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A nuclear reactor module includes a reactor vessel and a reactor housing mounted inside the reactor vessel, wherein the reactor housing comprises a shroud and a riser located above the shroud. The nuclear reactor module further includes a heat exchanger proximately located about the riser, and a rea

A nuclear reactor module includes a reactor vessel and a reactor housing mounted inside the reactor vessel, wherein the reactor housing comprises a shroud and a riser located above the shroud. The nuclear reactor module further includes a heat exchanger proximately located about the riser, and a reactor core located in the shroud. A steam generator by-pass system is configured to provide an auxiliary flow path of primary coolant to the reactor core to augment a primary flow path of the primary coolant out of the riser and into the shroud, wherein the auxiliary flow path of primary coolant exits the reactor housing without passing by the heat exchanger.

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1. A power module assembly comprising: a reactor housing;a reactor core located in a lower portion of the reactor housing;a heat exchanger proximately located about an upper portion of the reactor housing, the upper portion comprising a fluid exit for a primary coolant from the reactor housing, the

1. A power module assembly comprising: a reactor housing;a reactor core located in a lower portion of the reactor housing;a heat exchanger proximately located about an upper portion of the reactor housing, the upper portion comprising a fluid exit for a primary coolant from the reactor housing, the lower portion comprising a fluid entry for the primary coolant to the reactor housing; anda passageway provided in the reactor housing intermediate the lower portion and the upper portion, the passageway comprising a flow path for an auxiliary flow of the primary coolant to the fluid entry, the flow path adjustable based, at least in part, on a mode of operation of the power module assembly. 2. The power module assembly according to claim 1, wherein the flow path is in fluid communication with the fluid entry of the reactor housing without passing through the upper portion of the reactor housing. 3. 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 reactor housing comprises steam, and wherein the auxiliary flow of primary coolant comprises a mixture of two-phase coolant. 4. The power module assembly according to claim 1, wherein the flow path is closed or reduced based, at least in part, on a full power operation of the power module assembly. 5. The power module assembly according to claim 4, wherein the flow path is open based, at least in part, on a shut-down operation. 6. The power module assembly according to claim 5, wherein the shut-down operation comprises a loss of coolant accident or an over-pressurization event. 7. 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 housing during full power operation, and wherein the level of primary coolant is below the outlet during a shutdown operation. 8. The power module assembly according to claim 7, wherein the level of the primary coolant remains above the passageway during the shut-down operation. 9. A nuclear reactor module comprising: a reactor vessel;a reactor housing mounted inside the reactor vessel, wherein the reactor housing comprises a shroud and a riser located above the shroud;a heat exchanger proximately located about the riser;a reactor core located in the shroud; anda steam generator flow by-pass system configured to provide an auxiliary flow path, between the shroud and the riser, of primary coolant to the reactor core to augment a primary flow path of the primary coolant out of the riser and into the shroud, wherein the auxiliary flow path of primary coolant exits the reactor housing without passing by the heat exchanger, and the steam generator flow by-pass system comprises one or more baffles that adjustably control an opening area of the auxiliary flow path of the primary coolant. 10. The nuclear reactor module according to claim 9, wherein the auxiliary flow path of primary coolant exits the reactor housing due to a difference in hydrostatic forces on either side of the auxiliary flow path. 11. The nuclear reactor module according to claim 10, 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. 12. The nuclear reactor module according to claim 9, wherein the one or more baffles are controllable to rotate about a pivot to open or close. 13. The nuclear reactor module according to claim 9, wherein the steam generator flow by-pass system comprises a unidirectional valve. 14. The nuclear reactor module according to claim 9, wherein the steam generator flow by-pass system forms a passageway for coolant to exit the reactor housing during a loss of coolant accident or a depressurization event. 15. The nuclear reactor module according to claim 14, wherein the passageway opens due to a change in temperature within the reactor vessel. 16. The nuclear reactor module according to claim 15, wherein the steam generator flow by-pass system comprises a bi-metallic cover located over the passageway, and wherein the bi-metallic cover comprises materials having different thermal expansion properties. 17. The nuclear reactor module according to claim 14, wherein a difference in rate of thermal expansion between the shroud and the riser causes the passageway to open. 18. The nuclear reactor module according to claim 17, wherein the riser and the shroud are separately attached to the reactor vessel. 