NEUTRON SOURCESDOE-HDBK-1019/1-93Reactor Theory (Neutron Characteristics)NP-02Page 2Rev. 0A limited number of neutrons will always be present, even in a reactor core that has never beenoperated, 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 comparisonof 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 installedneutron sources.TABLE 1Neutron Production by Spontaneous FissionNuclideT (Fission)T (-decay)neutrons/sec/gram1/2 1/2U1.8 x 10years6.8 x 10 years8.0 x 102359217 8 -4U8.0 x 10years4.5 x 10 years1.6 x 102389215 9 -2Pu5.5 x 10 years2.4 x 10 years3.0 x 10239945 4 -2Pu1.2 x 10years6.6 x 10 years1.0 x 102409411 3 3Cf66.0 years2.65 years2.3 x 102529812Another 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 orincrease core life-time. Boron-11 (80.1% of natural boron) undergoes a reaction with the alphaparticle emitted by the radioactive decay of heavy nuclides in the fuel to yield a neutron as shownbelow.The boron-11 must be mixed with, or in very close proximity to, the fuel for this reaction becauseof 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 maybe produced by the interaction of a gamma ray and a deuterium nucleus. This reaction is commonlyreferred to as a photoneutron reaction because it is initiated by electromagnetic radiation and resultsin the production of a neutron. The photoneutron reaction is shown below.
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