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 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 could lead to a “blind” start, which is an unsafe condition. 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 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 sources can be of two types:
Primary sources, used for startup of a fresh reactor core; conventional neutron sources are used. The primary sources are removed from the reactor after the first fuel campaign, usually after few months, as neutron capture resulting from the thermal neutron flux in an operating reactor changes the composition of the isotopes used, and thus reduces their useful lifetime as neutron sources.
Californium-252 (spontaneous fission)
Plutonium-238 & beryllium, (α,n) reaction
americium-241 & beryllium, (α,n) reaction
polonium-210 & beryllium, (α,n) reaction
radium-226 & beryllium, (α,n) reaction
Boron-11 & nitrogen-