[nine_k]

You don't need the reactor working, or assembled, on the way to the destination. Consider a pebble-bed design.

Digging underground is a reasonable way to get a massive layer of radiation protection without carrying it with you. This is important when you are just starting a long-term habitat. Later designs can of course be different.

[NikolaeVarius]

1) The issue isn't limited to the existence of the reactor. It is linked to the public perception of anything nuclear being launched into the air. A commercial/proven pebble-bed reactor doesn't even exist yet, and last I heard they only started the first design in 2018.

Digging underground is reasonable if your only requirement is "block radiation". Its less reasonable when requirements also include 'get the digging machines into space', 'have it survive landing', and 'learn all the new fun techniques required to dig into lunar regolith in a low g environment with never before tested or used techniques and equipment'

[m4rtink]

There are already big suitable underground spaces in the form of lunar lava tubes. Those should help massively with bootstrapping of any undrground lunar colony.