Patent Number: 
Section: claims

1. An apparatus comprising:a generally cylindrical pressure vessel defining a cylinder axis, the generally cylindrical pressure vessel comprising a sealing top portion, an upper vessel portion, and a lower vessel portion;a nuclear reactor core disposed in the lower vessel portion of the generally cylindrical pressure vessel;a steam annulus,a feedwater annulus,a central riser disposed coaxially inside the upper vessel portion of the generally cylindrical pressure vessel, the central riser being hollow and having an end proximate to the nuclear reactor core to receive primary coolant heated by the nuclear reactor core and an open end distal from the nuclear reactor core discharging the primary coolant;a once-through steam generator (OTSG) comprising a plurality of tubes arranged parallel with the central riser and disposed in an annular volume defined between the central riser and the upper vessel portion of the generally cylindrical pressure vessel, the primary coolant discharged from the open end of the central riser flowing through the OTSG and heating secondary coolant also flowing through the OTSG, the primary coolant and the secondary coolant being disposed in separate flow paths in the OTSG;neutron-absorbing control rods; andan internal control rod drive mechanism (CRDM) configured to controllably insert and withdraw the control rods into and out of the nuclear reactor core, the internal CRDM having all mechanical and electromagnetomotive components including at least a motor and a lead screw disposed inside the lower vessel portion of the pressure vessel. 2. The apparatus as set forth in claim 1, wherein the internal CRDM is non-integral with the generally cylindrical pressure vessel and disposed between the OTSG and the nuclear reactor core. 3. The apparatus as set forth in claim 1, wherein the generally cylindrical pressure vessel is vertical with its cylinder axis oriented vertically, and the internal CRDM is disposed below the OTSG in the vertical pressure vessel. 4. The apparatus as set forth in claim 3, further comprising:internal primary coolant pumps arranged to circulate primary coolant within the pressure vessel, the internal primary coolant pumps having all mechanical and electromagnetomotive components including at least a motor and at least one impeller disposed inside the lower vessel portion of the pressure vessel, the internal primary coolant pumps arranged below the OTSG to receive primary coolant discharged from the OTSG. 5. The apparatus as set forth in claim 4, wherein the internal primary coolant pumps are disposed peripherally around the internal CRDM. 6. The apparatus as set forth in claim 5, wherein a vertical height of the internal primary coolant pumps in the lower vessel portion of the vertical pressure vessel overlaps a vertical height range of the internal CRDM. 7. The apparatus as set forth in claim 3, wherein the internal CRDM comprises a plurality of CRDM units that are staggered at two or more different vertical heights such that no two neighboring CRDM units are at the same vertical height. 8. The apparatus as set forth in claim 1, wherein the internal CRDM comprises a plurality of CRDM units that are staggered at two or more different distances from the nuclear reactor core such that no two neighboring CRDM units are at the same distance from the nuclear reactor core. 9. The apparatus as set forth in claim 1, wherein a portion of the open end of the central riser extends vertically beyond the top elevation of the OTSG and further comprises a plurality of perforations. 10. The apparatus as set forth in claim 4, wherein the internal primary coolant pumps are arranged above the nuclear reactor core and wholly immersed in primary coolant. 11. The apparatus as set forth in claim 1, wherein a portion of the feedwater annulus has a larger diameter than the steam annulus. 12. The apparatus as set forth in claim 1, wherein the steam annulus is located above the feedwater annulus.