Steam Generator for a Nuclear Reactor

A nuclear reactor steam generator, comprising:

• a top portion having three or more plenums disposed in a plane around a riser column, wherein:
• at least one plenum of the three or more plenums includes an
• approximately flat tubesheet that faces a bottom portion of the steam generator,
• and wherein
• the approximately flat tubesheet of the at least one plenum includes a plurality of perforations, and wherein
• the plurality of perforations changes in density between an area near an inner edge of the at least one plenum and an area near an outer edge of the at least one plenum.

A method of operating a nuclear reactor, comprising:

• conveying a working fluid from a first group of three or more plenums to a plurality of flowpaths;
• vaporizing the working fluid in at least some of the plurality of flowpaths, the vaporizing resulting, at least in part, from coupling thermal energy from a reactor coolant to the at least some of the plurality of flowpaths;
• and
• transferring the vaporized coolant to a second group of three or more plenums.