Nuclear fission is the splitting of a heavy atomic nucleus into smaller nuclei. In this process, energy, new neutrons and fission products are released. A nuclear power reactor produces heat by keeping fission events under control; The resulting heat is then converted into electricity through the steam turbine and generator.
How does fission begin?
When a neutron is absorbed by a fission-competent nucleus, the nucleus may become unstable and break into two smaller pieces. The resulting pieces move at high speed. This kinetic energy interacts with the surrounding material and turns into heat.
Other neutrons may also be released during fission. If some of these neutrons create new fission events in other nuclei, a chain reaction occurs.
What does chain reaction mean?
The behavior of the chain reaction can be described in a simplified form in three cases:
- Decreasing reaction: The number of fissions occurring in one generation decreases in the next.
- Stable reaction: The fission number remains constant on average and the reactor power can be kept stable.
- Increased reaction: The number of fission increases from generation to generation and the reactor power increases.
In normal operation of power reactors, the aim is to control the reaction at the desired power level. Increasing and decreasing reactor power is accomplished by established procedures, measurement systems and reactivity control tools.
What is the role of the moderator?
Neutrons born during fission have high energy. In many thermal reactor designs, neutrons must be slowed to increase the likelihood of new fission. The material that performs this task is called a moderator.
Light water, heavy water and graphite can be used as moderators in different reactor designs. Not every reactor has the same moderator; The same approach of slowing down neutrons is not used in fast reactors.
What do control sticks do?
Control rods contain materials that can absorb neutrons. As they get further into the nucleus, they reduce the number of neutrons that can participate in the chain reaction. When they are removed from the nucleus, reactivity may increase.
Reactivity is not governed solely by control sticks. In some designs, changes in coolant and moderator temperature, dissolved neutron absorbers, and fuel over time also affect reactor behavior.
Does being critical mean being dangerous?
In reactor physics, the word “critical” is not used in the everyday sense of “dangerous”. The critical state describes the physical state in which the chain reaction remains stable on average. Using the term in the correct context is important in nuclear energy communication.
What is the difference between fission and fusion?
Fission is based on the splitting of heavy nuclei, while fusion is based on the combination of light nuclei. Today, commercial nuclear power plants use fission. Continuous and commercial electricity production from controlled fusion is a different field of technology development.
Resources
- IAEA, Fundamentals of Nuclear Power: https://nucleus-qa.iaea.org/sites/graphiteknowledgebase/wiki/Guide_to_Graphite/Fundamentals%20of%20Nuclear%20Power.aspx
- IAEA, Reactor Physics Training Material: https://nucleus-qa.iaea.org/Pages/training-rp.aspx
- IAEA, Design of a Nuclear Reactor: https://gnssn.iaea.org/main/bptc/BPTC%20Module%20Documents/Module04%20Design%20of%20a%20nuclear%20reactor.pdf