National 5PhysicsSQARevision NotesRadiationNuclear RadiationFission

Fission (SQA National 5 Physics): Revision Note

Exam code: X857 75

Leander Oates

Written by: Leander Oates

Reviewed by: Katie M

Updated on

Fission

  • Nuclear fission is defined as:

The splitting of a large, unstable nucleus into two smaller nuclei

  • There is a lot of energy stored within the nucleus of an atom

  • This energy can be released in a nuclear reaction such as fission or fusion

  • Nuclear fission is used in nuclear power stations to generate electricity

Nuclear fission reactions

  • During a fission reaction:

    • a neutron collides with an unstable nucleus

    • the nucleus splits into two smaller nuclei (called daughter nuclei)

    • and two or three neutrons are released

      • Gamma rays are also emitted

      • Energy is released

Nuclear fission of uranium-235

A neutron is absorbed by a uranium-235 atom which splits into two daughter nuclei and 2 neutrons, IGCSE & GCSE Physics revision notes
Large nuclei can decay by fission to produce smaller nuclei and neutrons with a lot of kinetic energy

  • The products of fission move away very quickly

    • Nuclear energy stored in the parent nucleus is transferred to the kinetic energy of the fission products

    • Energy is also transferred to the surroundings by radiation (gamma rays)

  • The energy released in a nuclear fission reaction far exceeds that released in a chemical reaction in which a similar mass of material reacts

Chain reactions

  • Only one extra neutron is required to induce fission in a uranium-235 nucleus

  • During the fission, it produces two or three neutrons which move away at high speed

  • Each of these new neutrons can start another fission reaction, which again emits further neutrons

    • This process can start a chain reaction

  • A chain reaction occurs when a neutron emitted from the splitting of a nucleus causes further nuclei to split

  • Then the neutrons emitted from these reactions cause further fission reactions, and so on

Diagram of a nuclear chain reaction showing U-235 atom splitting into smaller atoms and releasing neutrons, causing further reactions.
A neutron from a previous fission reaction, causes a new fission reaction which releases other neutrons, which all then go on to cause new fission reactions, and so on. This is a chain reaction.

Examiner Tips and Tricks

Students often lose marks for not showing understanding that the neutrons, released in one fission reaction, go on to initiate another fission reaction in a different nucleus.

Make sure you learn the roles of the neutrons in fission chain reactions.

Worked Example

A nuclear reactor on board a research vessel provides a constant power output of 95 space MW to sustain operations.

Each nuclear fission reaction releases 3.5 cross times 10 to the power of negative 11 end exponent space straight J of usable energy.

Determine the minimum number of fission reactions that must occur in the reactor during one day of continuous operation.

Answer:

Step 1: Determine the time in seconds

  • 1 day = 24 h

  • 1 h = 60 min

  • 1 min = 60 s

t space equals space 1 space cross times space 24 space cross times space 60 space cross times space 60

t space equals space 8.64 cross times 10 to the power of 4 space straight s

Step 2: Calculate the total amount of energy produced

  • Write out the appropriate relationship

P space equals fraction numerator space E over denominator t end fraction

  • Rearrange to make E the subject

E space equals space P t

  • Convert power into SI units

    • mega = 106

95 space MW space equals space 95 cross times 10 to the power of 6 space straight W space equals space 9.5 cross times 10 to the power of 7 space straight W

  • Substitute in the known values to calculate

E subscript t o t a l end subscript space equals space open parentheses 9.5 cross times 10 to the power of 7 close parentheses space cross times space open parentheses 8.64 cross times 10 to the power of 4 close parentheses

E subscript t o t a l end subscript space equals space 8.208 cross times 10 to the power of 12

Step 3: Calculate the minimum number of fission reactions required

  • The number, N, of fission reactions is the total energy divided by the energy released per fission reaction

  • Energy released by each fission reaction, E subscript f i s s i o n end subscript space equals space 3.5 cross times 10 to the power of negative 11 end exponent space straight J space

N space equals space E subscript t o t a l end subscript over E subscript f i s s i o n end subscript

  • Substitute in the known values to calculate

N space equals space fraction numerator 8.208 cross times 10 to the power of 12 over denominator 3.5 cross times 10 to the power of negative 11 end exponent end fraction

N space equals space 2.345 cross times 10 to the power of 23

Step 4: Round to an appropriate number of significant figures

  • The least precise input value was 2 s.f.

  • Therefore, the answer can only be given to the same precision

N space equals space 2.3 cross times 10 to the power of 23 space open parentheses 2 space straight s. straight f. close parentheses

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Physics Content Creator

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Katie M

    Reviewer: Katie M

    Expertise: Physics Content Creator

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.