Patent Number: 
Section: claims

1. A power module assembly comprising:a reactor vessel that comprises a substantially sealed enclosure;a primary fluid coolant enclosed in the sealed enclosure of the reactor vessel;a reactor core located in a lower portion of the reactor vessel, the reactor core comprising a primary fluid coolant outlet near an upper end of the reactor core and a primary fluid coolant inlet near a lower end of the reactor core;a riser conduit that extends from near the top of the reactor core to an upper portion of the reactor vessel;a heat exchanger located about a portion of the riser conduit in the upper portion of the reactor vessel;a fluid bypass path defined between a lower end portion of the riser conduit and the upper end of the reactor core and substantially enclosed within the sealed enclosure of the reactor vessel between the primary fluid coolant outlet of the reactor core and the heat exchanger, the fluid bypass path hydraulically coupling a fluid coolant path that extends between the primary fluid coolant inlet of the reactor core and the primary fluid coolant outlet of the reactor core with an annulus between the riser conduit and the reactor vessel; anda flow restriction positioned within the fluid bypass path and between the lower end portion of the rise conduit and the upper end portion of the reactor core. 2. The power module assembly according to claim 1, wherein during a loss of coolant accident, the flow of primary coolant out of the upper portion of the riser conduit comprises steam, and wherein a flow of primary coolant through the fluid bypass path comprises a mixture of two-phase coolant. 3. The power module assembly according to claim 1, wherein the fluid bypass path is closed or reduced during a full power operation of the power module assembly. 4. The power module assembly according to claim 3, wherein the fluid bypass path is configured to open during a shut-down operation. 5. The power module assembly according to claim 4, wherein the shutdown operation comprises a loss of coolant accident or an over pressurization event. 6. The power module assembly according to claim 1, wherein a level of the primary coolant is above an outlet of the upper portion of the reactor vessel during full power operation, and wherein the level of primary coolant is below the outlet during a shut-down operation. 7. The power module assembly according to claim 6, wherein the level of the primary coolant remains above the fluid bypass path during the shut-down operation. 8. The power module assembly according to claim 1, wherein the flow restriction comprises a unidirectional valve that defines a flow path only from the fluid coolant path that extends between the primary fluid coolant inlet of the reactor core and the primary fluid coolant outlet of the reactor core to the annulus. 9. The power module assembly according to claim 1, wherein the flow restriction comprises one of:an always-open valve; ora modulating valve that comprises an actuator configured to adjust the modulating valve between an open position and a closed position. 10. A nuclear reactor module comprising:a reactor vessel that comprises a substantially sealed enclosure;a reactor housing mounted inside the reactor vessel, the reactor housing comprising a shroud and a riser located above the shroud within the sealed enclosure of the reactor vessel;a heat exchanger proximately located about the riser;a reactor core located in the shroud;a fluid bypass path defined between a lower end portion of the riser and an upper end portion of the shroud and substantially enclosed within the sealed enclosure of the reactor vessel between a fluid outlet of the shroud and the heat exchanger, the fluid bypass path hydraulically coupling a fluid coolant path that extends between a fluid inlet of the reactor core and the fluid outlet of the shroud with an annulus between the riser and the reactor vessel; anda flow restriction positioned within the fluid bypass path and between the lower end portion of the riser and the upper end portion of the shroud. 11. The nuclear reactor module according to claim 10, wherein an auxiliary flow of primary coolant exits the reactor housing due to a difference in hydrostatic forces in the fluid bypass path between the fluid coolant path that extends between the fluid inlet of the reactor core and the fluid outlet of the shroud and the annulus. 12. The nuclear reactor module according to claim 11, wherein the primary coolant exits the reactor housing as a result of a decrease in rate of the primary flow path of the primary coolant out of the riser. 13. The nuclear reactor module according to claim 10, wherein the fluid bypass path forms a passageway for coolant to exit the reactor housing during a loss of coolant accident or a depressurization event. 14. The nuclear reactor module according to claim 10, wherein the shroud comprises a nozzle-shaped member, the fluid outlet of the shroud being smaller than a fluid inlet of the shroud. 15. The nuclear reactor module according to claim 14, wherein the fluid inlet of the shroud is substantially the same size as a fluid outlet of the reactor core. 16. The nuclear reactor module according to claim 10, wherein the flow restriction comprises a unidirectional valve that defines a flow path only from the fluid coolant path that extends between the fluid inlet of the reactor core and the fluid outlet of the shroud to the annulus. 17. The nuclear reactor module according to claim 10, wherein the flow restriction comprises one of:an always-open valve; ora modulating valve that comprises an actuator configured to adjust the modulating valve between an open position and a closed position. 