Abstract: In a liquid metal cooled fast breeder nuclear reactor of the pool type, the hot and cold pools (13, 19) are separated by an intermediate plenum (24) which, under steady state conditions, encloses a volume of substantially stagnant liquid metal to form a thermal barrier between the hot and cold pools (13, 19). To avoid undesirably large temperature differentials developing for example in the event of large temperature excursions within the hot pool (13) as a result of a reactor trip, the intermediate plenum (24) is designed to permit, in such circumstances, rapid interchange of liquid metal coolant between the hot pool and the intermediate plenum, for example by way of a thermal siphon arrangement (30).

Abstract: A tube-in-shell heat exchanger wherein the tube bundle has a central spine which carries a series of bracing grids for the tubes. The grids are resiliently mounted on the spine so that differential thermal expansion of one group of tubes relative to other groups of tubes and structure can be accommodated without inducing severe thermal stress.