Nuclear fission and nuclear reactors

In a nutshell

Nuclear fission is the splitting of a large nuclei into two or more smaller nuclei releasing energy in the process. Nuclear power plants have several key components in order to allow the safe extraction of energy from nuclear fuel. Nuclear power plants have lots of advantages but also have some disadvantages too.

Nuclear fission

Nuclear fission is the splitting of large nuclei into two or more smaller nuclei. The smaller nuclei are sometimes called the daughter nuclei.

This process can happen spontaneously which means the nucleus splits by itself, or it can happen if an external factor causes the nucleus to split. This is called induced fission and is how we can harness the power of the atom.

Energy is released in fission due to the daughter nuclei having a higher binding energy per nucleon and a lower total mass.

The larger a nucleus is the more likely the nucleus will undergo spontaneous fission.

Note: The fact that larger nuclei have increased chances of undergoing spontaneous fission means that the number of elements that can exist is finite due to the nucleus only being able to hold so many nucleons.

Physics; Nuclear physics; KS5 Year 12; Nuclear fission and nuclear reactors

1	Unstable nucleus
2	Energy

When a neutron is introduced to a uranium nucleus, it must be low energy and slow moving in order to be absorbed by the nucleus. These neutrons are called thermal neutrons.

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 spontaneous fission of californium-238 into cadmium-115 and tin is shown below:

$_{98}^{238}Cf \rightarrow _{48}^{115}Cd+_Z^ASn + 2_{0}^{1}n$ 

Calculate the proton number and nucleon number for the $Sn$ nucleus.

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

$A_{right}=115+A+(2\times 1) \newline \\[0.1in]A_{right}=117+A \newline \\[0.1in]$ 

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

$A_{left}=A_{right} \newline \\[0.1in]238=117+A \newline \\[0.1in]A=238-117 \newline \\[0.1in]A=121$ 

Repeat for the proton number:

$Z_{left}=Z_{right} \newline \\[0.1in]98=48+Z+(2\times 0) \newline \\[0.1in]Z=98-48 \newline \\[0.1in]Z=50$ 

So the full nuclear equation for the spontaneous fission of californium-238 is:

$_{98}^{238}Cf \rightarrow _{48}^{115}Cd+_{50}^{121}Sn + 2_{0}^{1}n$ 

Chain reaction

When some nuclei undergo nuclear fission, they break into smaller daughter nuclei but also release a number of neutrons in the process. These neutrons can then go on to induce more fission reactions. If these reactions are not controlled then the number of fission reactions and hence amount of energy being released can quickly run out of control.

This is called a chain reaction and is defined as when the reaction is self sufficient and will keep going. Nuclear reactors use a controlled chain reaction to harness the energy from nuclear fission.

Nuclear bombs use an uncontrolled chain reaction to release a lot of energy very quickly.

Physics; Nuclear physics; KS5 Year 12; Nuclear fission and nuclear reactors

1	Stage 1
2	Stage 2
3	Stage 3
4	Neutron

Curiosity: The number of fission reactions per "stage" can be calculated using the following formula; $3^{n-1}$ . The first stage $n=1$ has one fission reaction due to only having one neutron, second stage $n=2$ has 3 fission reactions. The 100th stage has $1.72\times 10^{47}$ fission reactions!

Nuclear reactors

There are lots of different types of nuclear fission reactors depending on the country which commissioned them, however they will all share some common features which make them work safely and efficiently.

Nuclear fission reactors will use enriched uranium fuel rods. Most naturally occurring uranium is $^{238}U$ , it undergoes enrichment to make it richer in $^{235}U$ which is what is used for the fission process.

Physics; Nuclear physics; KS5 Year 12; Nuclear fission and nuclear reactors

1	Moderator
2	Fuel rod
3	Control rod
4	Heat exchanger
5	Steam
6	Pressuriser
7	Reactor
8	Coolant pump

Fuel rods	The fuel rods are in the middle of the reactor core and are made from enriched uranium. The fuel rods use a supercritical mass of $^{235}U$ which means that each fission reaction will trigger multiple new fission reactions.
Moderator	The moderator is to slow the neutrons down enough, and turn them into thermal neutrons, to be absorbed by other nuclei. The moderator is normally either graphite or water. Note: Thermal neutrons are slow moving neutrons which have a similar kinetic energy to the particles in the reactor.
Control rods	Control rods control the rate of fission by absorbing excess neutrons released during the fission process. These are normally made of cadmium or boron.
Coolant	The coolant is normally very high pressure water. This coolant loop is closed and does not get cycled into other parts of the reactor or outside environment due to the proximity it has with the core.
Pressure vessel	The whole nuclear reactor is housed in a thick steel pressure vessel which can withstand the temperatures and pressures created by the nuclear reactor. This is then encased in concrete to absorb as much radiation as possible.

The control rods are the main safety feature within the fission reactor. They are lowered into the reactor to absorb more neutrons and therefore slow down the rate of fission. If they are raised then the rate of fission increases as there are more neutrons to induce more fission.

In an emergency the control rods are dropped into the reactor and they absorb all the neutrons until there are no more neutrons to induce anymore fission. This is the only way a nuclear reactor can be stopped.

Advantages and disadvantages of nuclear power

Nuclear power plants are very complex achievements of science but are on the forefront of pollution-free energy. When considering nuclear power there are several factors which need to be considered.

Nuclear fission doesn't produce much green house gases which contribute to global warming. Compared to a fossil fuel power station the emissions are negligible as the fuel isn't combusted. It can also provide a continuous energy supply unlike most renewable energy sources.

However the main problems are the nuclear waste storage and the social stigma of nuclear power.

Nuclear waste is a major problem for nuclear power plants. When the waste is initially removed from the reactor, it must sit in cooling ponds as the fuel rods are still extremely active and produce a lot of heat. After the cooling ponds there are some different processing plants and recycling facilities, but the majority of the high level nuclear waste is buried deep below the ground.

This is still incredibly risky and costly as the nuclear waste won't be safe for many thousands of years and if the waste were to leak into the surrounding environment it could poison entire ecosystems.

Nuclear power disasters are often at the forefront of energy production controversy.

Example

in 2011, an Earthquake and subsequent tsunami interfered with the electrics and cooling systems in the Fukushima nuclear power plant. This then lead the reactor core to meltdown and release many tonnes of contaminated waste into the surrounding sea and environment.

In 1987 Chernobyl nuclear power plant carried out some tests which catastrophically failed leading to the explosion of reactor number 4. This explosion released an incredible amount of radioactive fallout across Europe.

Disasters such as these lead many people to oppose nuclear power stations being built, even though the deaths associated with fossil fuel power stations are significantly higher.

Finally, decommissioning a nuclear power plant is not as straight forward as decommissioning a fossil fuel power station. Decommissioning nuclear power plants is expensive and a lengthy process and the land cannot be repurposed for a long period of time due to the radioactivity of the site.