Nuclear fusion is the process of two nuclei joining together to form a heavier nucleus. It is accompanied by the release or absorption of energy. The fusion of Deuterium and tritirium to produce helium is the reaction that is constantly going on in the sun. When deuterium and tritirium fuse, a neutron is released, plus a positron. The positron meets a nearby electron and the two annihilate, producing two gamma rays.
Binding energy is the energy released, or absorbed in a nuclear fusion reaction. Nucleons with greater binding energies are more stable then nucleons with smaller binding energies. Iron and nickel have the greatest binding energies and therefore are the most stable. The fusion (see Nuclear Fission and Fusion – Difference and Comparison) of nuclei lighter then iron release energy while the fusion of nuclei heavier then iron absorbs energy. The mass of an atom is always less than the sum of the mass of protons and neutrons that constitute the atom. This missing mass is the measure of the atom’s binding energy. To calculate this binding energy is to simply subtract the mass of an atom from the mass of total number of neutrons and protons. The calculated defect mass is then converted into its energy equivalent using Einstein’s famous equation, E = mc.
This gives the binding energy formula:
E = (Z.Mp + N.Mn – Mb) x c
E = binding energy (MeV)
Z = number of protons
Mp = mass of a proton (amu)
N = number of neutrons
Mn = mass of a neutron (amu)
Mb = mass of the bound nucleus (amu)
c = the speed of light squared (931.494)
The binding energy of deuterium and tritirium can be considered, for example:
E = (2 x Mp + 2 x Mn – Mb) x c
E = (2 x 1.008 + 2 x 1.009 4.003) x 931.494
E = 28.876 MeV
Teller-Ulam Design of a Fusion Bomb
The fusion of the two isotopes of hydrogen, deuterium and tritirium, to produce helium is considered the most promising reaction for producing fusion power, and is used in fusion bombs. The fusion of deuterium and tritirium releases enormous amounts of energy. For the two nucleons to fuse, they must approach within 1.0E-15m so that the strong attraction between the nuclei overcomes the electrical repulsion between the protons. A lot of energy is required, in the form of kinetic energy, of the nuclei for them to be able to overcome each of their repulsive forces. The deuterium and tritirium must be stored at extremely high temperatures and pressures so the particles would achieve the required amount of kinetic energy for the reaction to initiate.
In the making of the bomb, the different particles had to be stored in special ways. Deuterium and tritirium are both gasses and are hard to store. To store deuterium, it was chemically combined with lithium. In short supply, tritirium has a short half life and if stored as is, would need to be continuously replenished. To solve this, it was found that tritirium didn’t actually have to be stored in the bomb, but produce from lithium. Lithium-6 plus a neutron yields tritirium and helium-4; lithium-7 plus a neutron yields tritirium, helium-4, plus a neutron. Neutrons from a fission reaction were used to create tritirium from lithium.
An implosion-type fusion bomb was used to start the fusion reaction. Most of the radiation given off in a fission reaction was X-rays. These X-rays provided the high temperatures and pressures necessary to initiate fusion.
The Teller-Ulam bomb consisted of an aluminium casing. Inside the casing of the bomb was an implosion fission bomb at one end. Further down inside the casing was a cylinder casing of U-238 for tamper. Inside the layer of tamper was the lithium dueteride (fuel) and a hollow rod of Pu-239 in the centre of the cylinder. The two separate parts were held in place by polystyrene foam. When the bomb detonated, it went through the following procedure:
– The fission bomb exploded, releasing large amounts of heat and X-rays, heating the interior of the bomb, and the tamper.
– The tamper shield prevented pre-mature detonation of the fuel.
– The heat caused the tamper to expand, exerting pressure inwards against the fuel.
– The compression shock waves initiated the plutonium rod to fission, releasing heat, radiation, and neutrons.
– The neutrons went into the lithium dueteride, combined with the lithium, and made tritirium.
– The extreme temperature and pressure made conditions sufficient for fusion reactions to initiate, producing even more heat, radiation, and neutrons.
– The neutrons from the fusion reactions induced nuclear reactions in the pieces of U-238 tamper, causing it to fission, producing even more heat, and radiation.
– The bomb exploded.
It took 600 nanoseconds for the bomb to explode; 550 nanoseconds for the fission bomb to explode, and 50 nanoseconds for the fusion explosion. The explosion was massive. It was more that 700 times greater then the little boy explosion, and had a 10,000 kiloton yield.