Patent Number: 
Section: claims

1. An apparatus comprising:a pressurized water reactor (PWR) including:a pressure vessel including a cylindrical side wall defining an external annular recess, the pressure vessel defining and a sealed volume containing primary coolant water,a nuclear reactor core disposed in a lower portion of the pressure vessel and immersed in the primary coolant water, anda control rod drive mechanism (CRDM) disposed in the pressure vessel above the nuclear reactor core and immersed in the primary coolant water; andan annular steam generator disposed outside of and secured coaxially with the pressure vessel within the external annular recess, the annular steam generator including:tubes having ends in fluid communication with the sealed volume of the pressure vessel so that the primary coolant water flows therethrough,an annular wall being coaxial with the cylindrical side wall of the pressure vessel that together with the external annular recess of the pressure vessel defines an annular secondary coolant flow volume that is coaxial with the side wall of the pressure vessel, the annular secondary coolant flow volume containing the tubes and not being in fluid communication with the sealed volume of the pressure vessel, anda feedwater inlet and a stream outlet in fluid communication with the secondary coolant flow volume. 2. The apparatus of claim 1, wherein the annular steam generator encircles the nuclear reactor core and encircles the CRDM. 3. The apparatus of claim 2, wherein the PWR further comprises:an internal pressurizer disposed at the top of the pressure vessel and including a steam bubble volume and heaters for generating steam in the steam bubble volume, the annular steam generator not encircling the steam bubble volume. 4. The apparatus of claim 1, further comprising:an annular inlet tube sheet connecting upper ends of the tubes of the annular steam generator with the sealed volume of the pressure vessel; andan annular outlet tube sheet connecting lower ends of the tubes of the annular steam generator with the sealed volume of the pressure vessel;the annular inlet and outlet tube sheets, together with the annular wall and the pressure vessel, defining the secondary coolant flow volume,wherein the annular inlet and outlet tube sheets are coaxial with the cylindrical side wall of the pressure vessel. 5. The apparatus of claim 1, wherein the tubes of the steam generator comprise straight tubes and the annular steam generator is a straight-tube once-through steam generator (OTSG). 6. An apparatus comprising:a pressurized water reactor (PWR) including:a cylindrical pressure vessel defining a sealed volume containing primary coolant water and having a cylindrical side wall defining an external annular recess, anda nuclear reactor core disposed in a lower portion of the cylindrical pressure vessel and immersed in the primary coolant water, andan annular steam generator disposed outside of and coaxially secured with the cylindrical pressure vessel in the external annular recess of the cylindrical pressure vessel, the annular steam generator including:straight tubes disposed in the annular recess of the pressure vessel and having their upper and lower ends in fluid communication with the sealed volume of the pressure vessel via annular tube sheets at respective upper and lower ends of the external annular recess so that the primary coolant water flows through the straight tubes,an annular secondary coolant flow volume defined at least in part by an annular wall that is coaxial with the side wall of the pressure vessel and not in fluid communication with the sealed volume of the pressure vessel, the annular secondary coolant flow volume containing the straight tubes, anda feedwater inlet and a steam outlet in fluid communication with the secondary coolant flow volume. 7. The apparatus of claim 6 further comprising:secondary coolant water disposed in the feedwater inlet and a lower portion of the secondary coolant flow volume; andsecondary coolant steam disposed in the steam outlet and an upper portion of the secondary coolant flow volume. 8. The apparatus of claim 6 wherein the annular steam generator encircles and vertically overlaps the nuclear reactor core.