Nuclear reactions as a source of energy
5 marks · HardA nuclear power plant uses uranium-235 as its fuel. The process of fission for a single uranium-235 nucleus is represented by the equation:
Explain, step by step, how the products of this reaction lead to the release of energy. In your answer, refer to how the energy is distributed between the different products.
Describe what happens to the neutrons that are produced in this reaction in a working nuclear reactor. Include the roles of the moderator and control rods in your answer.
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(a) Most of the energy is carried away as the kinetic energy of the two lighter nuclei (barium and krypton); the rest is carried by the neutrons and gamma radiation. · must show kinetic energy of fission fragments, energy of emitted neutrons, gamma emission
(b) Neutrons cause further fissions or are absorbed. The moderator slows neutrons to increase fission likelihood, and the control rods absorb some neutrons to control the chain reaction rate. · must show neutron absorption, role of moderator, role of control rods
Common mistakes: Omitting mention of kinetic energy of the fission fragments; Incorrectly stating all energy is from gamma radiation; Not mentioning absorption of neutrons by control rods
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Marking points
M1 (a) · 1 mark · Describes fission fragments receive most of the energy as kinetic energy
M2 (a) · 1 mark · Mentions neutrons and gamma radiation carry the rest of the energy
A1 (a) · 1 mark · Explains why energy is released (mass of products is less than reactants, lost mass converted to energy)
M3 (b) · 1 mark · Correctly describes role of moderator in slowing neutrons
M4 (b) · 1 mark · Correctly describes role of control rods in absorbing neutrons to control chain reaction
Worked solution
a) In fission, the uranium-235 nucleus absorbs a neutron and splits into two lighter nuclei (barium and krypton). These fission fragments move apart very quickly and thus have high kinetic energy; this is where most of the energy from the reaction goes. Some additional energy is released as gamma rays, and the emitted neutrons also carry away kinetic energy. The combined mass of the products is less than the mass of the reactants, so the lost mass is converted to energy according to .
b) The neutrons released can go on to strike other uranium-235 nuclei, leading to more fission events — this can result in a chain reaction. The moderator (usually graphite or water) slows down these neutrons to make them more likely to cause further fission. The control rods (made of materials such as boron) absorb some of the neutrons, reducing their number and thus controlling the rate of the chain reaction to prevent the reaction from running out of control.