Unit 5 Revision- Rate of reaction in nuclear reactor

[jmorris]

Can you remember how to explain how the rate of reaction in a reactor is controlled? Post replies.

[Sophie Mills]

The rate of the reactor is controlled by the number of neutron being absorbed by the control rods, if the control rods are lowered into the reactor then this will slow the reaction down, if the control rods are raised then the reaction speeds up.

[jmorris]

The control rods do control rate of reaction by controlling the number of neutrons able to undergo further fission.
What is fission and can you explain what thermal neutrons are?

[feedthegoat10]

The control rods do control rate of reaction by controlling the number of neutrons able to undergo further fission.
What is fission and can you explain what thermal neutrons are?

Fission is when neutrons are fired into a nucleus which causes it to split into two. Thermal neutrons are neutrons with low energy which makes fission more likely to happen.

[Joseph Wardley]

Fission is the splitting up of a nucleus by a neutron colliding with it. This then splits up the nucleus into two smaller nuclei. Also 3 neutrons are emitted as part of the reaction. For a chain reaction to occur at least one of these neutrons must collide with another nucleus and induce another fission reaction.

Rate of reaction is controlled in a nuclear reactor by:

Control rods
•Made from boron or cadmium.
•These elements absorb neutrons.
•When placed into the reactor the surface area of the rods in the reactor increases.
• As surface area increases the amount of neutrons absorbed at any one time increases.
•This slows down the rate of the reaction.
• When taken out of the reactor the surface area of the rods in the reactor decreases.
• As surface area decreases the amount of neutrons absorbed at any one time decreases.
•This speeds up the rate of the reaction.

Critical mass
•As mass increases for Uranium the surface area to volume ratio decreases.
•This means fewer neutrons escape the mass as they have longer to travel inside the mass to get out.
•If fewer neutrons escape then there is a higher chance that a neutron from other fission reactions will collide with the element and produce a fission reaction.

Moderator
•To increase the probability neutrons inducing a reaction they are need to be slowed down
•This is done by passing them through a moderator, such as water.
•The water molecules transform the kinetic energy of the neutrons into thermal energy of the water.
•This transfer of energy happens as energy can neither be create nor destroyed only transformed from one form to another.
•Neutrons that are slower are called thermal neutrons
•Thermal neutrons have an increased chance of inducing fission as they are less likely to skim off the nucleus and have more time in contact with the nucleus.

Fuel rods
•Fuel is placed into different rods as when a neutrons passes through a moderator the chance that it will come back along the same path is unlikely.
•Fuel is split up into rods so thermal neutrons can then collide with the uranium, more readily.
•So once more fission can be induced.

Enriched uranium
•Enriched uranium has a higher percentage of fissionable Uranium-235 than in natural occurring uranium
•Uranium-235 is more likely to cause a fission reaction when a neutron collides with it than Uranium-238
•Enriched Uranium has around 5% Uranium-235
•Natural Uranium has around 0.7% Uranium-235

Why you need to control the rate of reaction?

What you do want:
•You want a chain reaction.
•So at least one of the three neutrons emitted from the nucleus during fission should go on to create another fission reaction.

What you don’t want:
•The reaction to go out of control.
•So all three neutrons that are emitted then create three more fission reaction.
•This then goes on to create three more which goes on to create another three and so on...
•This is dangerous as a large amount of energy is being released at one time.
•This could cause a meltdown or explosion.
•This in turn could cause radiation to contaminate the environment.

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