E.4.2 Nuclear reactors Notes

Components of a Nuclear Reactor and Their Functions

Consider standing in the control room of a nuclear power plant.

1. Giant monitors display the status of various systems, while engineers carefully adjust controls to maintain balance in a process that powers entire cities.
2. At the heart of this system lies the nuclear reactor, a machine that harnesses the energy of nuclear fission.
3. But how does this machine work, and what ensures its safety and efficiency?

Control Rods: Regulating the Chain Reaction

1. In a nuclear reactor, fission reactions release neutrons, which can collide with uranium-235 nuclei, triggering further fission events.
2. This chain reaction is self-sustaining, but if left unchecked, it can lead to overheating or even a meltdown.
3. Control rods are the components that prevent this from happening.
   1. Made of materials like boron or cadmium, which are highly effective at absorbing neutrons.
   2. By inserting these rods deeper into the reactor core, more neutrons are absorbed, slowing the chain reaction.
   3. Pulling the rods out allows more neutrons to sustain the reaction, increasing its rate.

How It Works:

1. If the reaction is too fast: Insert the control rods deeper to absorb more neutrons and slow the reaction.
2. If the reaction is too slow: Withdraw the control rods slightly to allow more neutrons to sustain the chain reaction.

Moderators: Slowing Down Neutrons for Effective Fission

1. The neutrons released during fission are fast-moving, but uranium-235 nuclei are more likely to capture slow-moving neutrons.
2. This is where the moderator plays a critical role.
3. The moderator slows down fast neutrons, increasing the likelihood of further fission reactions.

Common Moderator Materials:

1. Water: Used in pressurized water reactors (PWRs) and boiling water reactors (BWRs).
2. Graphite: Found in certain reactor designs, such as RBMK reactors.

How It Works:

1. The moderator slows neutrons through collisions.
2. As neutrons collide with the nuclei of the moderator material, they lose energy and slow down.
3. These slower neutrons are more likely to sustain the chain reaction.

Heat Exchangers: Converting Nuclear Energy into Usable Energy

1. The energy released during fission appears as the kinetic energy of the fission products and neutrons.
2. This energy is transferred to the reactor’s coolant, causing it to heat up.
3. The heat exchanger extracts this heat and converts it into usable energy.

How It Works:

1. Heat from the reactor core is transferred to a coolant (commonly water or gas, such as carbon dioxide).
2. The coolant carries the heat to the heat exchanger.
3. In the heat exchanger, the coolant transfers its heat to a secondary loop of water, turning it into steam.
4. The steam drives turbines, which generate electricity.