Controlling a nuclear reaction

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;
• circulating 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 fluidly or thermally coupled to the primary coolant flow path to generate electric power;
• and
• controlling a power output of the nuclear fission reaction independent of any control rod assemblies during the normal operation.

A pressurized water reactor (PWR), comprising:

• a control rod assembly-less reactor module that comprises:
• a reactor vessel comprising a volume sized to enclose a reactor core, a riser, and a steam generator without enclosing a control rod assembly system, and
• a containment vessel comprising a volume sized to enclose the reactor vessel;
• and
• a power generation system comprising a steam conduit in fluid communication with the steam generator, a steam turbine-generator, and a steam condenser.