A tube and shell steam generator for transferring heat from a primary fluid to a secondary fluid, the steam generator comprising: a primary fluid header closed at one end by a first side of a tube sheet and separated into an inlet plenum and an outlet plenum by a divider plate; a plurality of U-shaped hollow heat exchange tubes respectively having a diameter and a cold leg and a hot leg, with the cold leg and the hot leg connected by a U-shaped bend section at one end and terminating respectively in an inlet section of the hot leg and an outlet section of the cold leg at another end with the inlet section of the hot leg extending through the tube sheet and opening into the inlet plenum and the outlet section of the cold leg extending through the tube sheet and opening into the outlet plenum; a central tube lane on a shell side of the tube sheet, opposite the first side, and centered between and having a side respectively adjacent the hot legs and the cold legs of the plurality of U-shaped hollow heat exchange tubes, the central tube lane extending below the U-shaped bend section of the plurality of U-shaped heat exchange tubes; and characterized in that said generator further comprises: a plurality of elongated flow buffer rods extending within and on either side of the central tube lane in a direction substantially perpendicular to the tube sheet, the flow buffer rods not communicatin with the primary fluid in the primary fluid header and wherein the largest outside diameter of the flow buffer rods has substantially the same outside diameter as the U-shaped hollow heat exchange tubes at elevations along the U-shaped hollow heat exchange tubes and wherein the flow buffer rods are positioned to substantially shield the heat exchange tubes from being buffeted by water passage through the central tube lane. a primary fluid header closed at one end by a first side of a tube sheet and separated into an inlet plenum and an outlet plenum by a divider plate; a plurality of U-shaped hollow heat exchange tubes respectively having a diameter and a cold leg and a hot leg, with the cold leg and the hot leg connected by a U-shaped bend section at one end and terminating respectively in an inlet section of the hot leg and an outlet section of the cold leg at another end with the inlet section of the hot leg extending through the tube sheet and opening into the inlet plenum and the outlet section of the cold leg extending through the tube sheet and opening into the outlet plenum; a central tube lane on a shell side of the tube sheet, opposite the first side, and centered between and having a side respectively adjacent the hot legs and the cold legs of the plurality of U-shaped hollow heat exchange tubes, the central tube lane extending below the U-shaped bend section of the plurality of U-shaped heat exchange tubes; and characterized in that said generator further comprises: a plurality of elongated flow buffer rods extending within and on either side of the central tube lane in a direction substantially perpendicular to the tube sheet, the flow buffer rods not communicatin with the primary fluid in the primary fluid header and wherein the largest outside diameter of the flow buffer rods has substantially the same outside diameter as the U-shaped hollow heat exchange tubes at elevations along the U-shaped hollow heat exchange tubes and wherein the flow buffer rods are positioned to substantially shield the heat exchange tubes from being buffeted by water passage through the central tube lane. a plurality of elongated flow buffer rods extending within and on either side of the central tube lane in a direction substantially perpendicular to the tube sheet, the flow buffer rods not communicatin with the primary fluid in the primary fluid header and wherein the largest outside diameter of the flow buffer rods has substantially the same outside diameter as the U-shaped hollow heat exchange tubes at elevations along the U-shaped hollow heat exchange tubes and wherein the flow buffer rods are positioned to substantially shield the heat exchange tubes from being buffeted by water passage through the central tube lane.