Stan Ulam figured it out in 1947. He realized that it was hard enough keeping a chemical rocket from melting and trying to contain a reaction one million times more powerful was simply not worth the trouble. At tremendous expense and risk a Nuclear Thermal Rocket would be only a little over twice as efficient as a chemical rocket. Ulam did not just think outside the box… he threw the box away and conceived the idea of a propulsion system an order of magnitude more efficient and one that would become even more efficient the larger the scale used.
Nuclear Pulse Propulsion (bombs) has long been the only propulsion system capable of enabling Human Space Flight Beyond Earth and Lunar Orbit (BELO). It is the only option for the foreseeable future. Nothing else will work. Nuclear Thermal Rockets are a dead end and always have been.