Radioactivity and Particles
5 marks · HardA nuclear reactor uses uranium-235 as a fuel. The uranium-235 nucleus can undergo fission when it absorbs a neutron. The process produces energy, radioactive daughter nuclei, and more neutrons. Control rods and moderators play key roles in the reactor's operation. Some isotopes in the reactor can also emit gamma radiation.
Describe, using nuclear notation if helpful, the basic process of nuclear fission for uranium-235 and why more neutrons are produced in the process.
Explain how a chain reaction is controlled in a nuclear reactor. Your answer should mention the purposes of control rods and the moderator.
One of the fission products emits gamma radiation. Compare the nature and penetration of gamma radiation with that of beta particles.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Uranium-235 absorbs a neutron and splits into two smaller nuclei (e.g., barium and krypton), releasing energy and 2 or 3 more neutrons: . The extra neutrons come from the splitting nucleus.
(b) Control rods absorb excess neutrons to prevent the reaction from running out of control. The moderator slows down released neutrons so they are more likely to cause further fission.
(c) Gamma radiation is a highly penetrating electromagnetic wave with no mass or charge, while beta particles are fast-moving electrons with charge and little mass. Gamma can penetrate thick lead; beta is stopped by aluminium.
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Marking points
M1 (a) · 1 mark · Describes absorption of neutron by uranium-235 and splitting (fission), with notation or words.
A1 (a) · 1 mark · Correctly states emission of 2 or 3 neutrons and mention of energy or daughter nuclei.
B1 (b) · 1 mark · Explains function of control rods in absorbing neutrons.
B2 (b) · 1 mark · Explains moderator’s role in slowing down neutrons.
B3 (c) · 1 mark · Correct comparison between gamma and beta in terms of nature, mass/charge, and penetration.
Worked solution
a) Fission occurs when uranium-235 absorbs a neutron and splits into two smaller nuclei (such as barium and krypton), releasing energy and 2 or 3 further neutrons:
b) Control rods in the reactor absorb neutrons to keep their number steady, preventing the reaction from getting too fast. The moderator slows neutrons down so they are more likely to cause other uranium nuclei to split.
c) Gamma radiation is a wave, has no mass or charge, and can pass through thick lead; beta is a negatively charged electron, much less penetrating, and can be blocked by aluminium.