18. A nuclear reactor module comprising:a reactor vessel that comprises a substantially sealed enclosure;a reactor housing mounted inside the reactor vessel, the reactor housing comprising a shroud and a riser located above the shroud within the sealed enclosure of the reactor vessel;a heat exchanger proximately located about the riser;a reactor core located in the shroud; anda fluid bypass path defined between a lower end portion of the riser and an upper end portion of the shroud and substantially enclosed within the sealed enclosure of the reactor vessel between a fluid outlet of the shroud and the heat exchanger, the fluid bypass path hydraulically coupling a fluid coolant path that extends between a fluid inlet of the reactor core and the fluid outlet of the shroud with an annulus between the riser and the reactor vessel,wherein an auxiliary flow of primary coolant exits the reactor housing due to a difference in hydrostatic forces in the fluid bypass path between the fluid coolant path that extends between the fluid inlet of the reactor core and the fluid outlet of the shroud and the annulus. 19. The nuclear reactor module according to claim 18, wherein the shroud comprises a nozzle-shaped member, the fluid outlet of the shroud being smaller than a fluid inlet of the shroud, and the fluid inlet of the shroud is substantially the same size as a fluid outlet of the reactor core. 20. The nuclear reactor module according to claim 18, further comprising a flow restriction positioned within the fluid bypass path and between the lower end portion of the riser and the upper end portion of the shroud, the flow restriction comprising at least one of:a unidirectional valve that defines a flow path only from the fluid coolant path that extends between the fluid inlet of the reactor core and the fluid outlet of the shroud to the annulus;an always-open valve; ora modulating valve that comprises an actuator configured to adjust the modulating valve between an open position and a closed position. 21. A power module assembly comprising:a reactor vessel that comprises a substantially sealed enclosure;a primary fluid coolant enclosed in the sealed enclosure of the reactor vessel;a reactor core located in a lower portion of the reactor vessel, the reactor core comprising a primary fluid coolant outlet near an upper end of the reactor core and a primary fluid coolant inlet near a lower end of the reactor core;a riser conduit that extends from near the top of the reactor core to an upper portion of the reactor vessel;a heat exchanger located about a portion of the riser conduit in the upper portion of the reactor vessel; anda fluid bypass path defined between a lower end portion of the riser conduit and the upper end of the reactor core and substantially enclosed within the sealed enclosure of the reactor vessel between the primary fluid coolant outlet of the reactor core and the heat exchanger, the fluid bypass path hydraulically coupling a fluid coolant path that extends between the primary fluid coolant inlet of the reactor core and the primary fluid coolant outlet of the reactor core with an annulus between the riser conduit and the reactor vessel,wherein during a loss of coolant accident, the flow of primary coolant out of the upper portion of the riser conduit comprises steam, and wherein a flow of primary coolant through the fluid bypass path comprises a mixture of two-phase coolant. 22. A power module assembly comprising:a reactor vessel that comprises a substantially sealed enclosure;a primary fluid coolant enclosed in the sealed enclosure of the reactor vessel;a reactor core located in a lower portion of the reactor vessel, the reactor core comprising a primary fluid coolant outlet near an upper end of the reactor core and a primary fluid coolant inlet near a lower end of the reactor core;a riser conduit that extends from near the top of the reactor core to an upper portion of the reactor vessel;a heat exchanger located about a portion of the riser conduit in the upper portion of the reactor vessel; anda fluid bypass path defined between a lower end portion of the riser conduit and the upper end of the reactor core and substantially enclosed within the sealed enclosure of the reactor vessel between the primary fluid coolant outlet of the reactor core and the heat exchanger, the fluid bypass path hydraulically coupling a fluid coolant path that extends between the primary fluid coolant inlet of the reactor core and the primary fluid coolant outlet of the reactor core with an annulus between the riser conduit and the reactor vessel,wherein the fluid bypass path is closed or reduced during a full power operation of the power module assembly. 23. The power module assembly according to claim 22, wherein the fluid bypass path is configured to open during a shut-down operation, and the shutdown operation comprises a loss of coolant accident or an over pressurization event. 24. A power module assembly comprising:a reactor vessel that comprises a substantially sealed enclosure;a primary fluid coolant enclosed in the sealed enclosure of the reactor vessel;a reactor core located in a lower portion of the reactor vessel, the reactor core comprising a primary fluid coolant outlet near an upper end of the reactor core and a primary fluid coolant inlet near a lower end of the reactor core;a riser conduit that extends from near the top of the reactor core to an upper portion of the reactor vessel;a heat exchanger located about a portion of the riser conduit in the upper portion of the reactor vessel; anda fluid bypass path defined between a lower end portion of the riser conduit and the upper end of the reactor core and substantially enclosed within the sealed enclosure of the reactor vessel between the primary fluid coolant outlet of the reactor core and the heat exchanger, the fluid bypass path hydraulically coupling a fluid coolant path that extends between the primary fluid coolant inlet of the reactor core and the primary fluid coolant outlet of the reactor core with an annulus between the riser conduit and the reactor vessel,wherein a level of the primary coolant is above an outlet of the upper portion of the reactor vessel during full power operation, and wherein the level of primary coolant is below the outlet during a shut-down operation. 25. The power module assembly according to claim 24, wherein the level of the primary coolant remains above the fluid bypass path during the shut-down operation.