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.
)[1] The sources are positioned so that the neutron flux they produce is always detectable by the reactor monitoring instruments.
[2] The equilibrium level of neutron flux in a subcritical reactor is dependent on the neutron source strength; a certain minimum level of source activity has to be ensured in order to maintain control over the reactor when in strongly subcritical state, namely during startups.
Similarly, in modern reactors (after startup), delayed neutron emission from fission products suffices to sustain the amplification reaction while yielding controllable growth times.