Patent Number: 
Section: claims

1. A nuclear reactor comprising:a generally cylindrical pressure vessel defining a cylinder axis;a nuclear reactor core disposed in the generally cylindrical pressure vessel;a central riser disposed coaxially inside the generally cylindrical pressure vessel, the central riser being hollow and having a bottom end proximate to the nuclear reactor core to receive primary coolant heated by the nuclear reactor core, the central riser having a top end distal from the nuclear reactor core; anda once-through steam generator (OTSG) comprising tubes arranged parallel with the cylinder axis in an annular volume defined between the central riser and the generally cylindrical pressure vessel, primary coolant discharged from the top end of the central riser flowing inside the tubes toward the nuclear reactor core, the OTSG further including a fluid flow volume having a feedwater inlet and a steam outlet wherein fluid injected into the fluid flow volume at the feedwater inlet and discharged from the fluid flow volume at the steam outlet flows outside the tubes in a direction generally opposite flow of primary coolant inside the tubes;wherein the nuclear reactor has an operating state in which fluid comprising feedwater injected into the fluid flow volume at the feedwater inlet is converted by heat transfer from primary coolant flowing inside the tubes into steam that is discharged from the fluid flow volume at the steam outlet; andwherein the tubes of the OTSG are secured in a support including a pair of tubesheets made of steel and the tubes are supported at their ends by the tubesheets,wherein the tubes comprise a material having a higher coefficient of thermal expansion than steel and are in axial tension in a non-operating state of the nuclear reactor in which the tubes of the OTSG are at a temperature of less than 100° C. and are in axial compression in the operating state. 2. The nuclear reactor as set forth in claim 1, wherein:in the operating state of the nuclear reactor the primary coolant flowing in the tubes of the OTSG is at a temperature of at least 500° C. 3. The nuclear reactor as set forth in claim 2, wherein the tubes of the OTSG are made of an austenitic nickel-chromium-based alloy and the tubes are secured in the tubesheets which are attached to the central riser and the pressure vessel, wherein the central riser and pressure vessel are made of a steel, and wherein the austenitic nickel-chromium-based alloy has a higher coefficient of thermal expansion than the steel. 4. The nuclear reactor as set forth in claim 1, wherein:the tubesheets are attached to the central riser and the pressure vessel wherein the central riser and the pressure vessel are made of steel; andends of the tubes are expanded to secure to the tubesheets whereby the tubes are under axial tension due to the Poisson effect. 5. An apparatus comprising:a pressurized water nuclear reactor (PWR) including a pressure vessel, a nuclear reactor core disposed in the pressure vessel, and a vertically oriented hollow central riser disposed above the nuclear reactor core inside the pressure vessel; anda once-through steam generator (OTSG) disposed in the pressure vessel of the PWR, the OTSG including vertical tubes having a higher coefficient of thermal expansion than steel arranged in an annular volume defined by the central riser and the pressure vessel and secured in a support made of steel, the OTSG further including a fluid flow volume surrounding the vertical tubes;wherein the PWR has an operating state in which primary coolant at a temperature of at least 500° C. flows in the tubes of the OTSG and in which feedwater injected into the fluid flow volume at a feedwater inlet is converted to steam by heat emanating from the primary coolant flowing inside the tubes of the OTSG, and the steam is discharged from the fluid flow volume at a steam outlet; andwherein the tubes of the OTSG are in axial tension in a non-operating state of the PWR in which the tubes of the OTSG are at a temperature of less than 100° C. and are in axial compression in the operating state. 6. The apparatus as set forth in claim 5, further comprising:a flow diverter disposed in the generally cylindrical pressure vessel above the central riser, the flow diverter having a flow-diverting surface facing a top of the central riser that is at least one of sloped and curved to redirect primary coolant discharged from the top of the central riser toward inlets of the tubes of the OTSG. 7. The apparatus as set forth in claim 6, wherein the flow diverter divides the pressure vessel into an upper integral pressurizer volume and a remaining lower interior volume, and in the operating state the upper integral pressurizer volume contains fluid at a temperature greater than a temperature of the primary coolant disposed in the remaining lower interior volume of the pressure vessel. 8. The apparatus as set forth in claim 5, wherein the pressure vessel is divided into an upper integral pressurizer volume and a remaining lower interior volume, and in the operating state the upper integral pressurizer volume contains fluid at a temperature greater than a temperature of the primary coolant disposed in the remaining lower interior volume of the pressure vessel. 9. The apparatus as set forth in claim 8, further comprising:neutron-absorbing control rods; anda control rod drive mechanism (CRDM) configured to controllably insert and withdraw the control rods into and out of the nuclear reactor core;wherein no portion of the CRDM is disposed in or passes though the integral pressurizer volume. 10. The apparatus as set forth in claim 5, wherein:in the operating state of the nuclear reactor the primary coolant flowing inside the tubes is at a higher pressure than the fluid in the fluid flow volume. 11. The apparatus as set forth in claim 5, wherein the tubes of the OTSG are prestressed to place the tubes in axial tension in the non-operating state of the PWR in which the tubes of the OTSG are at a temperature of less than 100° C. by operations including:mounting the tubes in the tubesheets by expanding the tube ends to secure them to the tubesheets, whereby axial tension is imparted to the tubes. 12. An apparatus comprising:a pressurized water nuclear reactor (PWR) including a pressure vessel, a nuclear reactor core disposed in the pressure vessel, and a vertically oriented hollow central riser disposed above the nuclear reactor core inside the pressure vessel;a once-through steam generator (OTSG) disposed in the pressure vessel of the PWR, the OTSG including vertical tubes having a higher coefficient of thermal expansion than steel arranged in an annular volume defined by the central riser and the pressure vessel and secured in a support made of steel, the OTSG further including a fluid flow volume surrounding the vertical tubes;neutron-absorbing control rods; anda control rod drive mechanism (CRDM) configured to controllably insert and withdraw the control rods into and out of the nuclear reactor core;wherein the PWR has an operating state in which primary coolant at a temperature of at least 500° C. flows in the tubes of the OTSG and in which feedwater injected into the fluid flow volume at a feedwater inlet is converted to steam by heat emanating from the primary coolant flowing inside the tubes of the OTSG, and the steam is discharged from the fluid flow volume at a steam outlet;wherein the tubes of the OTSG are in axial tension in a non-operating state of the PWR in which the tubes of the OTSG are at a temperature of less than 100° C. and are in axial compression in the operating state;wherein the pressure vessel is divided into an upper integral pressurizer volume and a remaining lower interior volume, and in the operating state the upper integral pressurizer volume contains fluid at a temperature greater than a temperature of the primary coolant disposed in the remaining lower interior volume of the pressure vessel; andwherein no portion of the CRDM is disposed in or passes though the integral pressurizer volume. 13. The apparatus as set forth in claim 12, further comprising:a flow diverter disposed in the generally cylindrical pressure vessel above the central riser, the flow diverter having a flow-diverting surface facing a top of the central riser that is at least one of sloped and curved to redirect primary coolant discharged from the top of the central riser toward inlets of the tubes of the OTSG. 14. The apparatus as set forth in claim 13, wherein the flow diverter divides the pressure vessel into an upper integral pressurizer volume and a remaining lower interior volume, and in the operating state the upper integral pressurizer volume contains fluid at a temperature greater than a temperature of the primary coolant disposed in the remaining lower interior volume of the pressure vessel.