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21 Cards in this Set
- Front
- Back
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What is fission?
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The splitting of a large nucleus into two smaller fission fragment nuclei (there are usually some neutrons set free too!)
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What is induced fission?
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When we (humans) deliberately fire neutrons at a source of Uranium 235 nuclei in order to cause fission.
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What is natural fission?
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When fission occurs in nature. Evidence that it has occurred in Uranium seams has been found.
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What Uranium isotope is needed in fission fuel rods?
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Uranium 235
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When Uranium 235 absorbs a neutron into the nucleus what isotope do we get?
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Uranium 236.
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What is a fission fragment?
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When a nucleus splits into two smaller nuclei they are called fission fragments. They are not usually equally sized, but can be!
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What else, besides fission fragments, is produced when a nucleus splits?
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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.
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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)?
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It has to have its speed moderated - it has to be slowed down!
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What is a moderator?
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A substance that is put into the reactor vessel to make the released neutrons slow down.
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How does a moderator work?
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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.
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Give an example of a material that would make a good moderator.
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Graphite.
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What properties must a moderator possess?
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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.
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What is a chain reaction?
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A chain reaction occurs when the product of a reaction can then be used to instigate a further reaction. It is then self perpetuating.
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What are used to prevent the reaction from escalating out of control?
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Control rods.
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What do control rods do?
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They absorb the neutrons that are not required to produce fissions.
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Give an example of a material that control rods are made of.
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Boron.
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What properties must a control rod possess?
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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.
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Why must a reactor be shielded?
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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.
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What is used to shield the reactor?
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Thick concrete walls - several metres thick.
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What happens to the shielding over time?
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The neutrons absorbed by the concrete make some of the nuclei in the concrete into radioactive isotopes. It therefore become radioactive itself.
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What properties must the shielding material have?
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It must be dense, able to withstand high temperatures, strong and able to be made at great thickness.
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