Nuclear fission and fusion – IGCSE Physics exam questions with mark scheme

1Grade 6–77 marks

In a nuclear reactor, uranium-235 nuclei undergo fission.

(a) Describe what happens in a fission event of a uranium-235 nucleus. [5]

(b) State the purpose of the moderator in the reactor. [1]

(a) a (slow-moving / thermal) neutron is absorbed by the U-235 nucleus; allow a neutron collides with / hits the nucleus(1)
(a) any one further correct detail: the nucleus becomes unstable / forms uranium-236; OR the daughter nuclei (fission products) are radioactive; OR the released neutrons can be absorbed by / split other U-235 nuclei, giving a chain reaction(1)
(a) the nucleus splits into two (smaller) daughter nuclei; accept named examples e.g. barium and krypton(1)
(a) (two or three) neutrons are released; accept 'a small number of / a few / more neutrons released'(1)
(a) energy is released (as kinetic energy of the products / daughter nuclei and neutrons); allow heat / thermal energy released; allow gamma radiation emitted(1)
(b) slows down neutrons (so they are more likely to be absorbed by U-235 nuclei / cause further fission); reject 'absorbs neutrons'(1)
(c) absorb neutrons (to control / reduce the rate of fission / chain reaction); reject 'slow down neutrons'; ignore 'absorb radiation' unless neutrons stated(1)
Answer: (a) neutron absorbed, U-236 unstable, splits into two daughter nuclei, 2–3 neutrons and energy released (b) slows neutrons (c) absorb neutrons
Examiner tip: The moderator slows neutrons, it does not absorb them: confusing it with the control rods loses the mark.

2Grade 6–75 marks

The Sun releases energy by nuclear fusion.

(a) State what is meant by nuclear fusion. [1]

(b) Explain why fusion only happens at very high temperatures and pressures. [3]

(a) (two) light / small nuclei join to form a heavier / larger nucleus (releasing energy)(1)
(b) nuclei are (positively charged so they) repel each other(1)
(b) high temperature gives nuclei enough kinetic energy / speed to overcome the (electrostatic) repulsion and get close enough to fuse(1)
(b) high pressure / density makes collisions between nuclei frequent enough(1)
(c) very difficult to achieve / contain the high temperatures and pressures needed; accept more energy put in than is obtained(1)
Answer: (a) light nuclei join to form a heavier nucleus (b) to overcome electrostatic repulsion and make collisions frequent (c) extreme conditions hard to contain
Examiner tip: Fusion is about nuclei, not atoms: 'atoms join together' does not get the definition mark.

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