Nuclear pulse propulsion seemed to be quite actively researched up until the 1990's, with a few projects that saw some rather promising results (some that could theoretically reach over 4% light speed.)
However, it all seems to have gone a bit quiet since then - is there any ongoing research into this area, or have plans been shelved for now? For a very promising technology (on the surface at least) that could hugely cut journey times, it seems a shame if not much is being invested in it.