Term
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Definition
| The splitting of a large nucleus into two smaller fission fragment nuclei (there are usually some neutrons set free too!) |
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Term
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Definition
| When we (humans) deliberately fire neutrons at a source of Uranium 235 nuclei in order to cause fission. |
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Term
| What Uranium isotope is needed in fission fuel rods? |
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Definition
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Term
| When Uranium 235 absorbs a neutron into the nucleus what isotope do we get? |
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Definition
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Term
| What is a fission fragment? |
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Definition
| 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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Term
| What else, besides fission fragments, is produced when a nucleus splits? |
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Definition
| 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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Term
| 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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Definition
| It has to have its speed moderated - it has to be slowed down! |
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Term
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Definition
| A substance that is put into the reactor vessel to make the released neutrons slow down. |
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Term
| How does a moderator work? |
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Definition
| 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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Term
| Give an example of a material that would make a good moderator. |
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Definition
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Term
| What properties must a moderator possess? |
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Definition
| 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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Term
| What is a chain reaction? |
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Definition
| 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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Term
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Definition
| They absorb the neutrons that are not required to produce fissions. |
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Term
| What properties must a control rod possess? |
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Definition
| 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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Term
| Why must a reactor be shielded? |
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Definition
| 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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Term
| What is used to shield the reactor? |
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Definition
| Thick concrete walls - several metres thick. |
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Term
| What happens to the shielding over time? |
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Definition
| 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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Term
| Give an example of a material that control rods are made of. |
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Definition
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Term
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Definition
| They absorb the neutrons that are not required to produce fissions. |
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Term
| What are used to prevent the reaction from escalating out of control? |
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Definition
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Term
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Definition
| When fission occurs in nature. Evidence that it has occurred in Uranium seams has been found. |
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