Nuclear reactions
Fusion and fissionWe know from radioactive emissions that the nuclei of some elements can break up into smaller pieces which are emitted as alpha or beta radiation. Such changes to the nuclei are called nuclear reactions.
The emissions of alpha, beta and gamma radiations involve a lot of energy. The alpha and beta particles have kinetic energy. The gamma ray haselectromagnetic energy. We now study two types of nuclear reactions that can produce even more energy: fusion and fission.
We know that Uranium-238 can decay by emitting an alpha particle. Uranium-235 is an isotope of Uranium-238. The nucleus of each of them has 92 protons in its nucleus, but a Uranium-235 nucleus has 3 neutrons fewer than a Uranium-238 nucleus. Uranium-235 also decays by emitting alpha particles, but it can also undergo another type of reaction. If a neutron comes along and falls on the Uranium-235 nucleus, the nucleus will split into two nuclei and give out much more energy (202.5 MeV) than an alpha particle (3 - 7 MeV). This type of reaction is called nuclear fission. Here is one possible way that it can break up:
A neutron is represented by 10n, where the 0 means that there is no proton. In the decay products, there is a Barium nucleus and a Krypton nucleus. Both are much heavier than an alpha particle. Notice that there are also 3 neutrons produced. These 3 neutrons will play an important role in nuclear power stations.
The above reaction is a fission reaction in which a nucleus breaks into two parts. There is an opposite reaction in which two nuclei combine into one. This is called a fusion reaction. Here is an example:
21H + 31H → 42He + 10n + 17.6 MeV
21H is the nucleus of a deuterium atom. This is an isotope of hydrogen which has one proton and one neutron. 31H is the nucleus of a tritium atom. This is an isotope of hydrogen which has one proton and two neutrons. When they join together, they form the nucleus 42He. This is an isotope of Helium, with 2 protons and 1 neutron in the isotope. This reaction produces 17.6 MeV.
In the two examples above, it would seem that the fusion reaction produces less energy than the fission reaction. However, to compare them properly, we really need to look at how much energy is produced by the same mass of starting material. The mass number of 23592U is 235. The total mass number of 21H and 31H is 5. 23592U is 235/5 = 47 times heavier. To compare for the same mass of starting material, we should multiply the fusion energy of 17.6 MeV by 47. This gives 827.2 MeV, which is over four times the energy produced by the fission reaction above.This makes fusion reaction an even more attractive source of energy. Unfortunately, there is a lot of practical difficulties because of the huge amount of energy needed to bring two nuclei together against their repulsion. No practical fusion reactor has yet been built.
Fission reactor for use in a power station
When a Uranium-235 nucleus breaks up like this:
a lot more energy is given out than a chemical reaction, which usually releases just a few eV. This makes nuclear fission a very attractive source of energy. Today, nuclear power stations are used in may countries. To be able to use nuclear fission as an energy source, we must know how to control it - how to start the reaction and to make it go faster or slower. This is clearly important. We know the danger when nuclear reactions go out of control - nuclear power station melt down and nuclear weapon.
The above reaction starts when a neutron attaches itself to a uranium nucleus. The initial neutrons could come a neutron source, such as the americium-beryllium. Once started, the reaction itself produces more neutrons as we can see from the nuclear equation above. In fact, number of reactions can increase exponentially with time. One reaction produces 2 extra neutrons, 2 neutrons start 2 reactions which produce 4 neutrons, 4 neutrons start 4 reactions which produce 8 neutrons, ... So it goes 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × 2 × ... just like the story of the rice and chessboard. We call this a chain reaction, and it can easily produce so much energy in such a short time that we get a nuclear explosion. This is how a nuclear bomb works.
We do not want this to happen in a power station, so we need some way to slow down it down and avoid a chain reaction. The way it is done in a nuclear power station is to insert graphite rods into the uranium fuel. Graphite rods can absorb the neutrons. This reduces the neutrons captured by uranium nuclei and prevents a chain reaction. On the other hand, the reaction can go faster if we pull out some graphite rods. So by controlling the number of graphite rods inserted, we can adjust the temperature in the uranium fuel.
The
heat from the uranium fuel is then used to generate electricity in the
same way as using coal or oil - by boiling water to produce steam that
drives a turbine and electric generator.
Star formation and their energy production by fusion
We have also seen above that for the same weight of starting fuel, a fission reaction produces even more energy:
21H + 31H → 42He + 10n + 17.6 MeV
Man has been able to produce fusion reaction in the form of a hydrogen bomb or thermonuclear weapon. So far, it has not been possible to make a power station using nuclear fusion. But it is interest human beings have been usingnuclear fusion for energy ever since they existed - in the form of sunlight.
Energy from the sun and stars comes from nuclear fusion. The sun is made up mainly of protons. Pressure from gravitational force causes the protons to undergo fusion reaction to form deuterium, tritium and helium. This is the source of the sun's energy. It is also the source of energy from stars.
The theory is that a star is formed when particles in space attract each other and come together under their own gravitational attraction. When enough mass comes together, the pressure from gravitational force gets so large that the protons undergo fusion reaction and start to produce energy. A star is born. As more and more of the protons are used up, the star may go through a number of changes - expanding, shrinking, exploding, even turning into a black hole.References
Nuclear fission and fusionHow nuclear power works
Life cycles of stars