Storage of heat generating materials

Heat-generating material, such as radioactive waste is stored above ground level in steel-lined bore holes (12) in a block (16) of concrete. A system of ducting (24, 26, 27) provides a natural circulation path for fluid between an external air-cooled condenser (28) and evaporation ducting sections (18) which run alongside the bore holes (12). The ducting sections (18) are directly connected to the bore liner (20) or are linked to the bore liner (20) by thermally conductive elements such as fins (22) so that there is a heat flow path of low thermal resistance, provided by the fluid circulation p@ and the thermally conductive elements, between the bores and atmosphere. The fluid is one which changes phase from liquid to gas at temperatures which do not deleteriously affect the concrete, e.g. 80 DEG C. The fluid may be water/steam @ sub-atmospheric conditions. <IMAGE>