A steam generator primary circuit coolant header comprising a horizontal heat-exchange bundle of U-shaped tubes wherein the U-shaped tubes are grouped into banks (11, 12) and separated by vertical inter-tubular tunnels; a thick-wall welded vessel with a central cylindrical part perforated with holes configured for installation and fastening of the heat-exchange bundle of U-shaped tubes therein, the central cylindrical part having a drilling area exceeding an area of holes by at least 20 %, wherein a lower cylindrical part of the welded vessel is configured for welded connection with a steam generator vessel of the steam generator primary circuit, wherein an upper cylindrical part of the welded vessel comprises a conical adapter configured for flanged connection to a manhole with a lid, and wherein the primary circuit header has an outer diameter D head in the central cylindrical part configured in accordance with the following ratio: 2 ⋅ d + 5.5 ⋅ n 1 + n 2 π + 100 ≤ D head ≤ 2 ⋅ 2 ⋅ n 1 − 1 ⋅ S Γ + 2 2 ⋅ B 2 − 2 4 ⋅ n 1 + n 2 ⋅ S Γ − R Γ ⋅ tg π 8 + 100 where: S h is a spacing between U-shaped tubes in the horizontal heat-exchange bundle, mm, B 2 is a width of the heat-exchange bundle opposite to the coolant header, mm, d is an outer heat-exchange tube diameter, mm, n 1 and n 2 respectively indicate a quantity of tubes in the horizontal row of smaller and bigger heat-exchange tube banks (11, 12), mm, R b is a minimum bend radius of the U-shaped tubes, mm, and holes for fastening of heat-exchange tubes in the perforated central cylindrical part of the header are staggered so that a distance between edges of a adjacent holes is not less than 5.5 mm. a horizontal heat-exchange bundle of U-shaped tubes wherein the U-shaped tubes are grouped into banks (11, 12) and separated by vertical inter-tubular tunnels; a thick-wall welded vessel with a central cylindrical part perforated with holes configured for installation and fastening of the heat-exchange bundle of U-shaped tubes therein, the central cylindrical part having a drilling area exceeding an area of holes by at least 20 %, wherein a lower cylindrical part of the welded vessel is configured for welded connection with a steam generator vessel of the steam generator primary circuit, wherein an upper cylindrical part of the welded vessel comprises a conical adapter configured for flanged connection to a manhole with a lid, and wherein the primary circuit header has an outer diameter D head in the central cylindrical part configured in accordance with the following ratio: 2 ⋅ d + 5.5 ⋅ n 1 + n 2 π + 100 ≤ D head ≤ 2 ⋅ 2 ⋅ n 1 − 1 ⋅ S Γ + 2 2 ⋅ B 2 − 2 4 ⋅ n 1 + n 2 ⋅ S Γ − R Γ ⋅ tg π 8 + 100 where: S h is a spacing between U-shaped tubes in the horizontal heat-exchange bundle, mm, B 2 is a width of the heat-exchange bundle opposite to the coolant header, mm, d is an outer heat-exchange tube diameter, mm, n 1 and n 2 respectively indicate a quantity of tubes in the horizontal row of smaller and bigger heat-exchange tube banks (11, 12), mm, R b is a minimum bend radius of the U-shaped tubes, mm, and holes for fastening of heat-exchange tubes in the perforated central cylindrical part of the header are staggered so that a distance between edges of a adjacent holes is not less than 5.5 mm. where: S h is a spacing between U-shaped tubes in the horizontal heat-exchange bundle, mm, B 2 is a width of the heat-exchange bundle opposite to the coolant header, mm, d is an outer heat-exchange tube diameter, mm, n 1 and n 2 respectively indicate a quantity of tubes in the horizontal row of smaller and bigger heat-exchange tube banks (11, 12), mm, R b is a minimum bend radius of the U-shaped tubes, mm, and holes for fastening of heat-exchange tubes in the perforated central cylindrical part of the header are staggered so that a distance between edges of a adjacent holes is not less than 5.5 mm.