by Oppenheimer until March 15, 1945. In the end, 4 ABNER initiators out of a batch of 16 shipped to Tinian were used in Little Boy. These were fastened radially to the front end of the target assembly.
The gun was a 3" (inside diameter)
anti-aircraft barrel, 6.5" wide, and six feet long that had been
bored out to 4" to accommodate the bullet. It weighed about 450 kg,
and had a breech block weighing 34 kg. Cordite, a conventional artillery
smokeless powder, was used as the propellant, and the velocity achieved by
the bullet was 300 m/sec.