Nuclear fusion

In a nutshell

Nuclear fusion is the combining of lighter nuclei to form a heavier nucleus. In order to do this the nuclei must overcome the Coulomb repulsion. Fission releases more energy in a single reaction, but gram for gram, fusion releases more energy. Nuclear fusion is the green future of energy production with very minimal disadvantages.

Nuclear fusion

Nuclear fusion is the joining together of two lighter nuclei to make a heavier nucleus and release energy in the process. Nuclear fusion is the process which occurs in the centre of stars.

Physics; Nuclear physics; KS5 Year 12; Nuclear fusion

1	$^3_1H$
2	$^4_2He$
3	$^2_1H$
4	$^1_0n$
5	Energy

Nuclear fusion can only occur under extreme temperatures and pressures, such as those found in the middle of stars. This is because nuclei are positively charged due to the protons within them and when they are moved close together there is a electrostatic repulsion force called the Coulomb repulsion.

The nuclei must get within the range of the strong nuclear force in order to get close enough to be able to bind and form a larger nucleus.

In order to achieve this, the nuclei are heated to extreme temperatures which give them a huge amount of kinetic energy which will be enough to overcome the Coulomb repulsion.

The pressures are also very high so the chances of collisions between the light nuclei are increased.

A lot of energy is released in fusion reactions due to the fact that the nucleus which has been formed is heavier and has a much greater binding energy per nucleon than the original nuclei.

Nuclear equations

Nuclear equations are similar to any other equations whereby both sides of the equation must be balanced. Knowing this, different unknowns can be calculated from a nuclear equation:

Example

The nuclear equation for the fusion of neon and helium into magnesium is shown below:

$_{10}^{21}Ne+_2^4He \rightarrow _{Z}^{A}Mg+ _{0}^{1}n$ 

Calculate the proton number and nucleon number for the Mg nucleus.

First, write down the total nucleon number for the left hand side:

$A_{left}=21+4 \newline \\[0.1in]A_{left}=25 \newline \\[0.1in]$ 

Then again for the right hand side:

$A_{left}=A+1 \newline \\[0.1in]$ 

We know that the left and right hand sides must be equal:

$A_{left}=A_{right} \newline \\[0.1in]25=A+1 \newline \\[0.1in]A=24 \newline \\[0.1in]$ 

Repeat for the proton number:

$Z_{left}=Z_{right} \newline \\[0.1in]10+2=Z+0 \newline \\[0.1in]Z=12 \newline \\[0.1in]$ 

So the full nuclear equation for the fusion of neon-12 and helium-4 into magnesium is:

$_{10}^{21}Ne+_2^4He \rightarrow _{12}^{24}Mg+_{0}^{1}n$ 

Curiosity: The fusion of helium and neon happens in massive stars which are eight times the mass of the Sun.

Fission vs fusion

The energy released in a singular fusion reaction is less than that of a single fission reaction. This is because the nuclei used in fusion reactions are significantly less massive than those involved in fission.

A mole of the reactants in a fusion reaction is less massive than a mole of reactants in a fission reaction. This means in a gram of fusion fuel there is many more reactants, or more fuel, than in a gram of fission fuel.

So whilst the energy per reaction for fission is higher, there are many more reactions in a gram of fusion fuel which means a gram of fusion fuel will release much more energy than a gram of fission fuel.

Future of fusion

Scientists are actively trying to work on creating a fusion reactor as a sustainable source for energy. Fusion has no pollutants, nuclear waste and potentially no catastrophic failures.

The waste product helium can be collected and used elsewhere or even vented into the atmosphere where the majority will escape into space.

If a fusion reactor suffered a fault or containment issue then the plasma would escape the containment vessel, rapidly expand and cool which would stop the fusion reaction and hence stop the reactor.

The problem scientists are having is an energy efficient way of containing the hot fusion plasma which is in excess of $100\,000\,000 \degree C$ . Currently it takes more energy to contain the plasma than it can produce.