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The total mass of a system is not always conserved because according to Einstein’s theory of relativity, energy and mass are equivalent . Mass-energy is always conserved. . Nuclear Fission and Fusion .

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nuclear fission and fusion

The total mass of a system is not always conserved because according to Einstein’s theory of relativity, energy and mass are equivalent. Mass-energy is always conserved.

Nuclear Fission and Fusion

If a body gains an amount of energy E, its mass increases by an amount m, found by , where c is the speed of light.


Mass defect: The mass defect of a nucleus is equal to the difference between the total mass of the individual separate nucleons and the mass of the nucleus.

The total mass of the separated nucleons is greater than the mass of the nucleus.


The protons and neutrons in the nucleus are held together by the nuclear strong force, which is attractive at this range. In order to dismantle the nucleus, energy must be put into the system by doing work to separate the nucleons. They have an increased potential energy so the mass must increase by an amount Δm which is related to ΔE by the equation ΔE= Δmc2


Separated Nucleons

Mass defect = Mass of nucleons – the mass of the nucleus


Binding energy is the minimum energy needed to dismantle a nucleus into its separate nucleons. It is the energy equivalent of the mass defect.

If energy is released in a nuclear reaction, the binding energy of the system must have increased. This is because for energy to be released, the potential energy of the system must have decreased. The nucleons must have moved closer together so more energy will be required to separate them, hence an increased binding energy.

The stability of a nucleus depends on its binding energy per nucleon. Iron has the greatest stability because it has the largest binding energy per nucleon – lots of energy is required to separate its nucleons.

Decreased Potential Energy

Higher binding energy

+ energy

Low binding energy


Nuclear Fission and Fusion

Nuclear fission and nuclear fusion both release energy because both processes increase the binding energy per nucleon of the particles involved.

Nuclear fission is when a massive nucleus splits forming 2 smaller fragments. Large nuclei are unstable and some will spontaneously split into 2 smaller nuclei. We can induce nuclear fission if we bombard a nucleus with neutrons.

Nuclear fusion is when two light nuclei are joined together to create a larger nucleus.


Producing useful power with nuclear reactors…

The energy from a nuclear fission reaction can be harvested to produce useful power. Nuclear power stations provide about 20% of the nation’s electricity supplies.

  • Uranium is usually the fuel used in a nuclear power station
  • Only certain isotopes of these elements can undergo fission – they are described as fissile
  • A chain reaction must be established:

A single neutron strikes the uranium nucleus

The neutron is absorbed creating a larger unstable nucleus.

Highly unstable nucleus splits into 2 and a number of neutrons are released

These neutrons go on to induce fission of other uranium nuclei. The number of neutrons increases exponentially.

inside a fission reactor
Inside a fission reactor…
  • Uranium fuel rods in the reactor core
  • Fuel releases energy so that the core gets hot
  • Heat is transferred by the fluid coolant to the boiler area
  • Steam is generated which turns a turbine which turns a generator to produce electricity.

The fuel rods are positioned vertically. A coolant can flow between them, removing the heat produced by fission.

Fuel rods are surrounded by a material called the moderator. This may be a material such as graphite. The neutrons released in fission are very energetic – ‘fast neutrons’. They move so rapidly that they are unlikely to interact with uranium nuclei. As fast neutrons pass through the moderator material, they are slowed down. They are now slower moving ‘thermal neutrons’, which are more likely to cause induced fission of more uranium nuclei.

inside a fission reactor1
Inside a fission reactor…

Control rods are made of a neutron absorbing substance such as boron. To slow down or stop the chain reaction, they are lowered into the core. To speed up the reaction, they are partially withdrawn.

The chain reaction must continue on its own at a steady rate. The amount of fuel required for this is the critical mass. Any mass less than the critical mass (subcritical mass) and the reaction will just peter out. Nuclear reactors use a super critical mass, where several new fissions follow each fission.

The same nuclear fission reaction can be used to create a nuclear bomb. In a nuclear bomb, the reaction escalates so that more and more neutrons fly around in the uranium causing a rapid release of energy.

nuclear waste
Nuclear Waste
  • Waste products of nuclear fission usually have a larger proportion of neutrons than stable nuclei making them unstable and radioactive.
  • Some waste products decay rapidly, causing them to become hot. They must be put into cooling ponds until their temperature falls.
  • Some products decay very slowly over thousands of years, posing a long term hazard.
  • Radioactive material can be stored in containers on the surface where activity is monitored
  • Some radioactive material can be buried underground in lead and concrete canisters, but this raises concerns that containers may leak, polluting underground water sources.
nuclear fusion
Nuclear Fusion

Nuclear fusion is when two light nuclei are joined together to create a larger nucleus.

Nuclei can only fuse if they have enough energy to overcome the electrostatic force of repulsion between them and get close enough for the strong nuclear interaction to bind them.

  • Energy emitted from the sun and other stars comes from nuclear fusion reactions. Fusion can happen in these stars because the temperature in the star’s core is so high.
  • At these high temperatures, atoms do not exist. Negatively charged electrons are stripped away, leaving positively charged nuclei and free electrons. The resulting mixture is plasma.
  • Lots of energy is released in nuclear fusion because the new heavier nuclei have a higher binding energy per nucleon than the starting reactants.
  • The large amount of energy released helps t maintain the high temperature for further fusion reactions to occur.