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Primary and Secondary Source of Neutrons

External Source of Neutrons. Sometimes source neutrons must be artificially added to the system. An external source of neutrons contains a material that emits neutrons and cladding to provide a barrier between the reactor coolant and the material. External sources are usually loaded directly into the reactor core (e.g., guide thimble tubes).

  • Primary Source of Neutrons. The primary source of neutrons does not need to be irradiated to produce neutrons. These sources can be used especially in the case of a first core (i.e., a core consisting of fresh fuel only). The primary source of neutrons is based on the spontaneous fission reaction. The most commonly used spontaneous fission source is the radioactive isotope californium-252. Cf-252 and other spontaneous fission neutron sources are produced by irradiating uranium or another transuranic element in a nuclear reactor, where neutrons are absorbed in the starting material and its subsequent reaction products, transmuting the starting material into the SF isotope. (α,n) reactions can also be used to produce neutrons. The bombardment of beryllium by α-particles leads to the production of neutrons by the following exothermic reaction: 4He + 9Be→12C + n + 5.7 MeV. This reaction yields a weak source of neutrons with an energy spectrum resembling that from a fission source and is used nowadays in portable neutron sources. Radium, plutonium, or americium can be used as an α-emitter.
  • Secondary Source of Neutrons. The secondary source of neutrons must be irradiated to produce neutrons. These sources usually contain two components. The second component is a neutron emitter element, for example, 9Be, which has the lowest threshold (1.666 MeV) for neutron emission. The first component is a material, which can be irradiated by fission neutrons (e.g., antimony) and then emits particle capable of knocking out a neutron from the second component. The (γ,n) source that uses antimony-124 as the gamma emitter is characterized in the following endothermic reaction. The antimony-beryllium source produces nearly monoenergetic neutrons with the dominant peak at 24keV.

124Sb→124Te + β− + γ

γ + 9Be→8Be + n – 1.66 MeV

Geometry of Source Neutrons Assemblies

Since at PWRs the source range neutron detectors are usually placed outside the reactor (excore). A source neutrons assemblies should be placed at least a few migration lengths from core periphery. The main reason is that source range detectors should not register primarily the source neutrons.

Sb-Be Source – Antimony-Beryllium Source

Sb-Be neutron source is a typical external neutron source used in commercial nuclear reactors. It is usually loaded into fuel assemblies near the periphery of the core because the source-range excore detectors are installed outside the pressure vessel. Sb-Be source is a two-component source and, prior to irradiation, contains natural antimony (57.4% of 121Sb and 42.6% of 123Sb) and natural beryllium (100% of 9Be). The first component is a source of strong gamma rays, while the second is a neutron emitter element.

Thus the Sb-Be source is based on (γ,n) reaction (i.e., it emits photoneutrons). The neutron flux inside the core activates (σa = 0.02 barn) the isotope 123Sb resulting in 124Sb. 124Sb decays (with a half-life of 60.2 days) via beta decay to 124Te. The decay scheme of 124Sb shows two relevant groups of γ-ray energies, namely 1691 keV and 2091 keV, with absolute intensities of 0.484 and 0.057 per decay, respectively.

These γ-rays have sufficient energy to knock out a neutron from the second component, 9Be, which has the lowest threshold (1.66 MeV) for neutron emission. The (γ,n) source that uses antimony-124 as the gamma emitter is characterized in the following endothermic reaction.

124Sb→124Te + β− + γ

γ + 9Be→8Be + n – 1.66 MeV

The antimony-beryllium source produces nearly monoenergetic neutrons with the dominant peak at 23keV. Using the laws of energy and momentum conservation, one can derive that the 1691 keV and 2091 keV gamma rays produce two groups of neutrons:

  • 23 keV (~97%)
  • 378 keV  (~3%)

Sb-Be sources have three main disadvantages:

  • They have an extremely high photon to neutron ratio, complicating work with such sources.
  • The yield of neutrons is significantly lower than for (α,n) sources. On the other hand, (α,n) sources contain transuranic elements such as americium which can be converted into fissile isotope 242mAm. These sources are not appropriate for commercial reactors.
  • When loaded into a reactor, the Sb-Be source may contribute to the production of tritium. The most important source (due to releases of tritiated water) of tritium in nuclear power plants stems from the boric acid, but it can also be produced from beryllium via the following sequence of reactions:
    • 9Be(n,α)6He
    • 6He →6Li + e–   (β–;   T1/2=0.8s)
    • 6Li(n,α)3H
Source neutrons play an important role in reactor safety, especially during shutdown state and reactor startup. Without source neutrons, there would be no subcritical multiplication, and the neutron population in the subcritical system would gradually approach zero. That means each neutron generation would have fewer neutrons than the previous one because keff is less than 1.0.

With source neutrons, the population remains at levels that can be measured by the source range of excore neutron detectors so that operators can always monitor how fast the neutron population is changing (can always monitor the reactivity of subcritical reactor). Note that if neutrons and fissionable material are present in the subcritical reactor, fission will occur (even a deep subcritical reactor will always produce a small number of fissions).

The source neutrons enter the life cycle and experience the same environment that fission neutrons experience. It must be noted, source neutrons are produced at different energies (e.g., 24keV for Sb-Be source), usually below energies of fission neutrons.

 
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 above:

Subcritical Multiplication