Fission and Fusion

Nuclear fission involves the splitting of a larger nucleus into multiple smaller nuclei. The process produces several products but also huge amounts of energy. The process of a fission reaction can be seen below:

Several neutrons produced from each fission reaction may then cause another fission reaction to occur if they collide with another U-235 nuclei, triggering a chain reaction. This chain reaction is controlled, and the energy harnessed in a nuclear reactor. The number of neutrons produced is dependent on the element undergoing fission. 

It is important to note that the neutrons must be slow moving, because a fast moving neutron will rebound off of the U-235 nucleus without fission occurring. 

All fission reactions have a critical mass. This is the minimum mass of fuel (U-235) required for the reaction to be self-sustaining. Lower than the critical mass and the reaction will slowly come to a stop. Beyond the critical mass, the reaction becomes uncontrollable. 

Nuclear fusion is the process of two smaller nuclei fusing to form one larger nucleus, giving off huge amounts of energy in the process. 

Tritium and deuterium undergo nuclear fusion in the sun in a process which creates massive amounts of energy:

In this process, deuterium and tritium must have very large kinetic energies before they collide. If they have too little kinetic energy, the nuclei will never overcome the force of electrostatic repulsion. Once the nuclei get close enough, the strong nuclear force takes over, fusing the nuclei to form Helium and an emitted neutron. 

Although the process produces massive amounts of energy, it also requires huge amounts of energy in the form of heat and pressure which proves to be expensive to produce. Because of the need for extreme temperatures and pressure, the correct conditions are hard to maintain for the amount of time needed.

When comparing the stability of nuclei, it is useful to compare the binding energy per nucleon. The higher the binding energy per nucleon, the more stable the nuclei because more input energy is needed to split the nucleus into its components. 

This can be represented in a graph as shown below: 

From the graph we can see that iron-56 is the most stable with the highest binding energy per nucleon (8.9 \text{ MeV}). 

At low nucleon numbers, nuclei are least stable. Hydrogen and Helium have very small electrostatic forces holding the nuclei in place and therefore are unstable. 

Higher values of nucleon number such as U-235, the binding energy begins to decrease as the nucleus gets too large to remain stable. 

A nuclear reactor uses fission reactions to produce large amounts of thermal energy. The thermal energy is then used to heat water into high pressure steam which is pumped through a turbine to turn a generator and produce electricity. 

Within the nuclear reactor, there are three distinct components:

The reactor is surrounded by a thick insulator that keeps heat inside but also minimises radiation leaks through the walls of the reactor. 

The fuel rods, usually made of uranium, contain the radioactive nuclei used in the reaction. 

The control rods can be lowered and lifted to control the rate of the reaction. The control rods absorb neutrons. They are lowered to absorb neutrons and slow the rate of reaction and lifted to allow more of the chain reaction to occur. 

The coolant absorbs the thermal energy and is pumped to a boiler where water is heated into high pressure steam.