A method and apparatus for physically restraining an inlet mixer of a boiling water reactor (BWR) jet pump assembly. An inlet mixer clamp assembly is attached to the top of a transition piece of the BWR jet pump assembly, to provides a downward clamping force on the top of the inlet mixer. The clamping force restrains the inlet mixer from experiencing vibration and mitigates leakage between the inlet mixer and the transition piece sealing surfaces of the jet pump assembly. The clamping force also restrains the inlet mixer from undesirable rotation, especially during operation of the jet pump assembly. The inlet mixer clamp assembly may be used as an alternative to conventional jet pump beam assemblies. Alternatively, the inlet mixer clamp assembly may be used as a redundant physical restraint in addition to the conventional jet pump beam assemblies.