In this explainer, we will learn how to explain how energy is produced by nuclear fusion and fission in the context of nuclear power plants.
Nuclear energy is the energy released when atomic nuclei break apart or are combined. We can harness the energy from the nucleus to generate electrical current at nuclear power plants. Nuclear energy has advantages over energy derived from fossil fuels, and so it is a desirable method of electricity generation.
Let us investigate the two processes by which nuclear energy is released from nuclei.
Protons and neutrons are bound together in the nucleus of an atom and are prevented from flying apart by a very strong attractive force called the nuclear force or strong nuclear force. The energy required to work against the nuclear force to separate the nucleons (protons and neutrons) in the nucleus of an atom is called the nuclear binding energy.
Definition: Nuclear Binding Energy
It is the energy required to disassemble the nucleus of an atom into the protons and neutrons that make it up.
When protons and neutrons are separated, or when subatomic particles are added to nuclei, changes in the binding energy occur. A very large amount of energy is released in the form of nuclear energy.
Nuclear energy can be generated by two processes: fission and fusion. Let us investigate both these processes.
Fission reactions are used in nuclear power plants to generate electricity. The picture below shows a nuclear power plant.
Fission occurs when one heavy atomic nucleus is split into two or more lighter, smaller atomic nuclei.
Definition: Nuclear Fission
It is the splitting of one heavy atomic nucleus into two or more lighter atomic nuclei.
The diagram below shows a fission reaction that occurs in nuclear power plants. The uranium-235 isotope is one of the fuels for nuclear power. It is packed into rods called fuel rods. In the fuel rods, the atoms of uranium-235 are bombarded with high-speed neutrons. When the collision between a neutron and a uranium atom has enough energy to overcome the attractive forces in the nucleus that bind the nucleons together, the uranium nucleus is split into smaller, lighter nuclei: an atom of barium and an atom of krypton. Three neutrons are also produced.
Uranium-235 is known as a fissionable material because it can undergo fission reactions during which very large amounts of energy are released.
We can write the nuclear reaction for this equation as
Inside nuclear power plants, a continually repeating fission reaction, called a chain reaction, occurs.
A chain reaction is a reaction that continues over and over, in the same manner, each time. The same step or steps in the mechanism or process repeat indefinitely until the reactants are used up or until something interferes to stop the reaction.
Definition: Chain Reaction
It is a reaction that follows the same step or steps repeatedly and indefinitely, until the reactants are used up or the reaction is terminated.
Let us now use this same fission nuclear reaction as above to investigate how chain reactions occur. The diagram below shows how the three neutrons produced from the bombardment of a uranium-235 atom can go on to strike other uranium-235 atoms.
The reaction equation, occurs over and over repeatedly in a chain reaction. The diagram above shows how this chain reaction increases in intensity exponentially, generating increasing amounts of energy over time.
Example 1: Selecting the Correct Name for an Indefinitely Repeating Reaction
Although it is carefully controlled, once nuclear fission is initiated in a nuclear reactor, the reaction continues indefinitely until the fuel rods are spent. What is the name for a reaction of this type?
- Neutron reaction
- Decay reaction
- Geiger reaction
- Chain reaction
- Fusion reaction
When a nuclear reaction is started in a nuclear reactor at a power plant, it will continue spontaneously. This is because the products of the first fission reaction can go on to strike more reactant atoms, causing them to undergo fission too. This process occurs over and over repeatedly, and exponentially, until all the reactant material is used up or until the reaction is terminated.
The type of reaction where the process follows the same step or steps repeatedly and indefinitely until the reactants are used up or the reaction is terminated is called a chain reaction. The correct answer is D.
Fission chain reactions need to be carefully controlled. If they are not controlled, they stop or become unstable. If there is not enough uranium-235 and neutrons, there will not be enough collisions to sustain the chain reaction and the reaction will stop. Or if there are too many collisions between neutrons and uranium-235 atoms, the chain reaction can become uncontrolled. An uncontrolled chain reaction could ultimately lead to the meltdown of a reactor core.
