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Nuclear Fission

Q3. A rod made from uranium-238 () is placed in the core of a nuclear reactor where it absorbs free neutrons. When a nucleus of uranium-238 absorbs a neutron it becomes unstable and decays to neptunium-239 (), which in turn decays to plutonium-239 ().

(a) Write down the nuclear equation that represents the decay of neptunium-239 into plutonium-239.

[2 marks]

(b) A sample of the rod is removed from the core and its radiation is monitored from time t = 0 s. The variation of the activity with time is shown below:

 

(i) Show that the decay constant of the sample is about 3.4 × 10–6 s–1.

[2 marks]

(ii) Assume that the activity shown in the graph above comes only from the decay of neptunium.

Estimate the number of neptunium nuclei present in the sample at time t = 5.0 × 105 s.

[1 mark]

(c)

(i) A chain reaction is maintained in the core of a thermal nuclear reactor that is operating normally.

Explain what is meant by a chain reaction, naming the materials and particles involved.

[2 marks]

(ii) Explain the purpose of a moderator in a thermal nuclear reactor.

[2 marks]

(iii) Substantial shielding around the core protects nearby workers from the most hazardous radiations. Radiation from the core includes α and β particles, γ rays, X–rays, neutrons and neutrinos.

Explain why the shielding becomes radioactive.

[2 marks]

(Total 11 marks)