A NUCLEAR-REACTOR STRUCTURE IN WHICH THE REACTOR CORE IS HOUSED IN A METALLIC ENCLOSURE SERVING IN PART AS A PRESSURE VESSEL AND AT LEAST IN PART AS A RADIOACTIVE SHIELD. THE WALLS OF THE STRUCTURE ARE MADE UP OF PRESTRESSED CAST-IRON BODIES WHICH MAY HAVE INTERFITTING OR INTERLOCKING FORMATIONS WITH ANY GAPS FILLED BY PRESSURE-RETAINING COMPOUNDS. STRESSING MAY BE EFFECTED BY RRODS OR CABLES RUNNING THROUGH THE CAST-IRON ELEMENTS OR BY EXTERNALLY POSITIONED CABLES, WIRES, BANDS OR RINGS.