Nuclear processes - A Level Standard Questions.
Q5.
(a) When 235U undergoes fission, the average number of neutrons produced per fission is about 2.5. Describe what happens to these neutrons in a thermal reactor which is producing power at a constant rate.
When power is produced at a constant rate the neutron speed [ or kinetic energyof neutrons] has been reduced 
so that the kinetic energy of the neutrons are at thermal energy . This is achieved
by interaction with the moderator (or collisions).The neutrons are then at an energy wihich makes successful absorption by the uranium-235 nucleus more likely, and fission to occur.
Power production is constant when one fission produced neutron goes on to produce a further fission. The reactor is then in a critical state.The other neutrons are either absorbed by control rods, shielding or other nuclides or escapefrom the reaction vessel
(5 marks)
(b) Write brief notes on the safety aspects of the following in the use of nuclear reactors for power production.
(i) use of control rods in normal operation and in emergency shut down
Control rods absorb neutrons. The neutron flux [or power or reaction rate] is kept constantby raising or lowering control rods. When lowered into the reaction vessel they can absorb more neutrons. In an emergency they are rapidly dropped into the reaction vessel to stop the reaction
(ii) shielding
Neutrons and gamma radiation must be contained by concrete shielding
(iii) treatment of spent fuel rods
Spent fuel rods are kept underwater (in ponds within the nuclear facility) until the heat from the short half life isotopes has diminished. They are then stored securely for long periods as they remain radioactive (from the longer half-life isotopes) and therefore dangerous to living entities for many years
(8 MAX marks)
(c) Explain how artificial isotopes are produced
The common isotope of an element is bombarded with neutrons. The nucleus absorbs a neutron forming a different isotope. This proces can be repeated to obtain different isotopes.
(2 marks)
(Total 15 marks)
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