Control system for a small fission reactor

This patent describes a system for controlling the reactivity of a fission reactor including a reactor core containing a liquid coolant in which fission reactions occur producing energetic neutrons which initiate and sustain the fission reactions. The system consists of: 1.) neutron reflecting means positioned around the reactor core in spaced relation so as to define a space for reflecting energetic neutrons which escape from the reactor core back into the reactor core for sustaining the fission reactions; 2.) a variable length control system positioned within the space between the reactor core and the neutron reflector for absorbing neutrons which escape from the reactor core. The control system includes at least one flexible, helically coiled control element positioned around the periphery of the reactor core; and 3.) displacement coupled to the control system for varying the length of the control system positioned between the reactor core and the neutron reflector in regulating the number of energetic neutrons absorbed and controlling the reactivity of the fission reactor