Abstract: A pipework junction (10, 11) has internal insulation to limit the effect of thermal shock in the form of a two part (14, 15) internal sleeve in which the parts of the sleeve are joined (34) and are, for the most part of their external surfaces, spaced from the junction by a small clearance (20) adequate to ensure that they do not lock to the junction when subjected to thermal shock. Piston rings (62) may be provided to restrain flow along the small clearance and the sleeve may be stabilized in the junction by having an expanded part (63) engaging a recess (64), which also serves to restrain flow along the small clearance.

Abstract: A liquid metal cooled fast breeder nuclear reactor comprising a nuclear fuel assembly submerged in a pool of coolant contained in a primary vessel is housed in a concrete vault. A core cover structure suspended from the roof of the vault and immersed in the pool of coolant comprises a plurality of vertical tubes braced apart by perforated grid plates. An intermediate grid plate has a permeable shroud which constrains coolant passing through the perforations in the grid plate to flow transversely over the upper surface to reduce the transient temperature differential.