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Theme E — Nuclear and Quantum Physics

E.4.1 Spontaneous and Induced Fission — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 32 of 45 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key Gibbs Free Energy FormulaFast Exam Solution
ΔG = ΔH - TΔS | (ΔG < 0: تلقائي Spontaneous, ΔG > 0: غير تلقائي, ΔG = 0: اتزان)
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Lesson summary

E.4.1 Spontaneous and Induced Fission — IB PHYSICS SL notes: Distinguish spontaneous from induced fission and write balanced fission equations.Nuclear fission = the splitting of a large unstable nucleus into two smaller nuclei (fission fragments) with neutrons and a large release…

  • The products move away very quickly
  • Spontaneous fission
  • It is rare
  • It is uncontrolled
  • A slow (thermal) neutron is absorbed by uranium-235
  • Uranium-236 splits almost immediately

Part 1 of 7What fission is

What fission is
Nuclear fission
Nuclear fission the splitting of a large unstable nucleus into two smaller nuclei (fission fragments) with neutrons and a large release of energy.
  • Gamma rays are also emitted.
  • Isotopes of uranium and plutonium are used as fuels in power stations.
It occurs because very heavy nuclei lie on the falling side of the binding energy per nucleon curve.
  • The fragments have mass numbers of about 90 to 140 and are more tightly bound.
  • The increase in binding energy is released, about 200 MeV per fission of uranium-235.
The products move away very quickly
  • The products move away very quickly nuclear potential energy is transferred to their kinetic energy, then carried away as heat.

Part 2 of 7Spontaneous fission

Spontaneous fission
Spontaneous fission fission of a nucleus with no outside trigger.
  • It is random, like radioactive decay, and is described by a half-life.
It is rare
  • It is rare and more likely for heavier nuclei.
  • For uranium-238 the half-life is of order 1016 years.
  • For californium-252 it is a significant mode and the source emits neutrons steadily.
It is uncontrolled
  • It is uncontrolled so it is not useful for power, though the neutrons give portable neutron sources.

Part 3 of 7Induced fission

Induced fission
Induced fission
Induced fission fission triggered by a nucleus absorbing a neutron.
A slow (thermal) neutron is absorbed by uranium-235
  • A slow (thermal) neutron is absorbed by uranium-235 forming an excited, unstable uranium-236 nucleus.
  • A thermal neutron has kinetic energy of about 0.025 eV.
Uranium-236 splits almost immediately
  • Uranium-236 splits almost immediately uranium-235 itself has a very long half-life, so it releases energy only when neutrons are added.
Typical reaction
  • 10n + 23592U gives 14156Ba + 9236Kr + 3 10n + energy.
  • Nucleon number balances236 = 141 + 92 + 3.
  • Proton number balances92 = 56 + 36.
Many fragment pairs are possible
  • Many fragment pairs are possible about 2 to 3 neutrons are released per fission.
    • (mean 2.4 for uranium-235)

Part 4 of 7Writing and checking fission equations

Writing and checking fission equations
Add the nucleon numbers and proton numbers on the left.
Subtract those of the named products from the totals.
The remainder gives the missing particles — each neutron is nucleon number 1 and proton number 0.
1Both nucleon number and charge are conserved
  • Both nucleon number and charge are conserved but total rest mass of the products is less than that of the reactants.
2
The mass difference appears as energy.

Part 5 of 7Fissile fuels

Fissile fuels
Fissile nuclides such as uranium-235, plutonium-239 and uranium-233 are split by thermal neutrons.
Uranium-238 needs fast neutrons of at least about 1 MeV
  • Uranium-238 needs fast neutrons of at least about 1 MeV it does not fission easily with thermal neutrons.
Natural uranium contains about 99.3% uranium-238 and 0.72% uranium-235
  • Natural uranium contains about 99.3% uranium-238 and 0.72% uranium-235 so fuel is enriched to 3–5% uranium-235.
Plutonium-239 is produced in reactors when uranium-238 captures a neutron.
Fission probability for uranium-235 is far higher for slow neutrons than for fast ones.

Part 6 of 7After the split

After the split
Fragments are neutron-rich
  • Fragments are neutron-rich heavy nuclei have a higher neutron-to-proton ratio than stable medium nuclei.
  • They undergo a series of beta-minus decays with gamma emission until stable.
  • This causes the radioactive waste problem.
Neutrons released are fast
  • Neutrons released are fast and must be slowed before they can cause efficient further fission.
Liquid-drop picture
  • Liquid-drop picture surface tension from the strong force holds the nucleus together, while proton repulsion stretches it.
  • After absorbing a neutron the nucleus elongates, and it splits if repulsion wins in the neck.

Part 7 of 7More from the specification

Density
  • Mass per unit volume.
Shielding
  • Material that absorbs radiation.
Moderator
  • Material that slows neutrons.
Specific energy
  • Energy released per unit mass of a fuel, in J kg−1
Daughter nuclei
  • The smaller nuclei produced in fission.
Energy density
  • The energy available per unit mass or area of a resource.
Chain reaction
  • Neutrons from one fission trigger further fissions.
High-level waste
  • Highly radioactive, heat-generating waste such as spent fuel.

Key terms

TermMeaning
FissionSplitting of a heavy nucleus into lighter fragments.
Induced fissionFission caused by absorption of a neutron.
Spontaneous fissionFission occurring randomly with no external trigger.
Thermal neutronSlow neutron with energy about 0.025 eV.
FissileAble to be split by thermal neutrons.
Fission fragmentsThe two medium-mass nuclei produced.
EnrichmentRaising the proportion of uranium-235 in the fuel.

Avoid these mistakes

1
Thinking fission is the same as alpha decay — fission gives two comparable pieces.
2
Saying only an outside neutron can cause fission — spontaneous fission needs no trigger.
3
Leaving the released neutrons out of the nucleon-number balance.
4
Assuming every fission gives the same products — many fragment pairs occur.
5
Believing uranium-238 fissions easily with thermal neutrons — it needs fast neutrons.

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