Nuclear Fission Bombs

When a nucleus undergoes nuclear fission, the nucleus splits into two smaller nuclei, and two or three neutrons are ejected. An incredible amount of energy is released in the form of heat and gamma radiation. 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). Gamma radiation has no mass and no charge. It is a type of ionizing radiation, with a low ionization power, and high penetration power.

Uranium-235 is the only naturally occurring isotope that will spontaneously undergo fission. Uranium-233 and Plutonium-239 are both artificially produced, and also will spontaneously undergo fission. U-238 is the other naturally occurring isotope of uranium. This isotope is not fissionable because it will absorb neutrons to produce U-239, which decays to Neptunium-239, and then to Pu-239. A sample of natural uranium is only about 0.72% U-235, and 99.2% U-238. 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%. Bombarding fissionable material with neutrons will initiate fission. 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. The new isotope then splits into two smaller nuclei and neutrons and gamma rays are released.

The total mass of the fission products and the neutrons is always less then the original U-235 atom. The missing mass is the measure of energy that is released in a single fission and is called the disintegration energy. Using Einstein’s famous equation, E=mc, the energy equivalent of the missing mass can be calculated.

To calculate E, the following equation can be used:
E = (Z.Mp + N.Mn Mb) x c
(M representing mass)

E = energy released (MeV)
Mp = mass of parent nucleus (atomic mass units, or amu)
My = total mass of the two daughter nuclei (amu)
N = number of neutrons released
Mn = mass of a neutron (amu)
c = the speed of light squared (931.494)

The minimal amount of fissionable material to sustain a fission chain reaction is called critical mass. There is no increase in neutron population or heat during 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. Too many neutrons are lost at the surface of the mass so there aren’t enough neutrons to continue the reaction. Supercritical mass is when the amount of fissionable material exceeds critical mass. The reaction will heat up extremely fast and neutron population will increase extremely fast and the mass will explode. Supercritical mass is used for the explosion in nuclear fission bombs. The shape with minimal critical mass and the smallest physical dimensions is a sphere. 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.

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. The first nuclear bomb was a uranium gun-type device; code-name “Little Boy”, dropped on Hiroshima on the morning August 6th, 1945. The second was a plutonium implosion-type device; code-name “Fat Man”, dropped on Nagasaki three days later.
A supercritical mass of weapons grade uranium cannot be stored as it will explode by itself. 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. 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. A barometric pressure sensor was used to determine the appropriate altitude for detonation.

The bomb went through the following procedure:

– At the appropriate altitude, the pressure sensor triggered the explosives to fire.
– The bullet propelled down the barrel, into the sphere.
– Supercritical mass was achieved.
– The fission reaction initiated
– The bomb exploded.

Little Boy had a 14.5 kiloton yield (equal to 14,500 tons of TNT). 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. The explosion was huge, but the bomb was not very efficient.

The Fat Man was an implosion plutonium bomb, using the fission of plutonium-239. 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 constituted of multiple subcritical masses of Pu-239 held apart by separating material, encased in a beryllium shell, surrounded by explosives, encased in shell of U-235 for tamper. A polonium/beryllium core, as a neutron initiator, was contained at the centre of the bomb. A barometric pressure sensor was used to determine the appropriate altitude for detonation.
The bomb went through the following procedure:
– At the appropriate altitude, the pressure sensor triggered the explosives to fire, creating a shockwave.
– The shockwave propelled the plutonium pieces together into a sphere.
– A powerful inward pressure was exerted on the plutonium, squeezing it and increasing its density.
– The plutonium achieved supercritical mass.
– The plutonium pieces stroke the polonium/beryllium core.
– The core fired neutrons out at the plutonium.
– The nuclear reaction initiated, and the bomb exploded.

The fat man had a 23-kiloton yield with an efficiency of 17 percent. The bomb exploded in fractions of a second. The fission occurred in about 560 nanoseconds. This bomb was a lot more satisfactory then the Little Boy. The Fat Man was more efficient then the Little Boy because of the supercritical mass of Pu-239 used in Fat Man exceeded the super-criticality of the U-235 mass used in Little Boy, and thus had more free neutrons within the mass causing it to fission a lot faster.