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A pressurized water reactor (PWR) includes a vertical cylindrical pressure vessel having a lower portion containing a nuclear reactor core and a vessel head defining an internal pressurizer. A reactor coolant pump (RCP) mounted on the vessel head includes an impeller inside the pressure vessel, a pu

A pressurized water reactor (PWR) includes a vertical cylindrical pressure vessel having a lower portion containing a nuclear reactor core and a vessel head defining an internal pressurizer. A reactor coolant pump (RCP) mounted on the vessel head includes an impeller inside the pressure vessel, a pump motor outside the pressure vessel, and a vertical drive shaft connecting the motor and impeller. The drive shaft does not pass through the internal pressurizer. A central riser may be disposed concentrically inside the pressure vessel, and the RCP impels primary coolant downward into a downcomer annulus between the central ser and the pressure vessel. A steam generator may be disposed in the downcomer annulus and spaced apart from with the impeller by an outlet plenum, A manway may access the outlet plenum so tube plugging can be performed on the steam generator via access through the manway without removing the RCP.

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1. An apparatus comprising: a pressurized water reactor (PWR) including:a cylindrical pressure vessel with its cylinder axis oriented vertically, a nuclear reactor core disposed in the cylindrical pressure vessel, a separator plate disposed in the cylindrical pressure vessel that separates the press

