Transitional phrases are one of those tools writers unconsciously use as a crutch. They seem like a natural way to move from one scene to another, but often as not, they only make your writing sound amateurish. Sometimes they simply can't be avoided, or can be tweaked to sound fresh. Otherwise, the best way to deal with them is probably not to simply replace one over-used phrase with another (i.e., “just then” with “the words had barely left his mouth”) but to stand back and look at your approach to writing scene transitions in general.
In the scene cited, you introduce the new event with the phrase “an explosion rocked the prison.” This phrase as a piece of narration is problematic in a couple ways. First, it’s as much a cliché as “just then” or “the words had barely left his mouth.” For that reason alone you should probably cut it out. But the real problem is that it’s pure narrative summary, (aka, telling.)
The way to segue into a new scene without falling back on a tired transitional phrase is to show the new event as experienced through your POV character, rather than simply telling the reader it has occurred.
For example, I’m standing minding my own business in some prison when somewhere in the vicinity something explodes. How do I experience that? Of course, it depends on various factors, mainly proximity to the explosion. But one way to approach relating my experience is like this:
Joseph terminated the feed. “They’re insane! They just put the fate of humanity solely in the hands of Emperor Bastardus!” A loud POP! broke the air. Joseph rocked on his feet, ears ringing. A siren wailed. “What the Hell was that?” Prisoners ran from their cells. “Explosion!” one of them yelled.
Far from perfect, but I hope it shows what I mean. The reader experiences the explosion along with Joseph rather than simply being told about it. The fact it was an explosion at all could be expressed later or via dialog as above.
Anyhow, just my take on a very thorny technical issue. I hope it helps.
Deck