Has NASA tested any nuclear thermal rocket engine after dropping the NERVA program? Is there any current ongoing mission to use nuclear power to make a rocket fly?
As of 2012 (Nuclear and Emerging Technologies in Space meeting at LPI) the outline of development program is as follows:
See this pdf file for links to papers.
(note: only tangible test & evaluation items listed)
Manufacture and test fuel elements in two compositions: NERVA composite, and Uranium dioxide UO2 in tungsten ceramic-metallic "cermet". Testing will be conducted at DOE facilities and at Marshall (NTR Element Environmental Simulator)
Conduct safety tests at Nevada Test Site by injecting hot gas with Krypton-85 tracer to find out if future testing will not leak radioactive particles.
After that NASA and DOE will go for approval of testing by higher-ups in 2015.
If approved, full scale testing will be started at Nevada Test Site in late 2019.
Flight technology demonstration mission is scheduled for 2023.