127 It seems to me that they did get the powerplant to fit the aircraft, but they never flew with it on board and activated, and definitely never flew with it as a power unit. Long time memories, from when I was a kid.
According to
this the nuclear powered version was never built or converted, only the testbed to study the effects of the reactor on the instruments and pilots.
There is some speculation that the B-2 has a small nuclear reactor onboard, that suposedly powers an electrogravity system that reduces the mass of the plane and payload by 89%. See here