The fission of a nucleus involves splitting it into two more or less equal fragments. For example uranium, in which it yields two or more lighter nuclei and a large amount of energy. If an atom of U- 235 is given sufficient energy through the absorption of one neutron, it enters an excited state and begins to oscillate. When this becomes unstable then the nucleus split in to two similar nuclei of medium mass, emitting more neutrons in this process. These forms chain reactions.
E.g.1 & 2
Radio active decay is the process in which an unstable atomic nucleus looses energy by emitting radiation in the form of particles. This results in an atom of one type, called the parent nuclide. It’s a random process and so it is impossible to predict when a particular atom decays.
2nd section: The role of hydrogen and helium in synthesising elements in the stars:
The rate at which fusion proceeds depends on the temperature and pressure at the core and thus, ultimately, on the mass of the star. These fusion reactions are exothermic: the energy is ultimately released as radiation from the surface of the star.
The exothermic reaction provides enough energy for heavier chemical elements to be made by the process of nucleogenesis. This mainly occurs in starts. In the sun the closest start to the earth hydrogen is converted to helium in nuclear fusion reactions.
41H → 4He + subatomic particles
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.
For example: 24He → 8Be.
Later on, once the helium has been used up the star begins a new series of processes; initially combining the carbon nuclei. For e.g.:
The production of small amounts of hydrogen and helium makes it possible for the star to synthesise most of the elements. Lithium forms in this way:
Lithium:-
Section: 3 nuclear fission and nuclear fusion reactions
Nuclear fission involves splitting of the nucleus into to two more or less equal fragments. It is bombarding a neutron on to a heavier nucleus to release enormous amount of energy.
The production of energy from the nuclear fission can be understood in terms of the curve of binding energy. Where binding energy is the energy released when nucleons are made. Fission of heavy elements is an exothermic reaction which releases large amount of energy both as electromagnetic radiation and as kinetic energy of fragments.
For example, if one of the chain reactions produced in this way by nuclear fission reaction is:
For this chemical equation, the amount of energy released can be supplied is:
This transformation results in a liberation of about 200MeV per uranium atom fissioned. And for 1 kg of this uranium atom fissioned the amount of energy produced is 8 x 10^13J.
When creating fission the continuous chain reaction of a nuclear fission reactor depends upon at least one neutron from each fission being absorbed by another fissionable nucleus; the reaction can be controlled by using control rods of material which absorbs neutrons. Cadmium and boron are strong neutron absorbers and are the most common materials used in control rods. A typical neutron absorption reaction in boron is:
In the operation of a nuclear reactor, fuel assemblies are put into place and then the control rods are slowly lifted until a chain reaction can just be sustained.
Nuclear fusion reactions:
Nuclear fusion is a process by which multiple atomic particle join together to form a heavier nucleus, resulting in the liberation of energy. The production of energy results from the increase in biding energy of the product over the reaction of the reactants.
For example: Under right conditions deuterium and tritium atoms fuse for a helium atom and a neutron.
The excess energy is released by the fusion reaction because of the lower binding energy of the helium nucleus compared to tritium and deuterium, and the combines mass of the product is less than that of the reactants. The lost mass is converted in to energy according to E= mc2.
The reaction is controlled in a nuclear reactor, which contains a graphite moderator (used to slow down the neutrons so that they can cause fission reactions as they collide with the uranium) and a control rod for absorbing the neutrons to generate electricity.
Advantages of using fusion to generate electricity:
- The fuel for fusion reactions is readily available. (Deuterium and tritium)
- Its causes no harm to the nature as its sources don’t create pollution and it creates huge amount of energy.
Disadvantages:
- Scientists have not yet been able to contain a fusion reaction long enough for there to be a net energy gain.
- Many countries are carrying out fusion research because of the failure to reach a breakthrough yet.
Advantages and disadvantages of suing nuclear fission:
Advantages:
- Relatively little fuel is needed and the fuel is relatively inexpensive and available in trace amounts around the world.
- Fission is not believed to contribute to global warming or other pollution effects associated with fossil fuel combustion
Disadvantages:
- Possibility of nuclear meltdown from uncontrolled reaction--leads to nuclear fallout with potentially harmful effects on civilians
- Waste products can be used to manufacture weapons
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High initial cost because plant requires containment safeguards.
Section 4: outline the main challenges that scientists face in developing fusion power stations.
- The equipment used to contain the energy is too hot to handle.
- The hydrocarbon films created by the formation of reactive radicals, causes problems as they trap deuterium and tritium (fuels from water) in the walls of the vessel. This means that they are not circulating in the reacting plasma to produce any energy.
- Because of this, the film gets thicker; it begins to flake off resulting in dust particles which can be absorbed into the plasma, thus affecting the purity and the performance.
