I am watching a video on nuclear rocket engines, specifically the Nerva engine. It has a propulsion efficiency of 750-800 Isp versus chemical rockets which are only 350-400 Isp. The follow on NRX engine had a efficiency of 900 Isp.
Basically both were a graphite reactor surrounded by a beryllium belt. Liquid hydrogen was then pumped though the reactor and out the nozzle.
The engines were successfully tested on test stands. A working engine was built. IF the engine is so efficient, why has one never been built and used?
Is it as simple of the fear of having a huge dirty bomb on a launch pad?