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
• operating a control rod assembly system positioned in the reactor vessel to adjust a position of a plurality of control rods from a first discrete position of only two discrete positions to a second discrete position of the only two discrete positions, such that the plurality of control rods are fully withdrawn from the reactor core in the first discrete position and the plurality of control rods are fully inserted into the reactor core in the second discrete position.

A pressurized water reactor (PWR), comprising:

• a reactor module that comprises:
• a reactor vessel comprising a volume sized to enclose a reactor core, a riser, and a steam generator, and
• a containment vessel comprising a volume sized to enclose the reactor vessel;
• and
• a plurality of control rods mounted in the reactor vessel above the reactor core on a control rod manifold, the control rod manifold attached to a control rod actuator operable to release the control rod manifold to drop the plurality of control rods from a first position above the reactor core to a second position within the reactor core.