4.9 Nuclear fission
9 exam questions worth 24 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is nuclear fission?
Choose one answer.
Correct Not quite: the answer is A large nucleus splitting into two smaller nuclei
Mark scheme
B: A large nucleus splitting into two smaller nuclei
Model answer
A large nucleus splitting into two smaller nuclei (B).
What is the job of the control rods in a nuclear reactor?
Choose one answer.
Correct Not quite: the answer is To absorb neutrons
Mark scheme
B: To absorb neutrons
Model answer
To absorb neutrons (B). The moderator slows the neutrons down.
Describe what happens when a neutron is absorbed by a uranium-235 nucleus.
Mark scheme
the nucleus becomes unstable and splits into two smaller nuclei (1); two or three neutrons are released (1); energy is released (1)
Model answer
The uranium-235 nucleus becomes unstable and splits into two smaller nuclei. Two or three neutrons are released, along with energy.
Explain what is meant by a chain reaction.
Mark scheme
neutrons released by fission are absorbed by other nuclei (1); causing more fissions, which release more neutrons, and so on (1)
Model answer
Neutrons from one fission are absorbed by other uranium nuclei, which split and release more neutrons, and this carries on.
Complete the fission equation: ^{1}_{0}\text{n} + ^{235}_{92}\text{U} \rightarrow\; ^{139}_{56}\text{Ba} + ^{?}_{36}\text{Kr} + 3\;^{1}_{0}\text{n}. Calculate the mass number of the krypton nucleus.
Mark scheme
mass number total on the left = 236 (1); 236 − 139 − 3 = 94 (1)
Model answer
The left side has a total mass number of 1 + 235 = 236. So the krypton is 236 − 139 − 3 = 94.
Explain the purpose of the moderator in a nuclear reactor.
Mark scheme
neutrons are slowed down by collisions with the moderator (1); slow neutrons are more likely to be absorbed by uranium-235 nuclei so the chain reaction continues (1)
Model answer
The moderator slows the neutrons down. Slow neutrons are more likely to be absorbed by uranium-235 nuclei, so the chain reaction keeps going.
A nuclear reactor starts to overheat. Describe what should be done to the control rods and explain why.
Mark scheme
lower the control rods into the reactor (1); they absorb more neutrons (1); so fewer fissions take place and the rate of the reaction falls / the reactor cools (1)
Model answer
The control rods should be lowered further into the core. They absorb more neutrons, so fewer fissions occur and the rate of the reaction falls.
Describe how the energy released by fission in a nuclear power station is used to generate electricity.
Mark scheme
fission in the fuel rods heats the coolant (1); the hot coolant boils water to make steam (1); the steam turns a turbine (1); which turns a generator (1)
Model answer
Fission heats the coolant, which boils water to make steam. The steam turns a turbine, and the turbine drives a generator that produces electricity.
Evaluate the use of nuclear fission to generate electricity compared with burning fossil fuels.
Mark scheme
advantages (up to 3): no carbon dioxide when running / does not add to global warming (1); large amount of energy from small amount of fuel (1); reliable / not dependent on the weather (1); disadvantages (up to 3): radioactive waste must be stored safely for a long time (1); risk of accidents / contamination (1); expensive to build and to decommission (1); a balanced conclusion (1)
Model answer
Nuclear power does not release carbon dioxide while it is running, uses very little fuel and is reliable. However, it produces radioactive waste that has to be stored for thousands of years, accidents are very serious, and power stations are expensive to build and decommission. Fossil fuel stations are cheaper to build, but they release carbon dioxide and add to global warming.
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