and in stoppage of the associated pump as indicated in the foregoing the vessel of the nuclear boiler is provided with at least one residual heat exchanger 100 which is illustrated in greater detail in fig4 the design function of this exchanger is to remove the residual power when the normal heat extraction circuits are not available preferably the complete installation comprises four residual power exchangers 100 suspended from small shield plugs such as the plug 102 which pass through the roof slab 26 in leak tight manner the heat exchanger 100 comprises an outer shell 104 which is directly suspended from the shield plug 102 said shell is endowed with high mechanical strength in order to protect the internal components of the heat exchanger 100 in the event of a power excursion within the reactor vessel this heat exchanger is clearly of the sodium sodium type the secondary sodium is admitted through the duct 106 then into the lower space 108 which is limited by the tube plate 110 and passes through the heat exchange tubes 112 then finally passes out through the pipe 114 the heat is extracted from the secondary sodium through a sodium air exchanger of conventional type as represented by the stack 116 whilst the blower 118 and the tube coils 120 form part of the secondary sodium circuit the outer shell 104 has a lower extension in the form of a duct constituted by a first portion 122a which is rigidly fixed to the shell 104 and by a second portion 122b which is rigidly fixed to.