Water and energy for everyone · Volume one
12 The reaction is controlled
A4 – More splits under control
Some neutrons make other nuclei split. In the reactor, this reaction is controlled.
Enlarged and simplified drawing
Look, connect the ideas and tell the story
- Neutrons
- Control rods
- Checks
Words and symbols to help you tell the story. Sizes and distances are not realistic.
Fission releases more neutrons too. Some can make other nuclei split. The splits follow one another: this is a chain reaction. Experts control it in the reactor. Some materials absorb neutrons and help regulate it.
What can start another split?
12 Let’s discover together
A split can release neutrons. Some may meet other nuclei and cause more splits. This allows the process to continue. Experts must regulate it.
In the drawing, we also see materials that absorb neutrons. They help control the reaction. They are not a magic wand. The power station needs cooling and barriers too. Safety involves several jobs. When we learn the name of one protection, we also ask what it does and which other protections are needed.
- Released neutrons
- More fissions are possible
- A reaction to regulate
A model to help us follow the idea. The colours and shapes are not measurements. Where arrows appear, they show the path described in the text.
12 Let’s draw and tell the story
Little helpers for my drawing
- Neutrons
- Control rods
- Checks
Look at these symbols. Then make your own drawing in the empty space.
The adult draws two possible paths for a neutron: towards a suitable nucleus, or towards a material that absorbs it. The child follows the lines with a finger.
Beside them, draw small symbols and write three words: control, cool, contain. Find the control rod’s job. We do not make objects fall in a chain. We want to understand regulation, rather than act out a process that seems impossible to stop.
The story in my drawing
The adult can write down something the child says here.
12 For adults
Look, connect the ideas and tell the story
- Neutrons
- Control rods
- Checks
The symbols help you read the diagram. The text explains the choices and limits.
The reaction is controlled
Control rods are one part of the regulating and shutdown systems. The whole nuclei in the picture show places where new fissions may occur, not a complete account of their products. Controlling the reaction does not replace cooling, containment or other defences.
Regulating the reaction is not the whole of safety. Stopping the chain reaction and removing decay heat are distinct functions. Keep that distinction explicit. Do not present inserting the rods as a guarantee that every problem has been solved.
The figure is conceptual and simplifies the products and paths of neutrons. A more technical reading requires material about the reactor type being considered. The paper activity introduces two outcomes without assigning real frequencies to them. Return to the question ‘what can lead to another fission?’ A useful answer refers to the neutron, rather than a flame passing from one nucleus to another.
The question and one possible answer
What can start another split?
Some released neutrons can meet other suitable nuclei and cause new fissions.
Welcome gestures and unfinished answers too. Return to just one part of the picture if the connection is not yet clear.
Sources for further reading F2 F4 · Final section of the book
Explore with an adult · F2 U.S. Department of Energy
https://www.energy.gov/ne/articles/nuclear-101-how-does-nuclear-reactor-work
Explore with an adult · F4 International Atomic Energy Agency
https://www.iaea.org/publications/10885/safety-of-nuclear-power-plants-design
