Controlling a power output of a nuclear reaction without control rods

A 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 generate a nuclear fission reaction;
• a riser positioned above the reactor core;
• 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;
• a primary coolant that circulates through the primary coolant flow path to receive heat from the nuclear fission reaction and release the received heat to generate electric power in a power generation system fluidly or thermally coupled to the primary coolant flow path;
• a chemical injection system in fluid communication with the primary coolant flow path;
• and
• a control system communicably coupled to the chemical injection system, wherein the control system is configured to perform operations to control one or more parameters of the chemical injection system to control a power output of the nuclear fission reaction independent of any control rod assemblies during the normal operation, the operations comprising:
• determining that the power output of the nuclear fission reaction is greater than an upper value or less than a lower value;
• based on the determination, adjusting an amount of a chemical injected into the reactor core from the chemical injection system to adjust the power output of the nuclear fission reaction, wherein the operation of adjusting the amount of the chemical injected into the reactor core from the chemical injection system comprises at least one of:
• increasing the amount of the chemical injected into the reactor core from the chemical injection system to decrease the power output of the nuclear fission reaction;
• or
• decreasing the amount of the chemical injected into the reactor core from the chemical injection system to increase the power output of the nuclear fission reaction;
• and
• subsequent to the adjustment, determining that the power output is within a range between the upper and lower values.

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, wherein the reactor vessel does not include any control rod assemblies therein,
• a riser positioned above the reactor core,
• 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;
• and
• a chemical injection system in fluid communication with the primary coolant flow path;
• 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 by:
• determining that the power output of the nuclear fission reaction is greater than an upper value or less than a lower value;
• based on the determination, adjusting an amount of a chemical injected into the reactor core from the chemical injection system to adjust the power output of the nuclear fission reaction;
• and
• subsequent to the adjustment, determining that the power output is within a range between the upper and lower values.

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, wherein the reactor core comprises one or more nuclear fuel assemblies configured to generate a nuclear fission reaction, and wherein the reactor vessel includes 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;
• a chemical injection system in fluid communication with the primary coolant flow path;
• and
• a containment vessel comprising a volume sized to enclose the reactor vessel and the chemical injection system;
• a power generation system comprising a steam conduit in fluid communication with the steam generator, a steam turbine-generator, and a steam condenser;
• and
• a control system communicably coupled to the power generation system and the chemical injection system, wherein the control system is configured to control a power output of the nuclear fission reaction independent of any control rod assemblies by controlling one or more parameters of at least one of the power generation system or the chemical injection system, and wherein the control system is configured to perform operations to control one or more parameters of the chemical injection system, the operations comprising:
• determining that the power output of the nuclear fission reaction is greater than an upper value or less than a lower value;
• based on the determination, adjusting an amount of a chemical injected into the reactor core from the chemical injection system to adjust the power output of the nuclear fission reaction;
• and
• subsequent to the adjustment, determining that the power output is within a range between the upper and lower values.