A clamp assembly (19) for securing a thermal sleeve (9) and an elbow (11) in a pipe assembly in a nuclear reactor (1), and a nuclear reactor (1) with such a clamp assembly is described. The clamp assembly comprises a sleeve clamp (37) adapted to be arranged surrounding the elbow (11) at the first joint (17) between the elbow (11) and the thermal sleeve (9), and comprising sleeve support means (25) adapted to extend across the first joint (17) between the elbow (11) and the thermal sleeve (9). The clamp assembly (19) comprises at least one clamp arm (27) with a first end being arranged to rest against at least one of the core shroud (7) and the core plate support ring (6), to provide a support for the positioning of the sleeve clamp (37).