Physics and Technology of Nuclear Reactors

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The following presentation was created by me (Paul Callaghan) in order to demonstrate learning on the Physics and Technology of Nuclear Reactors Course I attended from Autumn 2007 to Spring 2008 at The University of Birmingham.

Text of Physics and Technology of Nuclear Reactors

  • 1. Physics and Technology of Nuclear Reactors Paul Callaghan Consultant Engineer

2. A bit about me

  • (2008 Present)Consultant Engineer Atkins (Glasgow/Epsom/Bristol)
  • (2006-2008)Stress Engineer Rolls-Royce Submarines (Derby)
  • (2005-2006)Planning/Manufacturing Engineer Rolls-Royce Submarines (Derby)
  • (2004)Undergraduate Engineer Rolls-Royce AR&O (East Kilbride)
  • (2003)Undergraduate Engineer Rolls-Royce AR&O (East Kilbride)
  • (2002)Undergraduate Engineer Rolls-Royce AR&O (East Kilbride)
  • (2000 2004) B.Eng (Hons) Aeronautical Engineering (University of Glasgow)

3. Purpose

  • The following presentation was created by me (Paul Callaghan) in order to demonstrate learning on the Physics and Technology of Nuclear Reactors Course I attended from Autumn 2007 to Spring 2008 at The University of Birmingham.
  • I delivered this presentation to a selection of my peers to satisfy the requirements of Further Learning (Engineering and Science Deepening) for the IMechE.
  • The presentation was created in order to demonstrate my understanding of nuclear physics and the physics which underpins the operation of Nuclear Reactors.

4. Contents

  • General Nuclear Physics
  • Fission Processes
  • Transport Theory
  • Point Kinetics Equations
  • Reactor Systems


  • General Nuclear Physics
  • Interactions of neutrons with matter
  • Cross-Sections
  • Resonance Effects
  • U 235Absorption Cross Section vs Energy
  • Scattering
  • Importance of Xenon transients

Learning Outcomes: 6. Interactions of Neutrons with matter (1)

  • The energy released from a nuclear reaction is much higher than from a chemical reaction e.g. burning coal, oil or gas
  • Burning coal releases 4 eV per reaction whereas a (nuclear) fission reaction produces 200 million eV (MeV).

Prompt Energies Daughter nuclei of fission fragments ~169 MeV K.E of (2.5) neutrons ~5 MeV Gamma ray photons ~7 MeV Delayed Energies Beta (from decay) ~6.5 MeV Anti-neutrinos ~8.8 MeV Delayed Gamma Emission ~6.3 MeV 7. Interactions of neutrons with matter (2)

  • Nuclear reactionsinvolve collisions of a nucleus with a particle
  • Neutrons are ideal for use as incident particle as they areelectrically neutral
  • According to theCompound Nucleusmodel - a nuclear reaction occurs in 2 stages:
    • Incident particle absorbed by target nucleus creating a compound nucleus
    • Compound nucleus disintegrates expelling a particle (or photon) leaving a recoil nucleus.
  • Radiative Captureis the process whereby a particle is captured and the excess energy is emitted as radiation

8. Cross-sections (1)

  • Definition:A measure of the probability of occurrence of a particular nuclear reaction under prescribed conditions i.e. the probability of collision
  • MicroscopicCross-Section-
    • Applies to a particular process on a single nucleus
  • Macroscopi c Cross-Section-
    • Is volumetric and is for a collection of nuclei
    • Related toby= N.
      • Where N = Number of nuclei per cm 2
  • Nuclear cross-sections commonly of the order 10 -22to 10 -26cm 2per nucleus
  • Unit of measurement is the barn
    • 1 barn = 10 -24cm 2per nucleus
  • Different types of macroscopic cross-section for different nuclear processes
    • Absorption Cross-Section( a ) - neutrons lost to the system
    • Fission Cross-Section( f ) behaviour of incident particle leads to generation of new particles
    • Scatter Cross-Section( s) transfer of energy from one particle to another