Neutron moderation and neutron absorption are two processes used to control fission chain reactions.
At higher speeds, neutrons can pass straight through the nuclei of fissionable atoms. In neutron moderation, the speed of neutrons is reduced. This results in an increased number of successful collisions between the neutrons and the uranium-235 atoms and maintains the chain reaction. Carbon in the form of graphite and water is a suitable substance to use as a moderator.
An opposite outcome is sought in neutron absorption, as neutrons are prevented from colliding with uranium-235 atoms and the chain reaction slows down. Control rods made of elements such as cadmium are used to absorb neutrons.
Definition: Neutron Moderation
It is a process where neutrons are slowed to increase the number of successful collisions between neutrons and uranium-235 atoms and increase the chain reaction rate.
Definition: Neutron Absorption
It is a process where neutrons are absorbed by control rods to decrease the number of successful collisions between neutrons and uranium-235 atoms and reduce the chain reaction rate.
At the nuclear power plant, the chain reaction occurs inside the reactor. The diagram below shows the parts of the reactor. The fuel rods containing uranium-235 are inside the reactor core that is filled with water. Water is used to cool the rods and slow down neutrons via moderation. The reactor core also contains control rods for neutron absorption.
The number of control rods in the reactor core, and the depth to which they are inserted in the reactor core, can be adjusted in order to control the rate of fission.
Example 2: Identifying the Correct Term for the Chain Reaction Control Processes in Nuclear Reactors
In a nuclear reactor, it is important that the reaction does not proceed too quickly or too slowly.
- What is the name of the process by which neutrons are
slowed down to ensure more successful collisions occur?
- Neutron transmutation
- Neutron absorption
- Neutron ionization
- Neutron moderation
- Neutron activation
- What is the name of the process by which neutrons are blocked to prevent
collisions with the fuel rods?
- Neutron activation
- Neutron transmutation
- Neutron moderation
- Neutron absorption
- Neutron ionization
Fission chain reactions need to be controlled or they will either stop or continue in a hazardous and uncontrolled manner. To increase the rate of a fission reaction that is too slow, the speed of neutrons is reduced to increase the number of collisions between neutrons and the fissionable fuel atoms (for example, uranium-235 atoms) to increase the chain reaction rate. This process is called neutron moderation. So, the correct answer is D.
When a fission chain reaction is occurring too quickly, much energy is generated. The possibility of an uncontrollable reaction occurring, and even an explosion, increases. So, to decrease the fission reaction rate, control rods are inserted deeper into the reactor core, or more control rods are added, to absorb neutrons. With fewer free neutrons in the reactor core, fewer collisions occur between neutrons and fissionable fuel atoms, and the chain reaction rate is reduced. This process is called neutron absorption. So, the correct answer is D.
The diagram below shows how electrical energy is generated from fission in the reactor core.
The chain reaction generates a lot of heat energy that heats up the water in the reactor core. The hot water is pumped to a steam generator, or boiler, where it is converted to steam. The pressure from the steam spins a turbine that in turn generates electricity. The steam is cooled and condensed back into liquid water in large cooling towers. The large towers in the picture above are cooling towers. Finally, this cooled water is pumped back into the reactor core, and the cycle is repeated.
Example 3: Identifying the Correct Order of Energy Transfer in a Nuclear Power Plant
What is the order of transfer of the electrical energy generated by nuclear fission from the fuel rods through a power plant?
In a nuclear power plant, much heat energy is generated by fission reactions in the fuel rods, which sit inside the reactor core. This heat energy is used to convert the water in the reactor core into steam, either in the reactor core or in an attached boiler. The steam from the boiler spins turbines and electrical energy is then generated in the generator. So, the correct order of energy transfer is fuel rods, reactor, boiler, turbines, and lastly, generator. This order matches answer option B.
The process of electricity production in a nuclear power plant generates long-lived radioactive waste. The spent fuel rods are not the only hazardous waste products. Worker clothing and tools that came into contact with the nuclear fuel will also contain small amounts of radioactive material and will need to be disposed of carefully. Nuclear waste is first stored underwater to cool. After some time, uranium from spent fuel rods can either be recycled or buried underground.