19. A method of cooling a nuclear reactor comprising: circulating a primary coolant through a reactor housing comprising an upper riser and a lower shroud, wherein a primary flow path of the primary coolant passes by a heat exchanger proximately located about the riser, and wherein the primary coolant enters the lower shroud;detecting a loss of coolant accident or a depressurization event;decreasing a fluid level of the primary coolant below the top of the riser, wherein the primary coolant exits the riser as steam;adjustably controlling an auxiliary passageway provided in the reactor housing to form a fluid pathway through the reactor housing between the upper riser and the lower shroud;circulating an auxiliary flow path of the primary coolant from the auxiliary passageway and through the fluid pathway; andcombining the primary coolant from the auxiliary flow path with the primary coolant from the primary flow path that enters the lower shroud. 20. The method according to claim 19, wherein the primary coolant that exits the riser as steam condenses as liquid coolant before being combined with the primary coolant of the auxiliary flow path. 21. The method according to claim 20, wherein the primary coolant of the auxiliary flow path circulates through the auxiliary passageway due to a difference in hydrostatic forces on either side of the passageway. 22. The method according to claim 19, wherein circulating the auxiliary flow path of the primary coolant through the auxiliary passageway reduces a concentration of nonvolatile additives in the primary coolant within the reactor housing. 23. A power module assembly comprising: a reactor housing;a reactor core located in a lower portion of the reactor housing;a heat exchanger proximately located about an upper portion of the reactor housing, wherein primary coolant flows out of the reactor housing via the upper portion, and wherein the primary coolant flows into the reactor housing via the lower portion; anda passageway provided in the reactor housing intermediate the lower portion and the upper portion, the passageway configured to provide an auxiliary flow of primary coolant to the reactor core to augment the flow of the primary coolant out of the upper portion of the reactor housing and into the lower portion, and during a loss of coolant accident, the flow of primary coolant out of the upper portion of the reactor housing comprises steam and the auxiliary flow of primary coolant comprises a mixture of two-phase coolant. 24. A power module assembly comprising: a reactor housing;a reactor core located in a lower portion of the reactor housing;a heat exchanger proximately located about an upper portion of the reactor housing, wherein primary coolant flows out of the reactor housing via the upper portion, and wherein the primary coolant flows into the reactor housing via the lower portion; anda passageway provided in the reactor housing intermediate the lower portion and the upper portion, the passageway configured to provide an auxiliary flow of primary coolant to the reactor core to augment the flow of the primary coolant out of the upper portion of the reactor housing and into the lower portion, the passageway closed or reduced during a full power operation of the power module assembly. 25. A nuclear reactor module comprising: a reactor vessel;a reactor housing mounted inside the reactor vessel, wherein the reactor housing comprises a shroud and a riser located above the shroud;a heat exchanger proximately located about the riser;a reactor core located in the shroud; anda steam generator flow by-pass system configured to provide an auxiliary flow path of primary coolant to the reactor core to augment a primary flow path of the primary coolant out of the riser and into the shroud, wherein the auxiliary flow path of primary coolant exits the reactor housing without passing by the heat exchanger,wherein the steam generator flow by-pass system forms a passageway for coolant to exit the reactor housing during a loss of coolant accident or a depressurization event, and the passageway opens due to a change in temperature within the reactor vessel. 26. A nuclear reactor module comprising: a reactor vessel;a reactor housing mounted inside the reactor vessel, wherein the reactor housing comprises a shroud and a riser located above the shroud;a heat exchanger proximately located about the riser;a reactor core located in the shroud; anda steam generator flow by-pass system configured to provide an auxiliary flow path of primary coolant to the reactor core to augment a primary flow path of the primary coolant out of the riser and into the shroud, wherein the auxiliary flow path of primary coolant exits the reactor housing without passing by the heat exchanger,wherein the steam generator flow by-pass system forms a passageway for coolant to exit the reactor housing during a loss of coolant accident or a depressurization event, and a difference in rate of thermal expansion between the shroud and the riser causes the passageway to open. 27. A method of cooling a nuclear reactor comprising: circulating a primary coolant through a reactor housing comprising an upper riser and a lower shroud, wherein a primary flow path of the primary coolant passes by a heat exchanger proximately located about the riser, and wherein the primary coolant enters the lower shroud;detecting a loss of coolant accident or a depressurization event;decreasing a fluid level of the primary coolant below the top of the riser, wherein the primary coolant exits the riser as steam;circulating an auxiliary flow path of the primary coolant through an auxiliary passageway provided in the reactor housing, wherein the auxiliary flow path of the primary coolant exits the reactor housing without passing by the heat exchanger; andcombining the primary coolant from the auxiliary flow path with the primary coolant from the primary flow path that enters the lower shroud,wherein circulating the auxiliary flow path of the primary coolant through the auxiliary passageway reduces a concentration of nonvolatile additives in the primary coolant within the reactor housing.