9. Cross-Sections (2) Typical Reactor Material Values Source: The Elements of Nuclear Reactor Theory 2 ndEdition- Glasstone and Edlund Element Total - t (barns) Absorption - a (barns) Scatter - s (barns) H 20-80 0.32 20-80 D 2 0 15.3 0.00092 15.3 B 722 718 3.8 Zr 8.4 0.4 8.0 10. Fast and Slow Neutrons

  • Fast Neutron -a free neutron with a kinetic energy level of about 1 MeV (100 TJ/kg), hence a speed of 14,000 km/s.
  • Slow Neutron - a free neutron with a kinetic energy of about 0.03 eV (2.4 MJ/kg) (1/40) hence a speed of 2.2 km/s
  • Slow neutrons are often referred to asThermal Neutrons astheir energy corresponds to the most probable velocity at a temperature of 290 K/17C (Room Temperature)
  • Thermal neutronshave a different and often much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus

11. Resonance Effects (1)

  • Experimental studies have shown that bombarding different target elements with projectiles of specific energy values causes a sharp increase in reaction rate.
  • For certain energy values the probability that the incident particle will be captured and a compound nucleus formed is exceptionally large.
  • This phenomenon is attributed toresonance.

12. Absorption Cross-Section vs Neutron Energy for U 235(1) 13. Scattering (1)

  • Definition:The process in which the overall result is transfer of energy from one particle to another
  • Two kinds:
    • ElasticScatter Kinetic energy and momentum conserved
    • InelasticScatter Kinetic energy not conserved, momentum conserved.
  • Fast neutrons may be deprived of their kinetic energy and slowed down to become slow neutrons with an energy of ~0.03eV at room temperature.
  • The slowing down is performed by inelastic scatter in a process known as moderation

14. Scattering (2)

  • The medium used in this process is the moderator
    • Typically involves atoms of low mass number e.g. H 2 0 or D 2 0
    • Efficient moderators reduce the speed of fast neutrons in as few collisions as possible
  • After a number of scattering collisions, the kinetic energy of the neutrons is reduced such that it is similar to the moderator medium.
  • The new energy depends on the temperature of the medium and is thethermal energy .
  • Neutrons of this energy arethermal neutrons .
  • The process is thermalisation .

15. Importance of Xenon Transients (1)

  • Xenon-135 is a fission product poison produced during fission of U 235and U 238
  • Xenon-135 is formed from successive beta decays of its fission product precursors
    • 51 Sb 13552 Te 13553 I 13554 Xe 13555 Cs 13556 Ba 135
  • 135 Xe is of particular concern in a reactor as it has a half-life of 9.1 hrs compared with a 6.6 hr half-life of its precursor53 I 135
    • Thus53 I 135decays quicker to54 Xe 135than54 Xe 135can decay
    • Leads to increased concentration of54 Xe 135

16. Importance of Xenon Transients (2)

  • On restart after a shutdown, the Xenon transient becomes important as the reactivity must be greater than the absorbing effect of the Xenon to establish criticality.
  • Increases to reactivity are achieved by withdrawing the control rods
  • If the absorbing effect of Xenon concentration in core is greater than the reactivity that can be achieved by withdrawing the control rods criticality cannot be achieved!


  • Fission Processes
  • Binding energy curve
  • Number of neutrons per fission
  • Prompt and delayed neutrons
  • Delayed neutrons from fission products
  • Fission yield curve
  • Importance of reactor poisons

Learning Outcomes: 18. Binding Energy Curve 19. Number of Neutrons per fission

  • U 235undergoes fission with thermal neutrons as well as those of higher energies.
  • It has been observed that fission of U 235with slow neutrons produces 2.5 0.1 neutrons per fission
  • Not an integer as U nucleus splits in a number of different ways
    • Individually discrete
    • Mean may not be whole number

20. Prompt and Delayed Neutrons (1)

  • Two categories of neutron emitted from fission:promptanddelayed .
  • Promptneutrons
    • released in 10 -14sec
    • account for 99% of total fission neutrons
    • energies cover considerable range c.f Watt Spectrum
  • Delayedneutrons are emitted by one of the fission products anytime from a few milliseconds to a few minutes later

21. Prompt and Delayed Neutrons (2)

  • Delayed neutrons make it possible to run a r