What I have heard from others that work with that plant is it is looking like a steam explosion, but in a system outside the boiler.
I agree, it is all torn up under the turbine. The front wall is blown out in front of the maintenance pod. No one is saying much so it is hard to know. The whole plant may close because of this.
Creep failure on a main or reheat line?
If it is a steam line it is possibly a high pressure steam to the turbine line or a reheat line to a low pressure turbine. Lots of pressure in them needs to be bled off through a dump line. If overpressure, he pop valves should have lifted first unless a bad weld gave way.
Other causes could be starting a coal pulverizer mill, hydrogen explosion in the generator.