Nuclear Fission
Q6.
(a) Explain why, after a period of use, the fuel rods in a nuclear reactor become
(i) less effective for power production,
After a period of time the amount of fissionable uranium (235) in the fuel rod decreases making fissions less likely. 
Fission fragments and uranium-238 nuclei absorb neutrons. This prevents those neutrons from interacting with Uranium-235 nuclei.
(ii) more dangerous.
Fission fragments are radioactive (alpha, beta and gamma emitters are produced) and uranium-238 nuclei absorb neutrons making them transmute into radioactive isotopes that decay to produce further radioactive materials. These radioctive materials often have very short half lives and high activity, making the activity of the fuel rods higher and more dangerous to handle.
(3 marks)
(b) Describe the stages in the handling and processing of spent fuel rods after they have been removed from a reactor, indicating how the active wastes are dealt with.
For safety the spent fuel rods have to be moved by remote control as they are too radioactive for a person to get near to. They are placed in cooling ponds for several months to allow isotopes with short half lives to decay and the activity to decrease to a safer level before anything can be done with them .
Transport precautions, e.g. impact resistant flasks have to be taken when moving the rods from one location to another. Reprocessing involves the separation of uranium from active wastes .
Vitrification makes the storage and handling of the waste eaier and safer. It is the encasing of waste within glass blocks – these can then be cooled in ponds and later buried deep underground at a geologically stable site. Storage precautions need to be employed, e.g. shielded tanks have to be taken when disposing of the waste and it needs to be monitored carefully .
(5 marks maximum)
(Total 8 marks)
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