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A pressurized water reactor (PWR) comprises: a nuclear core comprising a fissile material; a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water; and a hollow cylindrical central riser disposed concentrically with a

A pressurized water reactor (PWR) comprises: a nuclear core comprising a fissile material; a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water; and a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel. A downcomer annulus is defined between the hollow cylindrical central riser and the cylindrical pressure vessel. The hollow cylindrical central riser has a radially expanding upper orifice that merges into an annular divider plate that separates an upper plenum above the annular divider plate from a lower plenum below the annular divider plate. The upper plenum is in fluid communication with the radially expanding upper orifice and the lower plenum is in fluid communication with the downcomer annulus. A weir may extend away from a bottom wall of the lower plenum into the lower plenum. An emergency core cooling system (ECCS) return line nozzle may be arranged to inject water into the upper plenum. A pump support plate spans the inner diameter of the cylindrical pressure vessel and forms a portion of the pressure boundary of the cylindrical pressure vessel, and reactor coolant pumps (RCPs) are supported by the pump support plate. Alternatively, reactor coolant pumps (RCPs) are supported by an arcuate annular ledge formed in the upper portion of the cylindrical pressure vessel.

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1. An apparatus comprising: a pressurized water reactor (PWR) including: a nuclear core comprising a fissile material,a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water, anda hollow cylindrical central riser disp

