Apparatus for supporting the lower elbow assembly of a jet pump in a reactor pressure vessel (RPV) of a nuclear reactor are described. The clamp apparatus includes a lower clamp element and an upper clamp element. The upper and lower elements are configured to be positioned at the interface between the thermal sleeve and the jet pump riser elbow. The upper and lower elements include extended ridges that are configured to fit in circumferential grooves precisely machined into the sleeve-elbow assembly on opposing sides of the interface.