[en] A thermal stress analysis of the 300-MW(e) gas-cooled fast breeder reactor (GCFR) grid plate was performed to (1) assess the ligament stresses of the grid plate due to the thermal load generated by gamma heating from the reactor core, (2) determine the temperature-induced deflection of the grid plate and the resulting deformations of the core for reactivity changes and for space envelope effects on interfacing components, and (3) evaluate irradiation-induced swelling of the grid plate. A parallel study was conducted in which a theoretical method and a finite-element analysis were employed. The theoretical approach used a simplified model for the grid plate, and analyses were carried out using the concept of an equivalent solid material. The solid rim effect was added to account for the in-plane force generated by the rim due to a thermal expansion which was greater than that of the interior portion of the grid plate. The temperature in the grid plate varied in the axial and radial directions in an axisymmetric pattern. In the finite-element analysis, the perforated portion of the grid plate was treated as an equivalent solid plate with effective elastic constants, and at the outer rim the solid material properties were retained. Discrete temperature data were assigned at each mode of the finite-element model. The results of both approaches are discussed