O-LevelPhysicsNuclear Physics5054/21 · Oct/Nov 2023 · Q89 Marks

Uranium-235 (²³⁵₉₂U) is the isotope of uranium that is used as the fuel in a nuclear power station.

(a)The isotope of uranium that is most abundant in nature is uranium-238 (²³⁸₉₂U). State how a neutral atom of uranium-235 differs from a neutral atom of uranium-238.[1]
(b)(i)In the reactor of a nuclear power station, a nucleus of uranium-235 absorbs a slow-moving neutron and undergoes fission. State what happens to the nucleus of an atom during fission.[1]
(b)(ii)Explain how fission can lead to a chain reaction in the reactor.[4]
(b)(iii)State the purpose of the moderator in the reactor.[1]
(b)(iv)Explain the action of the control rods in the reactor.[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[B1] it / uranium-235 has (3) fewer neutrons (in the nucleus)
[B1] the nucleus splits (and releases more neutrons)
[B1] neutrons released (when nucleus splits)
[B1] neutrons absorbed by other (uranium-235) nuclei
[B1] these nuclei split and release neutrons
[B1] this process continues (using released neutrons)
[B1] to slow down the neutrons (emitted by fission)
[B1] they / control rods absorb neutrons (without further fission)
[B1] to slow down / stop / control the chain reaction / limit the number of fissions

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