A method for sealing a leak between a control rod drive (CRD) stub tube (3) arranged in a reactor wall (2) and a tubular CRD housing (1) extending through the CRD stub tube (3) and the vessel wall (2). According to the invention, this sealing is obtained by deforming a suitably shaped element of memory metal below the transition temperature of the memory metal in such a way that it can be inserted, in the form of a sleeve (5) or a ring, into or around the CRD housing (1) where said housing passes through the vessel wall (2), or alternatively replace part of the CRD housing (1) at the passage through the vessel wall (2). Then when the memory metal is heated to above the transition temperature, it strives to resume its original shape which shall then have been such that the desired seal is obtained.