Root section (117, 217, 317) of a rotor blade (100, 200, 300) for interacting with working fluid (159, 259) upon rotating the rotor blade about a rotation axis oriented in an axial direction (155, 255, 355), the working fluid streaming in the axial direction, the root section of the rotor blade comprising: a curved cooling passage (221, 223) in an inside of the root section for guiding a cooling fluid within the root section from a radially inner end of the root section to a radially outer end of the root section, wherein a radial direction is perpendicular to the axial direction and pointing away from the rotation axis; a cooling fluid entry plenum (271, 371) having an entry aperture (273, 373) arranged at the radially inner end of the root section for introducing the cooling fluid into the cooling passage (221, 223); and a platform located at a radially outer end of the root section, the platform being in contact with the working fluid, wherein the curved cooling passage penetrates through the platform, wherein the following condition is satisfied in a portion of between 70% and 100% of a radial extent of the cooling passage: 0.25 * dr < rc < 1.5 * dr, wherein dr is a radial distance in the radial direction between the platform of the root section and the aperture (273, 373) of the entry plenum and rc is the radius of curvature of the curved cooling passage, wherein the cooling fluid is guided within the cooling passage from the radially inner end to the radially outer end of the root section such that the cooling fluid has a movement component in the axial direction and a movement component in the radial direction in a first, radially inner portion of the cooling passage, the cooling fluid has a movement component only in the radial direction in a second, radially middle portion of the cooling passage, and the cooling fluid has a movement component in a direction opposite to the axial direction and in the radial direction in a third, radially outer portion of the cooling passage. a curved cooling passage (221, 223) in an inside of the root section for guiding a cooling fluid within the root section from a radially inner end of the root section to a radially outer end of the root section, wherein a radial direction is perpendicular to the axial direction and pointing away from the rotation axis; a cooling fluid entry plenum (271, 371) having an entry aperture (273, 373) arranged at the radially inner end of the root section for introducing the cooling fluid into the cooling passage (221, 223); and a platform located at a radially outer end of the root section, the platform being in contact with the working fluid, wherein the curved cooling passage penetrates through the platform, wherein the following condition is satisfied in a portion of between 70% and 100% of a radial extent of the cooling passage: 0.25 * dr < rc < 1.5 * dr, wherein dr is a radial distance in the radial direction between the platform of the root section and the aperture (273, 373) of the entry plenum and rc is the radius of curvature of the curved cooling passage, wherein the cooling fluid is guided within the cooling passage from the radially inner end to the radially outer end of the root section such that the cooling fluid has a movement component in the axial direction and a movement component in the radial direction in a first, radially inner portion of the cooling passage, the cooling fluid has a movement component only in the radial direction in a second, radially middle portion of the cooling passage, and the cooling fluid has a movement component in a direction opposite to the axial direction and in the radial direction in a third, radially outer portion of the cooling passage.