Nuclear Fission
Q4. Natural uranium consists of 99.3% 
and 0.7% 
.
In many nuclear reactors, the fuel consists of enriched uranium enclosed in sealed metal containers.
(a)
(i) Explain what is meant by enriched uranium.
Enriched uranium has a greater proportion of U-235 than is found in natural uranium. 
(ii) Why is enriched uranium rather than natural uranium used in many nuclear reactors?
U-235 is the fissionable isotope - therefore induced fission is more probable with a higher proportion of U-235 in the mix.
(2 marks)
(b)
(i) By considering the neutrons involved in the fission process, explain how the rate of production of heat in a nuclear reactor is controlled.
For a steady heat output you need a steady rate of fission. This is achieved when one neutron per fission goes on to produce a further fission. If more than one neutron proceeeds form a fission to cause further fission the reaction will escalate and an increasing amount of haet will be produced each second. If less than one neutron proceeds to cause further fission the reaction will shut down.
Each fission produces two or three neutrons on average Some of the neutrons escape from the reaction vessel and some are absorbed by U-238 or fission fragments and do not cause fission to occur. Control rods are lowered into the reaction vessel just eanough to absorb excess neutrons to maintain steady rate of fission.
(ii) Explain why all the fuel in a nuclear reactor is not placed in a single fuel rod.
Neutrons need to pass through a moderator to slow them down enough so that they are of the optimum energy to be absorbed efficiently by the U-235 nucleus.Having the fuel in pellet form allows the fuel pellets to be interspersed with moderator material. It also makes it easier to replace the fuel in stages
(5 marks maximum)
(Total 7 marks)
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