Controlling a power output of a nuclear reactor without using control rods

A method for controlling a nuclear fission reaction, comprising:

• operating a nuclear power system to initiate a nuclear fission reaction, the nuclear power system comprising:
• a reactor vessel that comprises a reactor core mounted within a volume of the reactor vessel, the reactor core comprising one or more nuclear fuel assemblies configured to initiate and maintain the nuclear fission reaction during a normal operation,
• a riser positioned above the reactor core, and
• a primary coolant flow path that extends from a bottom portion of the volume below the reactor core, through the reactor core, within the riser, and through an annulus between the riser and the reactor vessel back to the bottom portion of the volume;
• passively circulating without pumping and during a normal operating state of the nuclear power system, a primary coolant through the primary coolant flow path to receive heat from the nuclear fission reaction;
• transferring the received heat into a power generation system that is fluidly, thermally, or both fluidly and thermally coupled to the primary coolant flow path to generate electric power;
• and
• operating a control rod assembly system positioned in the reactor vessel to adjust a position of a plurality of control rods of the control rod assembly system from a first discrete stationary position of only two discrete stationary positions to a second discrete stationary position of the only two discrete stationary positions, such that the plurality of control rods are fully withdrawn from the reactor core in the first discrete stationary position and the plurality of control rods are fully inserted into the reactor core in the second discrete stationary position,
• wherein the nuclear power system does not include any other control rods positioned within the reactor vessel and configured to control a power output of the nuclear fission reaction.