Your design seems like an absolute safety nightmare. I'll also note that your safety idea of the little fuel pellets separating when the air flow is removed rather depends on them not being so hot that they start melting, which rather defeats the whole point of the exercise.
The underlying idea, though, that if you remove the tiresome need to avoid melting your reactor down into deadly slag you can run it really hot and so gain all sorts of efficiency gains and various other benefits is a) quite true and b) something that people have looked into, though not necessarily for the purposes you were thinking of.
Operating such a thing inside a gravity well, such as on the surface of Earth, is a pretty terrible idea, because any sort of failure will ruin your reactor as a load of very hot, intensely radioactive gloop sloshes onto the floor of the containment structure and melts and burns away til it cools off enough, or alternatively intense radioactive fissioning gas will rise up and plate itself all over the ceiling, burning and melting its way through as it does. The former would be indistinguishable from a meltdown, and a lot harder to avoid, and the latter is a whole new and exciting kind of catastrophe.
In space, however, you don't have to worry about gravity or even convection, if you don't want to. Hell, in an emergency, there's the option of blowing out the reactor core into space where it won't do anyone any harm (not recommended in orbit around a planet. or possibly even in a planetary magnetosphere. or near other ships. etc).
Behold, the Open-cycle Gas Core Nuclear Thermal Rocket!
(image from Project Rho, more information on this and many other nuclear rocket designs at the link)
You can run your reactor at a fairly toasty 55000K, and the only compromise you have to make is that some of the uranium plasma will leak out in with the hydrogen, but you can't make an omelette without breaking a few eggs, eh?