Abstract: To provide intrinsic reactor safety, the fuel sub-assemblies of a nuclear reactor core are provided with reactivity control mechanisms operable, in response to temperature rises above desired limits, to interact with adjacent fuel sub-assemblies and effect radial dilation of the core in order to reduce reactivity. Various embodiments are disclosed in which differential thermal expansion of an operating device or devices (34; 40; 42, 44, 46; 50) is translated into radially outward displacement of a lever or levers (28) for contact with neighbouring sub-assemblies.

Abstract: A nuclear reactor construction comprising a fast reactor core submerged in a pool of liquid sodium in a primary vessel. A collecting tray for core debris is submerged in the pool below the core. The collecting tray comprises a base plate constituting a plane tube sheet while the wall of the tray constitutes an annular tube sheet. Coolant conducting tubes are end received in the tube sheets. An internal skirt of the primary vessel overlaps the wall of the tray.