Yet another fit of boredom inspired test here. Note I did use TMI but that's mainly because i'm not going to make a 6 chamber nuclear reactor bomb legit just to blow it up.
anyway, standard advice is to use 3 layers of reinforced stone to contain a blast, so I set up such, complete with water surrounding the thing as it would be in a legit setup and one small hole to shut the thing down. (so I could load fuel without it going *bang*) Anyway, it did contain the blast adequately and left a nice hollow shell. Didn't have chance to put a door in, had an oopsie with the water so it went off early
so yeah.. built a 2 layer reinforced stone box surrounding the reactor, complete with a mini-shroud covering the redstone shutoff, this time with a reinforced stone door. (one of the reactor chambers exposed for loading.
set that little bomb off and ran. The only real weak point was the redstone shutoff hole, the rest mostly held. There was a hole or two but no damage to the surrounding terrain.
as an aside: how much water helps? does it help putting water around the chambers as well or just the outer corners of the reactor?