“As stars like the sun evolve, they use up most of their hydrogen and begin a new series of fusion reactions, in which helium nuclei react to form beryllium, carbon, oxygen, neon and magnesium.”
Here are 2 reactions showing the formation of lithium through nucleogenesis:
Box 2 shows a 2-stage reaction; first the nuclear fusion of 1 Helium-3 atom and 1 Helium-4 atom to form the element Beryllium-7.
The second stage of this reaction shows a Beryllium-7 atom reacting with an electron to form Lithium. The electron causes the nucleus of the Beryllium-7 atom to chemically change, causing a proton in the nucleus to turn into a neutron.
Nuclear fission is the splitting of the nucleus of an atom into lighter nuclei. The amount energy released from this process is huge, and comes from the direct conversion of nuclear mass into energy. E (energy) = mc2 where m = mass and c = the velocity of light.
“If an atom of Uranium-235 is given sufficient energy through the absorption of one neutron, it enters an excited state and beings to oscillate. When the oscillations become unstable, the nucleus splits into two similar nuclei of medium mass, emitting more neutrons in the process.”
The neutrons emitted from the nucleus can cause further emissions in other nuclei, producing a chain reaction.
To control nuclear fission reactions, the nuclear reactor contains a graphite moderator and control rods, made from a boron-coated steel. The graphite moderator slows down the fast moving neutrons so they can undergo nuclear fission.
The control rods are able to absorb neutrons. When the rods are pushed entirely into the nuclear reactor, they absorb all the neutrons present, meaning no nuclear fission reactions can be initiated and all reactions stop. When the control rods are moved partly in and out of the reactor, they can control the rate at which fission reactions occur.
Nuclear reactors are also controlled with the presence of Uranium-238, which does not undergo fission in the reactor and absorbs neutrons, interrupting the chain reaction.
Nuclear fusion is the process by which multiple atomic particles join together to form a heavier nucleus.
“At normal temperatures on Earth, the repulsion between the two nuclei would be too great for them to fuse together. At higher temperatures, the nuclei are moving much more quickly and collide with so much energy that this repulsive energy barrier can be overcome.”
Deuterium and tritium can fuse to form a helium atom and a neutron:
The reaction is exothermic as the nuclear binding energy of the products is lower than that of the reactants: the mass of the reactants is 4.993 proton masses, whilst the mass of the products is 4.974. The mass that appears to be lost has been turned into energy. E (energy) = mc2 where m = mass and c = the velocity of light.
The advantages of using fission and fusion reactions to generate electricity are that they do not produce greenhouse gasses, as in the current methods of energy production which involve burning fossil fuels. They can also be used to solve the problems of energy demand, by being used as an alternative to fuel when fossil fuels are used up as they essentially have an unlimited supply of fuel.
The disadvantage of this method is that it produces nuclear waste. The byproducts of nuclear fission are radioactive and remain so for significant amounts of time, for example Plutonium-239, which has a half-life of 24,110 years.
For fusion reactions to happen, the particles must form a hot ionised gas called plasma. Once the temperature is high enough, a plasma of positive ions in a sea of delocalised electrons is created. On earth, this is very difficult to create as it requires extremely high temperatures.
In a fusion power station, the hot plasma is contained within a tokamak container made from carbon fibre. The plasma comes into contact with the walls at the bottom of this container to pump away excess helium ions. However, this causes the carbon tiles to be eroded by deuterium or tritium ions and neutral species, causing the formation of hydrocarbons, 1 – 3 carbon atoms in length.
As these hydrocarbons are bombarded with protons, they form reactive radicals which combine to form hydrocarbon films. This causes problems for the reaction as they trap tritium and deuterium ions, stopping their circulation in the plasma and preventing energy production. Also, if the film gets thicker it will flake and form dust particles, which reduces the purity and performance of the plasma.
