It was discovered that compressing subcritical masses of fissionable material together, into a sphere, is a good way to create a supercritical mass, that is denser then the supercritical mass used in Little Boy. The Fat Man was an implosion plutonium bomb, using the fission of plutonium-239. The explosion was huge, but the bomb was not very efficient. It had an efficiency of 1.5, meaning that 1.5 percent of the uranium was fissioned before the explosion carried the rest of the uranium away. Little Boy had a 14.5 kiloton yield (equal to 14,500 tons of TNT). ![]() – The bullet propelled down the barrel, into the sphere. – At the appropriate altitude, the pressure sensor triggered the explosives to fire. The bomb went through the following procedure: A barometric pressure sensor was used to determine the appropriate altitude for detonation. ![]() Little Boy constituted of a barrel with a ball of U-235 at one end, and the bullet at the other end with explosives behind it. To solve this problem, in the makings of Little Boy, a bullet of uranium was taken out of the sphere of supercritical mass to make two subcritical masses of U-235. The second was a plutonium implosion-type device code-name “Fat Man”, dropped on Nagasaki three days later.Ī supercritical mass of weapons grade uranium cannot be stored as it will explode by itself. The first nuclear bomb was a uranium gun-type device code-name “Little Boy”, dropped on Hiroshima on the morning August 6th, 1945. Two nuclear weapons have been detonated in the history of warfare both by the United States, both on Japan, on the closing dates of World War II. A sphere of Pu-239 has a critical mass of only 10kg, while a sphere of U-235 has a critical mass of 52kg, so less Pu-239 is required to make a nuclear bomb, making it more efficient then U-235. The shape with minimal critical mass and the smallest physical dimensions is a sphere. Supercritical mass is used for the explosion in nuclear fission bombs. The reaction will heat up extremely fast and neutron population will increase extremely fast and the mass will explode. ![]() Supercritical mass is when the amount of fissionable material exceeds critical mass. Too many neutrons are lost at the surface of the mass so there aren’t enough neutrons to continue the reaction. Subcritical is when the mass has a too small ratio of volume to surface area and there are only a low proportion of free neutrons inside the mass so it can’t sustain a chain reaction. There is no increase in neutron population or heat during the reaction. The minimal amount of fissionable material to sustain a fission chain reaction is called critical mass. My = total mass of the two daughter nuclei (amu) Mp = mass of parent nucleus (atomic mass units, or amu) To calculate E, the following equation can be used: Using Einstein’s famous equation, E=mc, the energy equivalent of the missing mass can be calculated. The missing mass is the measure of energy that is released in a single fission and is called the disintegration energy. The total mass of the fission products and the neutrons is always less then the original U-235 atom. The new isotope then splits into two smaller nuclei and neutrons and gamma rays are released. When a U-235 nucleus is striked by a neutron, it is temporarily absorbed by the nuclei and produces U-236, an unstable isotope with an extremely short half life. Bombarding fissionable material with neutrons will initiate fission. ![]() For a nuclear bomb to work, the uranium must be highly enriched, using the process of isotope separation, to weapon grade uranium, which has a U-235 composition of at least 90%. A sample of natural uranium is only about 0.72% U-235, and 99.2% U-238. This isotope is not fissionable because it will absorb neutrons to produce U-239, which decays to Neptunium-239, and then to Pu-239. U-238 is the other naturally occurring isotope of uranium. Uranium-233 and Plutonium-239 are both artificially produced, and also will spontaneously undergo fission. Uranium-235 is the only naturally occurring isotope that will spontaneously undergo fission. It is a type of ionizing radiation, with a low ionization power, and high penetration power. Gamma radiation has no mass and no charge. The radiation comes from the two new atoms they are left in an excited state and undergo a second decay to a lower energy state, by emitting a high-energy photon (a packet of electromagnetic radiation with a high frequency). An incredible amount of energy is released in the form of heat and gamma radiation. When a nucleus undergoes nuclear fission, the nucleus splits into two smaller nuclei, and two or three neutrons are ejected.
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