Abstract: A feedwater sparger assembly, and more particularly a thermal sleeve device, to prevent relatively cold feedwater from contacting the vulnerable surfaces of a feedwater nozzle in a boiling water nuclear reactor. Inner and outer concentric tubular thermal sleeve members (42,56) span the distance between the sparger (12) and a substantially cylindrical transition surface (34) machined into the feedwater nozzle (14) or pipe (22). At the first ends (46,64) of the tubular members, adjacent the transition surface (34), a packing means (50), is deformed against the transition surface (34). The second ends (60,62) of the tubular members are operatively connected to the sparger (12). One of the tubular members (42,56) is rigidly connected to the sparger (12) and the other tubular member is mounted on the sparger such that it can be longitudinally moved relative to the rigidly connected member.