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Key Characteristics of Prompt Neutrons

Key Characteristics of Prompt Neutrons

  • Prompt neutrons are emitted directly from fission, and they are emitted within a very short time of about 10-14 seconds.
  • Most of the neutrons produced in fission are prompt neutrons – about 99.9%.
  • For example, fission of 235U by thermal neutron yields 2.43 neutrons, of which 2.42 neutrons are prompt neutrons, and 0.01585 neutrons are the delayed neutrons.
  • The production of prompt neutrons slightly increases with incident neutron energy.
  • Almost all prompt fission neutrons have energies between 0.1 MeV and 10 MeV.
  • The mean neutron energy is about 2 MeV. The most probable neutron energy is about 0.7 MeV.
  • In reactor design, the prompt neutron lifetime (PNL) belongs to key neutron-physical characteristics of the reactor core.
  • Its value depends especially on the type of the moderator and the energy of the neutrons causing fission.
  • In an infinite reactor (without escape), prompt neutron lifetime is the sum of the slowing downtime and the diffusion time.
  • In LWRs, the PNL increases with the fuel burnup.
  • The typical prompt neutron lifetime in thermal reactors is on the order of 10-4 seconds.
  • The typical prompt neutron lifetime in fast reactors is on the order of 10-7 seconds.
Table of key prompt and delayed neutrons characteristics
Table of key prompt and delayed neutrons characteristics. Thermal vs. Fast Fission
Neutron Production - Prompt Neutrons
Most of the neutrons produced in fission are prompt neutrons. Usually, more than 99 percent of the fission neutrons are prompt neutrons. Still, the exact fraction is dependent on certain nuclides to be fissioned and is also dependent on an incident neutron energy (usually increases with energy).
Prompt Neutron Energy Spectra - Dependence on incident neutron energy.
Prompt Neutron Energy Spectra – Dependence on incident neutron energy.
Source: Madland, David G., New Fission-Neutron-Spectrum Representation for ENDF, LA-9285-MS, April 1982. http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/14/718/14718824.pdf
Slowing Down and Diffision Times - Prompt Neutron Lifetime

Slowing Down and Diffision Times for Thermal Neutrons in an Infinite Medium

Source: Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.

Dependencies of asymptotic time period on the reactivity required for different reactor types with different prompt neutron lifetimes. Source: http://www.hindawi.com/journals/ijne/2014/373726/
Table of main kinetic parameters.
Table of main kinetic parameters.

Interactive chart – Infinite Multiplying System Without Source and Delayed Neutrons

Press the “clear and run” button and try to stabilize the power at 90%.

Look at the reactivity insertion you need to insert to stabilize the system (of the order to a tenth of pcm).

Do you think that such a system is controlable?

 
References:
Nuclear and Reactor Physics:
  1. J. R. Lamarsh, Introduction to Nuclear Reactor Theory, 2nd ed., Addison-Wesley, Reading, MA (1983).
  2. J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.
  3. W. M. Stacey, Nuclear Reactor Physics, John Wiley & Sons, 2001, ISBN: 0- 471-39127-1.
  4. Glasstone, Sesonske. Nuclear Reactor Engineering: Reactor Systems Engineering, Springer; 4th edition, 1994, ISBN: 978-0412985317
  5. W.S.C. Williams. Nuclear and Particle Physics. Clarendon Press; 1 edition, 1991, ISBN: 978-0198520467
  6. G.R.Keepin. Physics of Nuclear Kinetics. Addison-Wesley Pub. Co; 1st edition, 1965
  7. Robert Reed Burn, Introduction to Nuclear Reactor Operation, 1988.
  8. U.S. Department of Energy, Nuclear Physics and Reactor Theory. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.

Advanced Reactor Physics:

  1. K. O. Ott, W. A. Bezella, Introductory Nuclear Reactor Statics, American Nuclear Society, Revised edition (1989), 1989, ISBN: 0-894-48033-2.
  2. K. O. Ott, R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985, ISBN: 0-894-48029-4.
  3. D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993, ISBN: 0-894-48453-2. 
  4. E. E. Lewis, W. F. Miller, Computational Methods of Neutron Transport, American Nuclear Society, 1993, ISBN: 0-894-48452-4.

See previous:

Description of Prompt Neutrons

See above:

Prompt Neutrons

See next:

Prompt Gamma Rays