Nuclear Fission and Fusion






This website attempts to explain the major differences between the fission and fusion processes with a little physics in to explain why energy is released in the background and to derive this amount. The viability of each process is also discussed and throughout the page the facts are made clear about the good and bad points of each of the nuclear reactions.

Contents



Differences between fission and fusion


The main difference that there is between fission and fusion is that fission is the splitting of an atom to form two smaller ones while fusion is the fusing of two smaller atoms to form a larger atom. In each of these the binding energy of the final product or products is higher than that of the prerequisites. This is explained more fully in the next section (Binding Energy).


Fission itself is a nuclear process which does not usually occur naturally in nature (at least not that we know of) as it requires a large mass and an incident neutron to start the reaction. However this would not last for a long time as the reaction would explode and produce a result such as in an atomic bomb. This volatile nature is what prevents fission from occurring for any length of time naturally.


Fusion on the other hand is a process which occurs quite naturally. In fact, if fusion did not occur naturally we would not be around today. This is due to stars being the result of the fusion process, the fusion of mainly light atoms into heavier atoms. Fusion also releases a lot more energy in comparison to fission in relation to the mass input. Put simply you will get more energy released from the fusion of 1kg of hydrogen ions than you will from fission of 1kg of Uranium.



Binding Energy

Binding energy is a measure of the amount of energy held within the bonds of the atom in question. As a result the more energy which is held in the bonds the more stable the atom is. The most stable nucleon is that of the Iron atom which neither splits nor fuses with other atoms with respect to a nuclear reaction. By referencing to the graph shown below we are able to determine what size nucleon is required for fusion and fission to occur.


As we can see iron is at the top of the binding energy curve. Any nucleon which has a greater nucleon number than that of iron will split by fission and cannot fuse. This is because each atom tries to become more stable by increasing its binding energy. The same goes with fusion with all atoms with a smaller nucleon number than that of iron will fuse to form larger nucleus' as they try and increase the amount of binding energy in them to become more stable. The closer various nucleons numbers are, however, to that of iron there is much less of a likelihood that they will split or fuse with other atoms easily.




Calculating the energy released


In order to be able to calculate the amount of energy released in a nuclear reaction there are several prerequisites which you need to be aware of.

  1. Atomic Masses (in u)

  2. Einsteins formula E=mc2

  3. How to convert to eV from Joules


In any nuclear reaction there is always a very small loss in mass in the final result. This is because the total atomic mass of the products is less than that of the individual atomic weights of the constituent parts. This loss in mass is lost as energy and is shown in Einsteins formula E=mc2 in which he was able to show that energy and mass are interchangeable.


If we take, as an example, the atomic mass of Deuterium 2.014102u, tritium 3.016049u and a proton 1.007825u we are able to work out the energy released in the following equation


On the left hand side we have two deuteron's with total atomic mass of 4.028204u

On the right hand side we have a triton and a proton which have a combined atomic mass of 4.023874u.


If we take the difference of these two atomic masses we end up with 4.028204 – 4.023874 = 0.00433u. It is this difference in atomic mass which we use to calculate the energy release. At this point you can equate the difference in atomic mass to kilograms and work out the answer or use the following energy conversion 1J = 931.5MeV/u. It will be shown using the latter of those two methods.


Taking the atomic mass of 0.00433u and the fact that there are 931.5MeV/u we can show that the amount of energy released in this fusion reaction is:

0.00433 x 931.5 = 4.03MeV.

This same principle can be applied to fission just by simply equating the difference in mass on both sides of the equation.



Fission


As has been said earlier fission is the process in which an atom with a large nucleon decays into that of two smaller nucleons. We shall be looking at Uranium in these examples relating to fission.


For fission to occur there need to be a few conditions which need to be satisfied. These are mainly:

  1. There being a critical mass of the substance (the minimum amount of mass required for fission to be self sustaining)

  2. A relatively slow neutron to be fired at the mass of the fissile material.


The neutron is fired at the fissile material in order to make the material even more unstable. It then splits so that the resultant atoms are more stable as they have more binding energy along with the release of more neutrons. The products which occur from the split of Uranium always occur in pairs with certain pairs having a higher likelihood of occurring than other pairs. This can be seen quite clearly in the diagram below. The pairs always occur at corresponding points on the opposite side of the chart so that the total mass numbers always adds up to the mass number of that of the atom which split. As shown in the diagram below a possible combination of products from Uranium-235 are Sr and Xe with mass numbers respectively of 95 and 140.



In a typical nuclear reaction involving Uranium-235 and a neutron we get the following series of equations:


followed by





Fusion


In the process of fusion we get two light atoms and combine them to form larger atoms. On earth the most likely fusion reaction which we will be able to get going is Deuterium – Tritium reaction. Deuterium and Tritium are both isotopes of hydrogen. Deuterium having a neutron in the nucleus with Tritium having two neutrons in the nucleus. It needs to be noted that in order for a D-T reaction to occur in the sun it requires a temperature in the region of 10million Kelvin but on Earth a temperature of 100million K is required. This is because in the sun the pressure is so great that it overcomes the natural repulsive charges but on Earth it is needed to heat the D-T mixture up high enough into a plasma (an electrically neutral mixture of charged ions) where-by the energy given to the constituent parts means that it can overcome the repulsion. The D-T reaction is demonstrated below.




The only problem with this is that Tritium is very hard to manufacture and is slightly radioactive. There is, however, a much easier way of manufacturing Tritium which also helps to contribute to the overall fission reaction. By firing a neutron into a Lithium-6 nucleus you can cause it to become unstable and form a triton and helium4 and also release 4.8MeV of energy. This method is particularly effective as the earths crust is full of lithium and releases energy. It also helps avoid any expensive method of fabricating tritium.


In comparison to fission, fusion releases much more energy per mass that you input. If you were to use 1kg of Uranium as fuel in a fission reaction and 1kg of Deuterium and Tritium combined to do a fusion reaction you would release far much more energy by the fusion reaction than the fission reaction.


Out of control nuclear reactions and by-products


We all know that the result of a nuclear fission reaction going out of control is the nuclear meltdown as was exhibited at the Chernobyl nuclear power plant. If the reaction is allowed to go out of control by not controlling the neutron emission rate we get the nuclear meltdown with lots of highly radioactive particles being released into the atmosphere. The difference with fusion is that if the reaction was to go out of control not only is the amount of radioactive materials produced in a fusion reaction very small with them only being weakly radioactive but also the reaction would stop as it cooled down and dissipated into nothing. The most damage which could happen is the vaporization of anything in the immediate vicinity of the reaction however this would only be a local effect and not on the same scale as a fission reaction meltdown.


As has been explained above we get highly radioactive materials released from a fission reaction while the by-products from a fusion reaction are very safe to be released back into the atmosphere or are very weakly radioactive. With regards to both these factors we can say that fusion itself is a much more environmentally friendly method of producing nuclear power.



More Information


If you want to know more about fission and fusion please visit these websites

Fusion Energy Educational Site
Wikipedia Entry on Nuclear Fission
Wikipedia Entry on Nuclear Fusion
Site Demonstrating Basic Nuclear Fission
Jet Nuclear Reactor Main Website