Rules Q for Skaven

[Antony]

To paraphrase:
A plague furnace is unbreakable so long as the priest is alive or it has some monks to push it.

a) Is this a correct rules interpretation? If not what happens when all the monks and priest are killed?
b) If it is what happens when it is broken in combat?

Cheers
Ant

[Simon]

I forgot to tell u, it was FAQed. Furnace and screamig bell are both unbreakable