Reactor Theory (Neutron Characteristics)
Neutron Sources Summary
Intrinsic neutron sources are sources of neutrons from materials that are in
the reactor for other purposes such as fuel, burnable poison, or moderator.
Installed neutron sources are materials or components placed in the reactor
specifically for the purpose of producing source neutrons.
Examples of intrinsic neutron sources are listed below.
Spontaneous fission of heavy nuclides in fuel, such as uranium-238,
uranium-235, and plutonium-239, results in fission fragments and
Boron-11 mixed with the fuel undergoes an alpha-neutron reaction
and becomes nitrogen-14.
Deuterium present in the reactor coolant undergoes a gamma-
neutron reaction and becomes hydrogen-1.
Examples of installed neutron sources are listed below.
Spontaneous fission of californium-252 results in fission fragments
and free neutrons.
Beryllium-9 undergoes an alpha-neutron reaction (alpha from the
decay of plutonium, polonium, or radium) and becomes carbon-12.
Beryllium-9 undergoes a gamma-neutron reaction (high energy
gamma from decay of antimony-124) and becomes beryllium-8.
Many startup sources of this type use antimony and beryllium because after activation with
neutrons the radioactive antimony becomes an emitter of high energy gammas. The photoneutron
sources of this type are constructed somewhat differently from the ( ,n) types. One design
incorporates a capsule of irradiated antimony enclosed in a beryllium sleeve. The entire assembly
is then encased in a stainless steel cladding. A large reactor may have several neutron sources of
this type installed within the core.
The important information in this chapter is summarized below.