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Nuclear Fission

Q8.

(a) In the reactor at a nuclear power station, uranium nuclei undergo induced fission with thermal neutrons. Explain what is meant by each of the terms in italics.

Fission occurs when a large nucleus splits into two smaller nuclei. Induced fission means that the split is brought about by bombardment or collision with thermal neutrons. Thermal neutrons have low energies or speeds (< 1 eV)

(3 marks)

(b) A typical fission reaction in the reactor is represented by:

+ + +

(i) Calculate N.

Left hand side of equation: 235 + 1 = 236

Right hand side of equation: 92 + 141 + N = 233 + N

236 = 233 + N

N = 3

(ii) How do the neutrons produced by this reaction differ from the initial neutron that goes into the reaction?

The neutrons produced by the fission have high(er) energies or speeds than the initial neutron that goes into the reaction.

(iii) Calculate the energy released in MeV when one uranium nucleus undergoes fission in this reaction.

Use the following data:

mass of neutron = 1.00867 u

mass of 235U nucleus = 234.99333 u

mass of 92Kr nucleus = 91.90645 u

mass of 141Ba nucleus = 140.88354 u

Δm = 234.99333 + 1.00867 - [(91.90645 + 140 88354) + (3 x 1.00867)]

Δm = 234.99333 - [(91.90645 + 140 88354) + (2 x 1.00867)]

Δm = 0.186 u

energy released = 0.186 x 931.5 = 173 MeV

(5 marks)

(Total 8 marks)