4.10 Nuclear fusion
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What happens in nuclear fusion?
Choose one answer.
Correct Not quite: the answer is Two small nuclei join to form a larger nucleus
Mark scheme
B: Two small nuclei join to form a larger nucleus
Model answer
Two small nuclei join to form a larger nucleus (B).
Where does the Sun get its energy from?
Choose one answer.
Correct Not quite: the answer is Nuclear fusion of hydrogen
Mark scheme
C: Nuclear fusion of hydrogen
Model answer
Nuclear fusion of hydrogen (C).
State the type of particle that is released together with energy when deuterium and tritium fuse to form helium.
Mark scheme
neutron
Model answer
A neutron.
Complete the equation for fusion: ^{2}_{1}\text{H} + ^{3}_{1}\text{H} \rightarrow\; ^{?}_{2}\text{He} + ^{1}_{0}\text{n}. State the mass number of the helium nucleus.
Mark scheme
total mass number on the left = 2 + 3 = 5 (1); 5 − 1 = 4 (1)
Model answer
The total mass number on the left is 5. The neutron has 1, so the helium has a mass number of 4.
Explain why fusion happens only at extremely high temperatures.
Mark scheme
nuclei are positively charged so repel each other (1); to fuse they must get very close together (1); high temperature gives the nuclei enough kinetic energy to overcome the repulsion (1)
Model answer
The nuclei are positively charged and repel each other. They have to be moving very fast to get close enough to join, so the temperature must be extremely high to give them enough kinetic energy.
Explain why a fusion reactor cannot be built from a solid container, and describe how the hot plasma is held in place.
Mark scheme
the plasma is at about 100 million °C which would melt any material (1); the plasma is held away from the walls (1); using strong magnetic fields (1)
Model answer
The plasma is at around 100 million °C, so any solid container would melt. It is held away from the walls by very strong magnetic fields.
Give two advantages and one disadvantage of using nuclear fusion, rather than nuclear fission, to generate electricity.
Mark scheme
advantages, any two from: fuel is almost unlimited / no carbon dioxide / no long-lived waste / no chain reaction (1 each); disadvantage: any one from very difficult / expensive to build / no working power station yet / needs a lot of energy to heat plasma (1)
Model answer
Fusion makes very little long-lived radioactive waste and the fuel, hydrogen from sea water, is almost unlimited. The disadvantage is that the conditions are very hard to achieve, so there is no working fusion power station yet.
Describe what happens to the mass of the nuclei in a fusion reaction, and where the energy released comes from.
Mark scheme
the mass of the nucleus formed is slightly less than the total mass of the nuclei that joined (1); the lost mass is converted to energy (1)
Model answer
The new nucleus has slightly less mass than the nuclei that joined. This mass is converted into energy.
Compare nuclear fission and nuclear fusion. Refer to the fuel, the conditions needed, the waste products and whether each is used to generate electricity today.
Mark scheme
fission: large nuclei split, fusion: small nuclei join (1); fission fuel uranium / plutonium, fusion fuel hydrogen isotopes (1); fission needs neutrons, fusion needs extremely high temperature and pressure (1); fission makes long-lived radioactive waste (1); fusion makes little / short-lived waste (1); fission is used in power stations now, fusion is only experimental (1)
Model answer
In fission a large nucleus such as uranium-235 splits, but in fusion small nuclei such as hydrogen isotopes join. Fission is started by neutrons, whereas fusion needs extremely high temperature and pressure. Fission produces radioactive waste that lasts a long time, while fusion makes very little. Fission is used in power stations today, but fusion is only used experimentally.
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