Given the effeciency of nuclear energy, and the violent forces released by a nuclear explosion, I would seriously doubt that the accident at Chernobyl was an explosion. A reactor out of control, overheating, etc.... but not an explosion.
If the containment domes are designed to withstand a [i]nuclear[/i] explosion, then they are some feat of engineering.
Also, in the snippet posted, there was no reference to an explosion of any type that I could see. It simply mentioned that a U.S. type containment dome would contain the [b]releases[/b] from the accident.
Now, as to the credibility of the threat, I have to go with nnciderr9 that no threat was made on the WTC (either time if I recall) and if these guys were going to do something, they wouldn't threaten, they'd just do it.