Heat-treating device for prolonging the life of a pressure vessel formed of thermally resistant material subject to time-dependent structural changes due to neutron fluence includes a heating device positionable within a pressure vessel adjacent to a wall portion thereof which is to be heat-treated; a mounting support for the heating device including a substantially cylindrical, heat-insulating holding member insertable into and removable from the pressure vessel through an opening in the pressure vessel; supply lines for an energy carrier supplying the heat; insulating means for keeping the heat away from heat-sensitive parts of the pressure vessel which are not to be heat-treated and for reducing heat losses during the heat treatment; a hollow substantially cylindrical thermal shield disposed in the pressure vessel and, with the inner peripheral surface of the pressure vessel, defining an annular gap; and a reactor chamber wherein at least part of the pressure vessel is located, the reactor chamber being otherwise empty during the heat treatment and having ceiling slabs disposed thereon for shielding the reactor chamber from the outside, the holding member being receivable in the pressure vessel in a location whereat it defines, with the inner peripheral surface of the thermal shield, a convection space communicating at upper and lower ends thereof with the annular gap, the heating device comprising a heating source and being secured to the holding member so that, with the heating device received in the pressure vessel, a flow of heating gas is traversible from the heating source through the convection space to an upper deflection space and, from the latter, downwardly through the annular gap to a lower deflection space and, from the latter, back to the heating source, in a closed circuit.