NEUTRON SOURCES
DOE-HDBK-1019/1-93
Reactor Theory (Neutron Characteristics)
NP-02
Page 2
Rev. 0
A limited number of neutrons will always be present, even in a reactor core that has never been
operated, due to spontaneous fission of some heavy nuclides that are present in the fuel. Uranium-
238, uranium-235, and plutonium-239 undergo spontaneous fission to a limited extent. Uranium-
238, for example, yields almost 60 neutrons per hour per gram. Table 1 illustrates a comparison
of the rate at which different heavy nuclides produce neutrons by spontaneous fission.
Californium-252 is not an intrinsic neutron source, but will be discussed in the section on installed
neutron sources.
TABLE 1
Neutron Production by Spontaneous Fission
Nuclide
T (Fission)
T ( -decay)
neutrons/sec/gram
1/2
1/2
U
1.8 x 10
years
6.8 x 10 years
8.0 x 10
235
92
17
8
-4
U
8.0 x 10
years
4.5 x 10 years
1.6 x 10
238
92
15
9
-2
Pu
5.5 x 10 years
2.4 x 10 years
3.0 x 10
239
94
5
4
-2
Pu
1.2 x 10
years
6.6 x 10 years
1.0 x 10
240
94
11
3
3
Cf
66.0 years
2.65 years
2.3 x 10
252
98
12
Another intrinsic neutron source is a reaction involving natural boron and fuel. In some reactors,
natural boron is loaded into the reactor core as a neutron absorber to improve reactor control or
increase core life-time. Boron-11 (80.1% of natural boron) undergoes a reaction with the alpha
particle emitted by the radioactive decay of heavy nuclides in the fuel to yield a neutron as shown
below.
The boron-11 must be mixed with, or in very close proximity to, the fuel for this reaction because
of the short path length of the alpha particle. For a reactor core with this configuration, this ( ,n)
reaction is an important source of neutrons for reactor startup.
In a reactor that has been operated, another source of neutrons becomes significant. Neutrons may
be produced by the interaction of a gamma ray and a deuterium nucleus. This reaction is commonly
referred to as a photoneutron reaction because it is initiated by electromagnetic radiation and results
in the production of a neutron. The photoneutron reaction is shown below.
