Nuclear Fission

00:00 9 marks

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Uranium-235 (235U) can undergo induced fission when it absorbs a slow neutron. One possible fission reaction is:
23592U + 10n → 14156Ba + 9236Kr + *x* 10n

Part a

1 minutes 1 marks

Question

Determine the number of neutrons, *x*, released in this reaction. [1]



Hint Hint

Hint

Apply the principle of conservation of nucleon number (mass number). The sum of the superscripts on the left must equal the sum of the superscripts on the right.

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Complete Answer

x = 3

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Part b

4 minutes 3 marks

Question

The following data are available:\nBinding energy per nucleon for 235U = 7.59 MeV\nBinding energy per nucleon for 141Ba = 8.31 MeV\nBinding energy per nucleon for 92Kr = 8.51 MeV\nCalculate the energy released in this fission reaction. [3]



Hint Hint

Hint

Energy released = (Total binding energy of products) – (Total binding energy of reactants). Calculate the total BE for each nucleus by multiplying BE per nucleon by the mass number (A). Remember the incident neutron has zero binding energy.

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Complete Answer

Energy released ≈ 171 MeV

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Part c

3 minutes 3 marks

Question

Explain the role of the moderator and the control rods in a nuclear fission reactor. [3]



Hint Hint

Hint

Consider what kind of neutrons are best for causing fission in U-235 and how the chain reaction is kept under control.

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Moderator: Slows down fast fission neutrons to thermal energies, increasing the probability of inducing further fission in 235U.
Control Rods: Absorb excess neutrons to control the rate of the chain reaction and prevent it from becoming supercritical.

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Part d

3 minutes 2 marks

Question

Outline why the fission fragments (Barium and Krypton) are likely to be radioactive. [2]



Hint Hint

Hint

Compare the neutron-to-proton ratio (N/Z) of the heavy U-235 nucleus with that of stable nuclei in the mass range of Ba and Kr. How do unstable nuclei achieve stability?

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Complete Answer

Fission fragments like Ba and Kr inherit a high neutron-to-proton ratio from the heavy parent nucleus (235U). This ratio is higher than that of stable nuclei with similar mass numbers. To achieve stability, these neutron-rich fragments undergo beta-minus (β⁻) decay, where a neutron converts into a proton, emitting an electron and an antineutrino.

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