The Convair NB 36H wasn't actually powered by the reactor, though; its sole job was to validate that it was possible to shield the crew from the reactor's radia

The Convair NB 36H wasn't actually powered by the reactor, though; its sole job was to validate that it was possible to shield the crew from the reactor's radiation (which it did). However, there were certainly successful tests of actual nuclear-powered jet engines, like General Electric's X39 (a modified J47, and slated to be slapped onto that NB 36H to make a Convair X-6) and J87 (a different design for a different nuclear-powered bomber project, the WS-125). A key issue is the shielding necessary to keep such an aircraft from irradiating everything around it (you think neighborhoods bitching about nearby airports are ornery now...); that shielding is heavy. Accidents also make such craft much riskier, which is indeed a cited reason (other than costs) why the USA and USSR both cancelled their nuclear-powered bomber programs.