Let us compare the advantages of nuclear energy from fission with that of the energy from fossil fuels. The table summarizes the important advantages and disadvantages of each.
|Fossil Fuels||Nuclear Fuel|
|The Volume of Fuel Needed to Release the Same Amount of Energy||Relatively large (1 tonne of coal)||Relatively small (6 grams)|
|Waste Products||, , compounds, compounds, and particulate matter (soot)||Uranium-238 , uranium-235 , and plutonium and other fission products|
|The Volume of Waste Products||Millions of tonnes of and hundreds of thousands of tonnes of ash per year||A few cubic metres per 1 000 000 people per year|
Firstly, nuclear reactions release far more energy than the energy released from burning fossil fuels. A much smaller volume of fuel is needed to generate nuclear energy. Both processes produce hazardous waste products. Burning fossil fuels generates more air pollutants, but nuclear waste is radioactive, sometimes for thousands of years. The volume of nuclear waste is much smaller than the volume of waste produced from burning fossil fuels. For this reason, nuclear energy is considered “greener” and more environmentally friendly by some countries. Other countries, however, have banned electricity generation from nuclear power.
Nuclear fusion occurs when two or more light atomic nuclei collide and combine with each other to form one different and larger atomic nucleus. The new nucleus that is produced is heavier than the two original nuclei.
Definition: Nuclear Fusion
It is the formation of one, heavier atomic nucleus from the collision and combination of two smaller, lighter atomic nuclei.
Example 4: Understanding the Difference between Fusion and Fission
Which of the following statements correctly contrasts nuclear fusion and nuclear fission?
- In nuclear fission, large nuclei split, while in nuclear fusion, small nuclei combine.
- In nuclear fusion, large nuclei split, while in nuclear fission, small nuclei combine.
Fusion is the process where two or more smaller, lighter nuclei fuse or combine into one, different, heavier, and larger nucleus. We can remember this by considering the words fusion and fuse, as fuse means to combine or join together. So, fusion is the joining together of nuclei.
Fission is the process where one heavier, larger nucleus is split into two or more smaller, lighter nuclei.
So, the correct answer is A.
Very large amounts of energy are released in this process. The energy released from the Sun is generated by fusion processes. Pressures in the Sun are very high, with temperatures of millions of degrees Celsius. These conditions enable atomic nuclei to collide and fuse with each other. Every second in the Sun, hundreds of millions of tons of hydrogen combine, to form helium.
Several different types of fusion reactions occur in the Sun. The simplest of these reactions is shown by this nuclear equation:
Four hydrogen nuclei collide and combine together to form one helium nucleus. Two positrons are also produced . During this process, some of the mass of hydrogen is converted into energy.
Fusion reactions generate much more energy than fission reactions and do not generate any long-lived radioactive waste. However, fusion reactions as a means of sustained energy production are not yet feasible. Nuclear power plants currently only use fission as the energy generation method and do not use fusion reactions to generate electricity. The reason for this is that fusion is incredibly difficult to achieve, and even more difficult to sustain, because of the extremely high temperatures and pressures needed to make positively charged nuclei fuse. Scientists cannot maintain the conditions necessary for sustained fusion reactions yet; however, scientists are researching how this could be done and perhaps in the future, nuclear fusion will also be used to generate electricity.
- Nuclear energy is the energy released from fusion or fission.
- Fission is the splitting of one heavy atomic nucleus into two or more lighter atomic nuclei.
- Fusion is the formation of one, heavier atomic nucleus from the collision and combination of two smaller, lighter atomic nuclei.
- Fission reactions are used to generate electricity at nuclear power plants.
- Nuclear power from fission produces more energy and less waste than burning fossil fuels but produces waste that is radioactive for a long time.
- Neutrons are slowed to increase the number of successful collisions between neutrons in a chain reaction rate in a process called neutron moderation.
- Neutrons are absorbed by control rods to decrease the number of successful collisions in a chain reaction rate in a process called neutron absorption.