Practise Nuclear fission. 14 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Name one element used as a fuel in nuclear reactors.
Uranium
Name the other element that is commonly used as a fuel for nuclear fission.
Plutonium
State one thing, other than energy and two smaller nuclei, that is released when a large nucleus undergoes fission.
Two or three neutrons (or gamma rays).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
(a) What is nuclear fission? Tick (✓) one box.[1]
The splitting of a large, unstable nucleus into two smaller nuclei
The joining of two light nuclei to form a heavier nucleus
The emission of an alpha particle from a nucleus
The absorption of an electron by a nucleus
(b) Name one element used as a fuel in nuclear reactors.[1]
(c) What must usually be absorbed by a uranium-235 nucleus before it undergoes fission?[1]
(d) As well as two smaller nuclei, what else is released when a nucleus undergoes fission? Tick (✓) two boxes.[2]
Two or three neutrons
Gamma rays
Alpha particles
Electrons from the atom
Show the answer and mark scheme
(a)Answer: The splitting of a large, unstable nucleus into two smaller nuclei
(b)Answer: Uranium
uranium / plutonium
(c)Answer: A neutron
a neutron
(d)Answer: Two or three neutrons; Gamma rays
Question 2Medium6 marks
Uranium-235 is used as the fuel in many nuclear power stations.
(a) Describe what happens when a uranium-235 nucleus undergoes fission.[4]
(b) Name the other element that is commonly used as a fuel for nuclear fission.[1]
(c) Spontaneous fission is rare. What does this tell you about how fission usually happens in a reactor?[1]
Show the answer and mark scheme
(a)Answer: The nucleus absorbs a neutron and splits into two smaller nuclei of roughly equal size, releasing two or three neutrons, gamma rays and energy; the products have kinetic energy.
the nucleus absorbs a neutron
it splits into two smaller nuclei (of roughly equal size)
two or three neutrons are released
gamma rays are emitted / energy is released
the fission products have kinetic energy
(b)Answer: Plutonium
plutonium
(c)Answer: Nuclei usually split only after absorbing a neutron.
fission usually happens only after the nucleus has absorbed a neutron (so neutrons are needed to start / keep the reaction going)
Question 3Hard8 marks
The chain reaction in a nuclear reactor must be controlled. The reactor core contains fuel rods, control rods and a moderator.
(a) Describe the job of the control rods.[2]
(b) Explain what happens to the rate at which energy is released if the control rods are lowered further into the reactor core.[2]
(c) The neutrons released by fission move very fast. Uranium-235 nuclei are much more likely to absorb slow-moving neutrons than fast-moving neutrons. The moderator slows the neutrons down. Explain why the reactor needs a moderator.[2]
(d) In a reactor working at a steady rate, on average one neutron from each fission goes on to cause another fission. Explain what would happen if, on average, two neutrons from each fission went on to cause further fissions.[2]
Show the answer and mark scheme
(a)Answer: They absorb neutrons, which controls the rate of fission.
the control rods absorb neutrons
so fewer neutrons are available to cause fission / the rate of fission is controlled
(b)Answer: More neutrons are absorbed, so there are fewer fissions and less energy is released each second.
more neutrons are absorbed, so fewer fissions happen
so the rate of energy release decreases
(c)Answer: Slowed-down neutrons are much more likely to be absorbed by uranium-235 nuclei and cause fission, so enough fissions happen for the chain reaction to keep going.
slowed-down neutrons are (much) more likely to be absorbed by uranium-235 nuclei (and cause fission)
so enough neutrons cause further fissions for the chain reaction to continue / without a moderator the chain reaction would stop
(d)Answer: The number of fissions would double every generation, so the energy release would run out of control.
the number of fissions would double in each generation / increase very rapidly
the rate of energy release would increase out of control, overheating the reactor / causing an explosion