Nuclear Processes - A Level Standard Questions
Q1.
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.
In enriched uranium the proportion of U-235 is greater than in natural uranium 
(ii) Why is enriched uranium rather than natural uranium used in many nuclear reactors?
Induced fission more probable with U-235 than with U-238
(2)
(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 rate of fission, one neutron per fission is required to go on to produce
further fission.
- On average each fission produces two or three neutrons.
- Some neutrons escape the reaction vessel or are absorbed by U-238 without fission occuring
- Control rods are inserted into the reaction vessel to a depth so that they absorb sufficient 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.
- To maximise efficiency the moderator needs to be interspersed between the fuel. Neutrons need to pass through a moderator to slow them (in order to cause further fissions or prevent U-238 absorbing them) - this can be inserted between fuel rods.
- It makes it easier to replace the fuel in stages
(MAX 5)
(Total 7 marks)
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