Just to be clear, uranium is not literally burned in a nuclear thermal rocket. Unless things are very very wrong and you’re not going to space today. Nuclear thermal propulsion is simply using a fission (or more speculatively fusion) reactor to heat hydrogen gas which is then allowed to escape at high velocities from the business end of the rocket. In theory the heat source could be anything, but very few things offer remotely as much W/kg as nuclear.
Saying that nuclear fuels "burn" when you cause them to undergo fission in a reactor is a common colloquialism. Hence standard terms like "burnup rates" when describing the efficiency of reactors.
