Compact nuclear reactor

An apparatus comprising:na 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.