Installed Neutron Sources

Reactor Theory (Neutron Characteristics) DOE-HDBK-1019/1-93 NEUTRON SOURCES Rev. 0 Page 3 NP-02 There is an abundant supply of high energy gammas in a reactor that has been operated because many of the fission products are gamma emitters. All water-cooled reactors have some deuterium present in the coolant in the reactor core because a small fraction of natural hydrogen is the isotope deuterium. The atom percentage of deuterium in the water ranges from close to the naturally occurring value (0.015%) for light water reactors to above 90% deuterium for heavy water reactors. Therefore, the required conditions for production of photoneutrons exist. The supply of gamma rays decreases with time after shutdown as the gamma emitters decay; therefore, the photoneutron production rate also decreases. In a few particular reactors, additional D O (heavy water) may be added to the reactor to increase the production of 2 photoneutrons following a long shutdown period. Installed Neutron Sources Because intrinsic neutron sources can be relatively weak or dependent upon the recent power history of the reactor, many reactors have artificial sources of neutrons installed. These neutron sources ensure that shutdown neutron levels are high enough to be detected by the nuclear instruments at all times. This provides a true picture of reactor conditions and any change in these conditions. An installed neutron source is an assembly placed in or near the reactor for the sole purpose of producing source neutrons. One strong source of neutrons is the artificial nuclide californium-252, which emits neutrons at the rate of about 2 x 10 neutrons per second per gram as the result of spontaneous fission. 12 Important drawbacks for some applications may be its high cost and its short half-life (2.65 years). Many installed neutron sources use the ( ,n) reaction with beryllium. These sources are composed of a mixture of metallic beryllium (100% beryllium-9) with a small quantity of an alpha particle emitter, such as a compound of radium, polonium, or plutonium. The reaction that occurs is shown below. The beryllium is intimately (homogeneously) mixed with the alpha emitter and is usually enclosed in a stainless steel capsule. Another type of installed neutron source that is widely used is a photoneutron source that employs the ( ,n) reaction with beryllium. Beryllium is used for photoneutron sources because its stable isotope beryllium-9 has a weakly attached last neutron with a binding energy of only 1.66 MeV. Thus, a gamma ray with greater energy than 1.66 MeV can cause neutrons to be ejected by the ( ,n) reaction as shown below.