Glossary of Nuclear Power

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What is fission?
The splitting of a large nucleus into two smaller fission fragment nuclei (there are usually some neutrons set free too!)
What is induced fission?
When we (humans) deliberately fire neutrons at a source of Uranium 235 nuclei in order to cause fission.
What is natural fission?
When fission occurs in nature. Evidence that it has occurred in Uranium seams has been found.
What Uranium isotope is needed in fission fuel rods?
Uranium 235
When Uranium 235 absorbs a neutron into the nucleus what isotope do we get?
Uranium 236.
What is a fission fragment?
When a nucleus splits into two smaller nuclei they are called fission fragments. They are not usually equally sized, but can be!
What else, besides fission fragments, is produced when a nucleus splits?
A few neutrons (usually two or three)... and a lot of energy in the form of the kinetic energy of the fragments and freed neutrons.
What has to be done to a neutron produced from fission before it is suitable for absorption by a Uranium nucleus (so that it can cause a fission)?
It has to have its speed moderated - it has to be slowed down!
What is a moderator?
A substance that is put into the reactor vessel to make the released neutrons slow down.
How does a moderator work?
The fast neutrons collide with the moderator atom lattice and loose kinetic energy on each empact. It takes about 50 collisions for a fast neutron to slow down enough to be a thermal neutron.
Give an example of a material that would make a good moderator.
What properties must a moderator possess?
It must allow neutrons to collide with it so they lose kinetic energy without a danger of them being absorbed by it. This is termed as having a low cross-section for neutron absorption.
What is a chain reaction?
A chain reaction occurs when the product of a reaction can then be used to instigate a further reaction. It is then self perpetuating.
What are used to prevent the reaction from escalating out of control?
Control rods.
What do control rods do?
They absorb the neutrons that are not required to produce fissions.
Give an example of a material that control rods are made of.
What properties must a control rod possess?
It must absorb neutrons - taking them out of the reaction so that they cannot go on to cause fissions. This is termed as having a high cross-section for neutron absorption.
Why must a reactor be shielded?
Because the fuel rods and products of fission are highly radioactive. They are giving out gamma, alpha, positron and beta radiation. There are also a lot of high energy neutrons being produced by the fission reaction. These are dangerous to humans because they are ionizing and/or cause cell damage.
What is used to shield the reactor?
Thick concrete walls - several metres thick.
What happens to the shielding over time?
The neutrons absorbed by the concrete make some of the nuclei in the concrete into radioactive isotopes. It therefore become radioactive itself.
What properties must the shielding material have?
It must be dense, able to withstand high temperatures, strong and able to be made at great thickness.

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