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Fission and fusion questions
A slow neutron can split the heaviest nuclei, and temperatures of millions of kelvin can fuse the lightest. Both routes move the products towards iron, both leave them lighter than the starting materials, and the mass difference, converted by E = mc², is the energy released.
18 original questions · 50 marks · the fission and fusion notes · Nuclear physics
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Describe what happens in induced nuclear fission.
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A heavy nucleus, such as uranium-235, absorbs a slow (thermal) neutron and becomes unstable (1); it splits into two smaller nuclei, releasing two or three neutrons and a large amount of energy (1).Explain what is meant by a chain reaction in a fissile material.
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Neutrons released by one fission are absorbed by other nuclei and cause further fissions (1); each fission releases more neutrons, so the reaction sustains itself and can grow (1).State what is meant by the critical mass of a fissile material.
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The minimum mass for a self-sustaining chain reaction (1); on average at least one neutron per fission goes on to cause another fission rather than escaping from the material (1).State what is meant by a thermal neutron.
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A slow neutron, with kinetic energy comparable to that of particles in thermal motion, which uranium-235 captures far more readily than a fast neutron (1).State the two number balances that must hold in any nuclear reaction equation.
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The total nucleon (mass) number is the same before and after the reaction (1); the total proton (atomic) number is the same before and after (1).State the form in which most of the energy released in a fission event appears.
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As kinetic energy of the two fission fragments (1).In one fission reaction, 23592U + 10n → 14156Ba + 9236Kr + x 10n. Determine the value of x and show that the proton numbers balance.
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Nucleon numbers: 235 + 1 = 141 + 92 + x (1)
236 = 233 + x, so x = 3 neutrons (1)
Proton numbers: 92 + 0 = 56 + 36 = 92, so they balance (1).A single fission event converts a mass of 3.5 × 10−28 kg into energy. Calculate the energy released in joules and in MeV.
c = 3.0 × 108 m s−1, 1 MeV = 1.60 × 10−13 JMark scheme
E = mc2 = 3.5 × 10−28 × (3.0 × 108)2 (1)
E = 3.15 × 10−11 J (1)
E = 3.15 × 10−11/(1.60 × 10−13) = 197 MeV (1)Explain why very high temperatures are needed for nuclear fusion to occur.
A fusion reaction releases 17.6 MeV of energy. Calculate the energy released in joules and the mass converted into energy.
c = 3.0 × 108 m s−1Explain why the fragments produced by the fission of uranium-235 are radioactive, and state the type of decay they undergo.
In one fusion reaction, two deuterium nuclei form helium-3 and a neutron:
21H + 21H → 32He + 10n
Nuclear masses: 21H 2.01355 u, 32He 3.01493 u, neutron 1.00867 u. Calculate the energy released, in MeV.
1 u = 931.5 MeVA reactor core produces thermal power at a steady 600 MW. Each fission releases about 200 MeV. Calculate the number of fissions occurring each second. Go on to determine the mass converted into energy in one day of operation.
1 MeV = 1.60 × 10−13 J, c = 3.0 × 108 m s−1, 1 day = 86 400 sEach fission of a uranium-235 nucleus releases about 200 MeV. Estimate the energy released by the complete fission of 1.0 kg of uranium-235.
NA = 6.02 × 1023 mol−1, 1 MeV = 1.60 × 10−13 JCompare nuclear fission and nuclear fusion in terms of the fuel used, the conditions required and the products formed.
Use the binding energy per nucleon curve to explain why energy is released in both fission and fusion.
The fusion of deuterium and tritium releases 17.6 MeV from nuclei totalling five nucleons. The fission of uranium-235 releases about 200 MeV from 236 nucleons (the nucleus plus the absorbed neutron). A journalist writes that fission must release more energy per kilogram of fuel, because 200 MeV is far greater than 17.6 MeV. Deduce whether the journalist is correct.
A fixed mass of fissile material can be formed into different shapes. Suggest why a sphere is the shape most likely to sustain a chain reaction.
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