Startup neutron source

RBMK reactor control rod positions at the moment of the Chernobyl disaster; blue=startup neutron sources (12), yellow=shortened control rods from the reactor bottom (32), grey=pressure tubes (1661), green=control rods (167), red=automatic control rods (12)

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. Without them, the reactor could suffer fast power excursions during startup from state with too few self-generated neutrons (new core or after extended shutdown). The equilibrium level of neutron flux in a subcritical reactor is dependent on the neutron source strength; a certain minimum level of source activity therefore has to be ensured in order to maintain control over the reactor when in strongly subcritical state, namely during startups.

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. When the reactor is in shutdown state, the neutron sources serve to provide signals for neutron detectors monitoring the reactor, to ensure they are operable.

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 plutonium-238-beryllium or americium-beryllium (α,n nuclear reactions). The primary sources are removed from the reactor after the first fuel campaign, usually after few months. Primary sources are subject to neutron capture; exposition to thermal neutron flux in an operating reactor reduces their lifetime.
• Secondary sources, originally inert, become radioactive and neutron-producing only after neutron activation in the reactor. Due to this, they tend to be less expensive. The exposition to thermal neutrons also serves to maintain the source activity (the radioactive isotopes are both burned and generated in neutron flux).
  • 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. The sources are often constructed as an antimony rod surrounded by beryllium layer and clad in stainless steel. Antimony-beryllium alloy can be also used.
  • Pb-Be source

Boron-11 can be added to the fuel; it emits neutrons by the (α,n) reaction to nitrogen-14. Deuterium in heavy water emits neutrons by (γ,n) reaction to ¹H.

A plutonium-238/beryllium primary source can be utilized. As Pu-238 would undergo neutron capture and transmutation when subjected to intense thermal neutron flux, which would dramatically shorten the source lifetime, the sources can be either affixed to some control rod and removed from the reactor when it is powered, or clad in a cadmium alloy, which is opaque for the thermal neutrons but transparent for the fast neutrons produced by the 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.