Patent Number: 
Section: claims

1. A power module comprising:a reactor vessel containing a coolant;a reactor core disposed in the reactor vessel and submerged in the coolant;a riser section disposed in the reactor vessel above the reactor core, the riser section comprising one or more outer walls defining an inner passageway;a coolant deflector shield comprising an ellipsoidal surface, the coolant deflector shield being disposed in the reactor vessel at a position above an open end of the inner passageway;a pressure control assembly, positioned in the reactor vessel in a pressurizer region between the coolant deflector shield and an upper end of the reactor vessel, to control a pressure in the pressurizer region, the pressure control assembly comprising one or more heaters positioned on opposite sides of the reactor vessel within the pressurizer region; anda fluid passageway that extends between the heaters from the coolant deflector shield to the upper end of the reactor vessel and is fluidly coupled to gaps between an inner surface of the reactor vessel and distal ends of the ellipsoidal surface for a flow of coolant from the riser section into the pressurizer region and through the fluid passageway. 2. The power module according to claim 1, wherein a bottom end of the reactor vessel comprises a second ellipsoidal surface, and wherein the second ellipsoidal surface directs the coolant towards the reactor core. 3. The power module according to claim 1, wherein the ellipsoidal surface comprises a surface that deflects the coolant that exits the riser section. 4. The power module according to claim 1, wherein the coolant deflector shield is supported by one or more control rod guide tubes or instrumentation structures. 5. The power module according to claim 4, wherein the one or more control rod guide tubes are attached to the upper end of the reactor vessel. 6. The power module according to claim 5, wherein the coolant deflector shield is suspended from the one or more control rod guide tubes such that the coolant deflector shield is substantially free from contact with the reactor vessel. 7. The power module according to claim 6, wherein the suspended coolant deflector shield is substantially free from changes in rates of thermal expansion of one or more components of the power module. 8. The power module according to claim 1, wherein the power module is configured to be submerged in a pool of liquid disposed in a reactor bay, the reactor bay disposed below a terranean surface. 9. The power module according to claim 1, wherein the ellipsoidal surface is configured to reduce a turning loss of the coolant directed towards a bottom end of the reactor vessel. 10. The power module according to claim 9, wherein the reduction in turning loss is between a factor of about 4 and about 5. 11. The power module according to claim 1, wherein the coolant deflector shield is disposed in the reactor vessel at a position below a fluid level of the coolant. 12. The power module according to claim 1, wherein the riser section comprises an inward facing portion at a top end of the riser section, a vertical cross-section of the inward facing portion comprising a substantially oval cross section that extends around a perimeter of the top end of the riser section. 13. The power module according to claim 12, wherein the coolant deflector shield is arranged at a vertical distance above the top end of the riser section that is substantially greater than a vertical distance between two distal ends of the ellipsoidal surface and the top end of the riser section, wherein a minor axis of the ellipsoidal surface comprises a distance between the top end of the riser section and a midpoint location between the two distal ends. 14. The power module according to claim 12, wherein the substantially oval cross section approximates an inverted teardrop. 15. The power module according to claim 14, wherein the perimeter of the top end of the riser section comprises a rounded rim of the inverted teardrop. 16. The power module according to claim 1, wherein the ellipsoidal surface comprises a single ellipsoidal surface that comprises a single midpoint location between two distal ends of the single ellipsoidal surface. 17. The power module according to claim 16, wherein the coolant deflector shield is disposed at the position such that the midpoint location of the ellipsoidal surface is substantially aligned with a centerline of the riser section. 18. The power module according to claim 16, wherein a distance between the midpoint location and a top end of the riser section comprises a first distance, and a diameter of the riser section comprises a second distance, and a ratio of the first distance to the second distance is between about 0.1 and about 2.0. 19. The power module according to claim 18, further comprising a first area defined by the diameter of the riser section, and a second area defined by an annulus between the riser section and the inner surface of the reactor vessel, and a ratio of the second area to the first area is between about 1 and about 10. 20. The power module according to claim 1, wherein the pressure control assembly further comprises a spray nozzle. 21. The power module according to claim 20, wherein the fluid passageway extends between the coolant deflector shield and the spray nozzle positioned at the upper end of the reactor vessel. 22. The power module according to claim 20, wherein the one or more heaters comprise a pair of heaters that are configured to heat the coolant that enters into the pressurizer region, such that the pressure control assembly controls the pressure in the pressurizer region to increase a flow rate of the coolant in the reactor vessel. 23. The power module according to claim 22, wherein the coolant in the pressurizer region comprises substantially saturated coolant with a higher temperature than the coolant below the coolant deflector shield that comprises relatively sub-cooled coolant. 24. The power module according to claim 20, wherein the pressure control assembly is configured to control the coolant to have a fluid level above the coolant deflector shield and the one or more heaters, such that an entire volume between the coolant deflector shield and a bottom end of the reactor vessel is full of the coolant.