1. An apparatus comprising: a pressurized water reactor (PWR) including: a nuclear core comprising a fissile material,a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water, anda hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel, a downcomer annulus being defined between the hollow cylindrical central riser and the cylindrical pressure vessel,wherein the hollow cylindrical central riser has a radially expanding upper orifice that extends radially outwardly from the hollow cylindrical central riser and merges into an annular divider plate that separates an upper plenum above the annular divider plate from a lower plenum below the annular divider plate and wherein the upper plenum is in fluid communication with the radially expanding upper orifice and the lower plenum is in fluid communication with the downcomer annulus. 2. The apparatus of claim 1 wherein an annular gap separates an outer perimeter of the annular divider plate from an inner wall of the cylindrical pressure vessel, the annular gap providing fluid communication between the upper and lower plenums, the annular gap being radially outboard of the downcomer annulus. 3. The apparatus of claim 1 wherein the annular divider plate forms an annular seal with an inner wall of the cylindrical pressure vessel, the annular divider plate including openings located radially outboard of the downcomer annulus providing fluid communication between the upper plenum and the lower plenum. 4. The apparatus of claim 3 further comprising: reactor coolant pumps (RCPs) arranged at the openings in the annular divider plate to pump reactor coolant water from the upper plenum into the lower plenum. 5. The apparatus of claim 4 wherein the PWR further comprising: a pump support plate spanning the inner diameter of the cylindrical pressure vessel and forming a portion of the pressure boundary of the cylindrical pressure vessel, the RCPs being supported by the pump support plate. 6. The apparatus of claim 5 wherein the pump support plate comprises a single-piece forged plate. 7. The apparatus of claim 3 further comprising: a steam generator disposed in the downcomer annulus, wherein downward primary coolant flow from the lower plenum through the steam generator heats upward secondary coolant flow through the steam generator that is in fluid isolation from the downward primary coolant flow. 8. The apparatus of claim 7 wherein the PWR further comprising: a pump support plate spanning the inner diameter of the cylindrical pressure vessel and forming a portion of the pressure boundary of the cylindrical pressure vessel, a plurality of RCPs being supported by the pump support plate. 9. The apparatus of claim 8 wherein: an upper portion of the cylindrical pressure vessel defines an internal pressurizer volume,the pump support plate separates the internal pressurizer volume from an operational PWR volume of the pressure vessel that includes at least the upper and lower plenums, the downcomer annulus, and the volume contained by the hollow cylindrical central riser, andthe pump support plate includes a surge line passing through the pump support plate. 10. The apparatus of claim 9 wherein the surge line is not located in the center of the pump support plate. 11. The apparatus of claim 9 wherein the PWR further comprises: a thermal insulator plate disposed above and spaced apart from the pump support plate. 12. The apparatus of claim 9 wherein the PWR further comprises: a flow diffuser disposed in the lower plenum to diffuse primary coolant flow from openings in the annular divider plate into the downcomer annulus. 13. The apparatus of claim 12 wherein the lower plenum is defined in part by a bottom wall that extends radially outboard from an upper end of the downcomer annulus, and the flow diffuser comprises: a weir extending away from the bottom wall of the lower plenum. 14. The apparatus of claim 13 further comprising: an emergency core cooling system return line nozzle arranged to inject water into the upper plenum. 15. The apparatus of claim 1 wherein the PWR further comprises: a flow diffuser disposed in the lower plenum to diffuse primary coolant flow from openings in the annular divider plate into the downcomer annulus. 16. The apparatus of claim 15 wherein the lower plenum is defined in part by a bottom wall that extends radially outboard from an upper end of the downcomer annulus, and the flow diffuser comprises: a weir extending away from the bottom wall of the lower plenum. 17. The apparatus of claim 16 wherein the weir comprises an annular weir surrounding the upper end of the downcomer annulus. 18. The apparatus of claim 15 further comprising: an emergency core cooling system return line nozzle arranged to inject water into the upper plenum. 19. An apparatus comprising: a pressurized water reactor (PWR) including: a nuclear core comprising a fissile material,a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water,a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel, a downcomer annulus being defined between the hollow cylindrical central riser and the cylindrical pressure vessel, and an annular divider plate separating an upper plenum above the annular divider plate from a lower plenum below the annular divider plate;wherein the upper plenum is in fluid communication with an upper orifice of the hollow cylindrical riser and the lower plenum is in fluid communication with the downcomer annulus; andwherein a flow path from the upper plenum to the lower plenum is disposed outboard of the downcomer annulus,the upper orifice of the hollow cylindrical central riser extends radially outwardly therefrom and merges into the annular divider plate. 20. The apparatus of claim 19 wherein a weir extends away from a bottom wall of the lower plenum into the lower plenum and the weir is radially outboard of the downcomer annulus andthe weir is radially inboard of the flow path from the upper plenum to the lower plenum. 21. The apparatus of claim 20 wherein the weir comprises an annular weir. 22. The apparatus of claim 20 wherein the apparatus further comprises: reactor coolant pumps (RCPs) arranged to pump primary coolant from the upper plenum into the lower plenum through the flow path from the upper plenum to the lower plenum that is disposed outboard of the downcomer annulus. 23. The apparatus of claim 22 wherein the PWR further comprises: a pump support plate spanning the inner diameter of the cylindrical pressure vessel and forming a portion of the pressure boundary of the cylindrical pressure vessel, the RCPs being supported by the pump support plate. 24. The apparatus of claim 23 wherein: an upper portion of the cylindrical pressure vessel defines an internal pressurizer volume,the pump support plate separates the internal pressurizer volume from an operational PWR volume of the pressure vessel that includes at least the upper and lower plenums, the downcomer annulus, and the volume contained by the hollow cylindrical central riser, andthe pump support plate includes a surge line passing through the pump support plate. 25. The apparatus of claim 24 wherein the surge line is not located in the center of the pump support plate. 26. The apparatus of claim 24 further comprising: a thermal insulator plate disposed above the pump support plate. 27. The apparatus of claim 26 further comprising: an emergency core cooling system return line nozzle arranged to inject water into the upper plenum. 28. An apparatus comprising: a pressurized water reactor (PWR) including: a nuclear core comprising a fissile material,a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water,a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel, a downcomer annulus being defined between the hollow cylindrical central riser and the cylindrical pressure vessel, anda pump support plate spanning the inner diameter of a top portion of the cylindrical pressure vessel above the hollow cylindrical central riser, wherein a portion of the pump support plate forms a portion of the pressure boundary of the cylindrical pressure vessel; andreactor coolant pumps (RCPs) supported by the portion of the pump support plate that forms a portion of the pressure boundary of the cylindrical pressure vessel, the RCPs being arranged to pump primary coolant water discharged from an upper orifice of the hollow cylindrical central riser downwardly into the downcomer annulus. 29. The apparatus of claim 28 wherein the pump support plate comprises a single-piece forged plate. 30. The apparatus of claim 29 wherein: an upper portion of the cylindrical pressure vessel defines an internal pressurizer volume,the pump support plate separates the internal pressurizer volume from an operation al PWR volume of the pressure vessel that includes at least the upper and lower plenums, the downcomer annulus, and the volume contained by the hollow cylindrical central riser, andthe pump support plate includes a surge line passing through the pump support plate. 31. The apparatus of claim 30 wherein the surge line is not located in the center of the pump support plate. 32. The apparatus of claim 30 further comprising: a thermal insulator plate disposed above the pump support plate, the thermal insulator plate providing a larger thermal barrier between the internal pressurizer volume and the operational PWR volume than is provided by the pump support plate. 33. The apparatus of claim 32 wherein the thermal insulator plate is spaced apart from the pump support plate and a gap between the thermal insulator plate and the pump support plate comprises a portion of the thermal barrier provided by the thermal insulator plate. 34. The apparatus of claim 33 wherein the gap between the thermal insulator plate and the pump support plate contains stagnant primary coolant water. 35. An apparatus comprising: a pressurized water reactor (PWR) including:a nuclear core comprising a fissile material, a cylindrical pressure vessel having a vertically oriented cylinder axis and containing the nuclear core immersed in primary coolant water,a hollow cylindrical central riser including a radially expanding upper orifice and being disposed concentrically with and inside the cylindrical pressure vessel, a downcomer annulus being defined between the hollow cylindrical central riser and the cylindrical pressure vessel, andan annular divider plate separating an upper plenum above the annular divider plate from a lower plenum below the annular divider plate the radially expanding upper orifice extending radially outwardly from the hollow cylindrical central riser and merging into the annular divider plate; andan emergency core cooling system return line nozzle arranged to inject water into the upper plenum.