The next ices up are ice 2 and ice 3, both of which have densities
   greater than that of water.

   So the pressure of frozen ice might not ever rise above 10^9 Pa, mostly
   staying a little over 10^8 at most. I could be totally wrong.
   If I wanted to design an ice bomb that never exploded, I might make
   sure there were a structural part that failed before the pressure got
   too high, not really sure.