Primary and Secondary Source of Neutrons
External Source of Neutrons. Sometimes source neutrons must be artificially added to the system. External source of neutrons contains a material, that emits neutrons and cladding to provide a barrier between reactor coolant and the material. External sources are usually loaded directly into the reactor core (e.g. into guide thimble tubes).
- Primary Source of Neutrons. Primary source of neutrons does not need to be irradiated to produce neutrons. These sources can be used especially in case of a first core (i.e. a core that consist of fresh fuel only). Primary source of neutrons are based on the spontaneous fission reaction. The most commonly used spontaneous fission source is the radioactive isotope californium-252. Cf-252 and all 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 be also 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. Secondary source of neutrons must be irradiated to produce neutrons. These sources usually contains 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 to knock 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-berylium source produces nearly monoenergetic neutrons with 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 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 serves as a source of strong gamma rays, while the second component 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 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 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 which complicates the work with such sources.
- Yield of neutrons is significantly lower than for (α,n) sources. On the other hand (α,n) sources contains transuranic elements as americium which can be converted into fissile isotope 242mAm. These source are not appropriate for commercial reactors.
- When loaded into a reactor, Sb-Be source may contribute to production of tritium. Most important source (due to releases of tritiated water) of tritium in nuclear power plants stems from the boric acid, but it can be also produced from beryllium via following sequence of reactions:
- 6He →6Li + e– (β–; T1/2=0.8s)
With source neutrons, the population remains at levels that can be measured by the source range 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 take place (even a deep subcritical reactor will always be producing 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.