Patent Number: 
Section: claims

1. 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; anda control rod assembly system positioned in the reactor vessel and configured to position a plurality of control rods in only two discrete stationary positions, such that the plurality of control rods are fully withdrawn from the reactor core in a first discrete position of the only two discrete stationary positions and the plurality of control rods are fully inserted into the reactor core in a second discrete position of the only two discrete stationary positions, and 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. 2. The nuclear power system of claim 1, wherein the control rod assembly is configured to adjust the plurality of control rods from the first discrete position to the second discrete position by at least one of:releasing the plurality of control rods to fall to the second discrete position from the first discrete position; orforcibly urging the plurality of control rods from the first discrete position to the second discrete position. 3. The nuclear power system of claim 1, wherein the plurality of control rods are sufficient to shut down the nuclear fission reaction or maintain the nuclear fission reaction at a sub-critical state in the second discrete position. 4. The nuclear power system of claim 1, further comprising a control system communicably coupled to the power generation system and configured to control a power output of the nuclear fission reaction independent of the control rod assembly system during a normal operation of the nuclear power system. 5. The nuclear power system of claim 4, wherein the control system is configured to perform operations to control one or more parameters of the power generation system 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, controlling the power generation system to adjust at least one of a turbine inlet steam valve or a feed water pump to adjust the power output of the nuclear fission reaction; andsubsequent to the adjustment, determining that the power output is within a range between the upper and lower values. 6. The nuclear power system of claim 5, wherein the operation of controlling the power generation system to adjust at least one of the turbine inlet steam valve or the feed water pump to adjust the power output of the nuclear fission reaction comprises at least one of:adjusting the turbine inlet steam valve toward a fully closed position to decrease the power output of the nuclear fission reaction, or adjusting the turbine inlet steam valve toward a fully open position to increase the power output of the nuclear fission reaction; ordecreasing an output flowrate of the feed water pump to decrease the power output of the nuclear fission reaction, or increasing the output flowrate of the feed water pump to increase the power output of the nuclear fission reaction. 7. The nuclear power system of claim 4, wherein the control system is configured to perform operations to control one or more parameters of a chemical injection system 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; andsubsequent to the adjustment, determining that the power output is within a range between the upper and lower values. 8. The nuclear power system of claim 7, 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; ordecreasing 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.