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A pressurized water reactor (PWR) includes a vertical cylindrical pressure vessel and a nuclear reactor core disposed in a lower vessel section. A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. A downcomer annulus is defined between the central riser and the

A pressurized water reactor (PWR) includes a vertical cylindrical pressure vessel and a nuclear reactor core disposed in a lower vessel section. A hollow cylindrical central riser is disposed concentrically inside the pressure vessel. A downcomer annulus is defined between the central riser and the pressure vessel. A reactor coolant pump (RCP) includes (i) an impeller disposed above the nuclear reactor core and in fluid communication with the downcomer annulus to impel primary coolant downward through the downcomer annulus, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a drive shaft operatively connecting the pump motor with the impeller. The PWR may include an internal steam generator in the downcomer annulus, with the impeller is disposed below the steam generator. The impeller may be disposed in the downcomer annulus. The RCP may further comprise a pump casing that with the impeller defines a centrifugal pump.

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1. An apparatus comprising: a pressurized water reactor (PWR) including:a vertically oriented cylindrical pressure vessel comprising upper and lower vessel sections that are secured together and having a cylinder axis of the cylindrical pressure vessel oriented vertically,a nuclear reactor core disp

1. An apparatus comprising: a pressurized water reactor (PWR) including:a vertically oriented cylindrical pressure vessel comprising upper and lower vessel sections that are secured together and having a cylinder axis of the cylindrical pressure vessel oriented vertically,a nuclear reactor core disposed in the lower vessel section, a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel wherein a downcomer annulus is defined between the hollow cylindrical central riser and the cylindrical pressure vessel, the downcomer annulus including (i) an upper portion with a relatively smaller inner diameter located in the upper vessel section, (ii) a lower portion with a relatively larger inner diameter located in the lower vessel section, and (iii) a transition region disposed between the upper portion and the lower portion, the inner diameter of the downcomer annulus transitioning in the transition region from the relatively smaller inner diameter of the upper portion to the relatively larger inner diameter of the lower portion, anda reactor coolant pump secured to the lower vessel section, the reactor coolant pump including (i) an impeller disposed above the nuclear reactor core and inside the pressure vessel in the transition region of the downcomer annulus to impel primary coolant downward through the downcomer annulus, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a horizontally oriented drive shaft operatively connecting the pump motor with the impeller. 2. The apparatus of claim 1 wherein the upper vessel section has a larger diameter than the lower vessel section. 3. The apparatus of claim 1 wherein the lower vessel section includes an overhang at which the reactor coolant pump is secured with (I) the impeller disposed inside the pressure vessel in the downcomer annulus above the overhang and (II) the pump motor disposed outside the pressure vessel below the overhang. 4. The apparatus of claim 3 wherein the lower vessel section includes a flange by which the lower vessel section is secured with the upper vessel section, the flange having a larger diameter than the remainder of the lower vessel section so as to define the overhang at which the reactor coolant pump is secured. 5. The apparatus of claim 1 wherein the impeller is disposed inside the pressure vessel above an overhang of the pressure vessel and the pump motor is disposed outside of the pressure vessel below the overhang of the pressure vessel and the drive shaft operatively connects the pump motor below the overhang with the impeller above the overhang. 6. The apparatus of claim 1 wherein the downcomer annulus proximate to the impeller is shaped to define a pump casing that cooperates with the impeller to impel primary coolant downward through the downcomer annulus. 7. The apparatus of claim 1 wherein: the reactor coolant pump comprises a plurality of reactor coolant pumps including a corresponding plurality of impellers disposed in the downcomer annulus and spaced apart around the hollow cylindrical central riser, and the downcomer annulus proximate to the plurality of impellers is shaped to define an annular pump casing that cooperates with the plurality of impellers to impel primary coolant downward through the downcomer annulus. 