1. An apparatus comprising: a pressurized water reactor (PWR) including:a cylindrical pressure vessel with its cylinder axis oriented vertically, a nuclear reactor core disposed in the cylindrical pressure vessel, a separator plate disposed in the cylindrical pressure vessel that separates the pressure vessel to define an internal pressurizer containing a pressurizer volume disposed above the separator plate and a reactor vessel portion defining a reactor volume disposed below the separator plate and containing the nuclear reactor core, wherein the separator plate restricts but does not completely cut off fluid communication between the pressurizer volume and the reactor volume, wherein the cross-section of the cylindrical pressure vessel includes a narrowed portion defining a recess of the internal pressurizer wherein the pump motor is disposed at least partially in the recess; anda reactor coolant pump including (i) an impeller disposed inside the pressure vessel in the reactor volume, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a drive shaft operatively connecting the pump motor with the impeller, wherein (1) at least a portion of the pump motor is disposed above the separator plate (2) no portion of the reactor coolant pump is disposed in the pressurizer volume and (3) the drive shaft of the reactor coolant pump is oriented parallel with the cylinder axis of the cylindrical pressure vessel. 2. The apparatus of claim 1, wherein the pump motor at least partially overlaps a cross-sectional area of the reactor vessel portion of the cylindrical pressure vessel in a plane transverse to the cylinder axis of the cylindrical pressure vessel. 3. The apparatus of claim 1, wherein the reactor coolant pump comprises N reactor coolant pumps spaced apart at 360°/N intervals around the cylinder axis of the cylindrical pressure vessel where N is an integer greater than or equal to 2. 4. The apparatus of claim 1, wherein: the PWR further includes a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel in the reactor volume; andthe impeller of the reactor coolant pump is configured to impel primary coolant water downward into a downcomer annulus defined between the hollow cylindrical central riser and the cylindrical pressure vessel. 5. The apparatus of claim 4, wherein: the PWR further includes an internal steam generator disposed in the downcomer annulus. 6. The apparatus of claim 5, wherein the reactor coolant pumps discharge primary coolant into an outlet plenum that spaces apart the reactor coolant pumps and the internal steam generator. 7. The apparatus of claim 6, wherein the reactor coolant pumps discharge primary coolant into the outlet plenum at a radial location respective to the cylinder axis of the cylindrical pressure vessel that is outboard from the radial location of the annular internal steam generator disposed in the downcomer annulus respective to the cylinder axis of the cylindrical pressure vessel. 8. The apparatus of claim 5, wherein the pressure vessel includes a closure at or below the separator plate and at or above the top of the internal steam generator wherein the internal pressurizer is removable to provide access from above to the internal steam generator for performing steam generator tube plugging. 9. The apparatus of claim 4, wherein the reactor coolant pump further comprises an annular pump casing containing the impeller, the annular pump casing having an inlet receiving primary coolant water from the top of the hollow cylindrical central riser and an outlet in fluid communication with the downcomer annulus. 10. The apparatus of claim 4, wherein the reactor coolant pump further comprises a 90° angled pump casing containing the impeller wherein the inlet faces horizontally and the outlet faces downwardly, the inlet being in fluid communication with the top of the hollow cylindrical central riser and the outlet being in fluid communication with the downcomer annulus. 11. The apparatus of claim 1, wherein the cylindrical pressure vessel includes an opening through which the drive shaft of the reactor coolant pump passes wherein said opening is too small for the impeller to pass through. 12. The apparatus of claim 11, wherein said opening through which the drive shaft of the reactor coolant pump passes includes a self-lubricating graphalloy bearing supporting the drive shaft. 13. The apparatus of claim 12, further comprising a manway disposed at or below the impeller that is large enough for the impeller to pass through. 14. The apparatus of claim 1, wherein the internal pressurizer disposed above the separator plate is of smaller cross-sectional diameter over its entire height than the reactor vessel portion defining the reactor volume disposed below the separator plate, and the smaller cross-sectional diameter of the internal pressurizer accommodates the pump motors. 15. An apparatus comprising: a pressurized water reactor (PWR) including: a nuclear reactor core, a pressure vessel having a closure by which an upper vessel head of the pressure vessel is removable, wherein the nuclear reactor core is disposed in the pressure vessel below the vessel head and wherein the upper vessel head includes an internal pressurizer defining a pressurizer volume with heaters configured to control PWR pressure, wherein the cross-section of the upper vessel head of the cylindrical pressure vessel includes a narrowed portion defining a recess of the internal pressurizer wherein the pump motor is disposed at least partially in the recess; anda plurality of reactor coolant pumps mounted on the vessel head such that the vessel head and plurality of reactor coolant pumps are removable as a unit, each reactor coolant pump including (i) an impeller disposed inside the pressure vessel below the internal pressurizer, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a drive shaft operatively connecting the pump motor with the impeller, wherein the drive shaft of each reactor coolant pump is oriented vertically andno portion of any reactor coolant pump is disposed in the pressurizer volume. 16. The apparatus of claim 15, wherein: the pressure vessel is cylindrical with it cylinder axis oriented vertically;the PWR further includes a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel; andthe impellers of the reactor coolant pumps are arranged to impel primary coolant water downward into a downcomer annulus defined between the hollow cylindrical central riser and the cylindrical pressure vessel. 17. The apparatus of claim 16, wherein: the PWR further includes an internal steam generator disposed in the downcomer annulus; andthe reactor coolant pumps discharge primary coolant into an outlet plenum that separates the reactor coolant pumps from the internal steam generator. 18. The apparatus of claim 17, wherein the reactor coolant pumps discharge primary coolant into the outlet plenum at a radial location respective to the cylinder axis of the cylindrical pressure vessel that is outboard from the internal steam generator disposed in the downcomer annulus respective to the cylinder axis of the cylindrical pressure vessel. 19. The apparatus of claim 15, wherein the upper vessel head and plurality of reactor coolant pumps are removable as a unit and the pressure vessel includes openings through which the drive shafts of the reactor coolant pumps pass wherein said openings are too small for the impellers to pass through. 20. The apparatus of claim 15, wherein the internal pressurizer is of smaller cross-sectional diameter over its entire height than the pressure vessel below the upper vessel head, and the smaller cross-sectional diameter of the internal pressurizer accommodates the pump motors. 21. An apparatus comprising: a pressurized water reactor (PWR) including:a nuclear reactor core comprising a fissile material, a cylindrical pressure vessel with its cylinder axis oriented vertically, the cylindrical pressure vessel having a lower portion containing the nuclear reactor core and an upper vessel head defining an internal pressurizer, wherein the cross-section of the upper vessel head of the cylindrical pressure vessel includes a narrowed portion defining a recess of the internal pressurizer wherein the pump motor is disposed at least partially in the recess; anda reactor coolant pump mounted on the vessel head and including (i) an impeller disposed inside the pressure vessel, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a vertical drive shaft connecting the pump motor and the impeller, the vertical drive shaft being oriented parallel with the vertically oriented cylinder axis of the cylindrical pressure vessel and not passing through the internal pressurizer. 22. The apparatus of claim 21, wherein the cylindrical pressure vessel includes a closure by which the upper vessel head is removable from the remainder of the pressure vessel. 23. The apparatus of claim 22, wherein the PWR further comprises: a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel, the impeller of the reactor coolant pump being configured to impel primary coolant water downward into a downcomer annulus defined between the hollow cylindrical central riser and the cylindrical pressure vessel. 24. The apparatus of claim 23, wherein the PWR further comprises: an internal steam generator disposed in the downcomer annulus;wherein the impeller of the reactor coolant pump and the internal steam generator are spaced apart by an outlet plenum. 25. The apparatus of claim 24, wherein the cylindrical pressure vessel includes a manway providing access to the outlet plenum. 26. The apparatus of claim 24, wherein the reactor coolant pumps are at a larger diameter than the internal steam generator disposed in the downcomer annulus. 27. The apparatus of claim 21, wherein the internal pressurizer is of smaller cross-sectional diameter over its entire height than the lower portion of the pressure vessel, and the smaller cross-sectional diameter of the internal pressurizer accommodates the pump motors.