Disclosed are various torsional restraint systems for torsionally restraining a jet pump assembly, wherein the jet pump assembly includes a riser pipe, a transition assembly on the riser pipe, and a restrainer bracket connecting a first and a second inlet mixer to the riser pipe. The torsional restraint system includes a first contact member operatively attached to a side of the jet pump assembly and a second contact member operatively attached to the side of the jet pump assembly and disposed from the first contact member such that the first and second contact members restrain the riser pipe from rotating about a centerline of the riser pipe.