8. The apparatus of claim 1 wherein the reactor coolant pump further comprises a pump casing containing the impeller, the pump casing also disposed inside the pressure vessel in the downcomer annulus, the pump casing and the impeller cooperatively defining a centrifugal pump with the pump casing defining the volute chamber of the centrifugal pump. 9. The apparatus of claim 8 wherein the reactor coolant pump is secured to the lower vessel section. 10. The apparatus of claim 1 wherein the impeller is accessible by separating the upper and lower vessel sections. 11. An apparatus comprising: a pressurized water reactor (PWR) including:a vertically oriented cylindrical pressure vessel comprising upper and lower vessel sections that are secured together and having a cylinder axis of the cylindrical pressure vessel oriented vertically,a nuclear reactor core disposed in the lower vessel section,a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel wherein a downcomer annulus is defined between the hollow cylindrical central riser and the cylindrical pressure vessel, the downcomer annulus including (i) an upper portion with a relatively smaller inner diameter located in the upper vessel section, (ii) a lower portion with a relatively larger inner diameter located in the lower vessel section, and (iii) a transition region disposed between the upper portion and the lower portion, the inner diameter of the downcomer annulus transitioning in the transition region from the relatively smaller inner diameter of the upper portion to the relatively larger inner diameter of the lower portion,a reactor coolant pump including (i) an impeller disposed in the transition region of the downcomer annulus above the nuclear reactor core and in fluid communication with the downcomer annulus to impel primary coolant downward through the downcomer annulus, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a horizontally oriented drive shaft operatively connecting the pump motor with the impeller, andan internal steam generator disposed in the downcomer annulus, the impeller of the reactor coolant pump being disposed below the internal steam generator. 12. The apparatus of claim 11 wherein the impeller is disposed inside the pressure vessel in the downcomer annulus below the internal steam generator to impel primary coolant discharged from the internal steam generator downward through the downcomer annulus. 13. The apparatus of claim 11, wherein the internal steam generator is disposed entirely in the upper vessel section with no portion of the steam generator being disposed in the lower vessel section. 14. An apparatus comprising: a pressurized water reactor (PWR) including:a vertically oriented cylindrical pressure vessel comprising upper and lower vessel sections,a hollow cylindrical central riser disposed concentrically with and inside the cylindrical pressure vessel wherein a downcomer annulus is defined between the hollow cylindrical central riser and the cylindrical pressure vessel, the downcomer annulus including (i) an upper portion with a relatively smaller inner diameter located in the upper vessel section, (ii) a lower portion with a relatively larger inner diameter located in the lower vessel section, and (iii) a transition region disposed between the upper portion and the lower portion, the inner diameter of the downcomer annulus transitioning in the transition region from the relatively smaller inner diameter of the upper portion to the relatively larger inner diameter of the lower portion,a nuclear reactor core disposed in the lower vessel section, anda plurality of reactor coolant pumps spaced apart around the hollow cylindrical central riser and secured to the lower vessel section wherein each reactor coolant pump includes (i) an impeller disposed inside the pressure vessel in the transition region of the downcomer annulus, (ii) a pump motor disposed outside of the pressure vessel, and (iii) a horizontally oriented drive shaft operatively connecting the pump motor with the impeller; anda steam generator disposed in the downcomer annulus;wherein the impellers are disposed below the steam generator and above the nuclear reactor core. 15. The apparatus of claim 14 wherein the downcomer annulus proximate to the plurality of impellers is shaped to define a common annular pump casing for the plurality of impellers that cooperates with the plurality of rotating impellers to impel primary coolant downward through the downcomer annulus. 16. The apparatus of claim 15 wherein the pressure vessel includes an overhang, the impellers of the reactor coolant pumps being disposed inside the pressure vessel above the overhang, the pump motors of the reactor coolant pumps being disposed outside of the pressure vessel below the overhang, and the drive shafts of the reactor coolant pumps being vertically oriented. 17. The apparatus of claim 14 wherein each reactor coolant pump further comprises a casing disposed inside the pressure vessel in the downcomer annulus and cooperating with the impeller to define a centrifugal pump with the pump casing defining the volute chamber of the centrifugal pump.