Startup neutron source

Startup neutron source is a neutron source used for stable and reliable initiation of nuclear chain reaction in nuclear reactors, when they are loaded with fresh nuclear fuel, whose neutron flux from spontaneous fission is insufficient for a reliable startup, or after prolonged shutdown periods. Neutron sources ensure a constant minimal population of neutrons in the reactor core, sufficient for a smooth startup.

The startup sources are typically inserted in regularly spaced positions inside the reactor core, in place of some of the fuel rods.

The sources are important for safe reactor startup. The spontaneous fission and cosmic rays serve as weak neutron sources, but these are too weak for the reactor instrumentation to detect; relying on them would lead to a "blind" start, with chance of going supercritical. The sources are therefore positioned so the neutron flux they produce is always detectable by the reactor monitoring instruments.

The sources can be of two types:

• Primary sources, used for startup of a fresh reactor core; conventional neutron sources are used, usually californium-252 (spontaneous fission) or americium-beryllium (α,n nuclear reaction). The primary sources are removed from the reactor after the first fuel campaign, usually after few months.
• Secondary sources, originally inert, become radioactive and neutron-producing only after neutron activation in the reactor
  • Sb-Be photoneutron source; antimony becomes radioactive in the reactor and its strong gamma emissions (1.7 MeV for ¹²⁴Sb) interact with beryllium-9 by an (γ,n) reaction and provide photoneutrons. In a PWR reactor one neutron source rod contains 160 grams of antimony, and stay in the reactor for 5-7 years.
  • Pb-Be source

Some neutron sources also serve as additional sources of delayed neutrons; these serve to dampen the response rate of reactor power to control rods and power transients, allowing safer and more stable operation.