Repair assemblies and methods for a jet pump diffuser in a nuclear reactor are described. In one particular embodiment, the diffuser has a substantially conical shape, and the larger diameter end of the diffuser is engaged to a shroud support plate of the reactor. The diffuser further has at least a first portion and a second portion welded together at a weld joint. The assembly, in one embodiment, includes a first c-ring having a first frusto-conical portion and a second substantially planar portion, and a second c-ring having a first frusto-conical portion and a second substantially planar portion. The first and second c-rings are configured, in the one embodiment, so that when the first and second c-rings are operatively positioned, first and second openings in the first c-ring are substantially aligned with first and second openings in the second c-ring, respectively.