NO!
Stackpoles, as such explosions have become affectionately termed by the community after a particular author who had a fetish for the mechs going sky high every time they died, are not possible. I refer you to the Tech Manual, pages 36-37, where it debunks the myth of the fusion reactor explosion in a humorous, clear passage titled "Fusion Engine Explosions: The Great Myth" that starts with the words:
"Fusion engine explosions: an urban legend that won't die. Let's see if I can kill it on this planet, at least. Where to start?" They elaborate by going into great detail about the function of a fusion reactor, and why it is literally impossible for it to explode like that, and how the engines function within the B-tech universe as well.
So no. Absolutely no. No huge explosions. When mechs die, they fall or sag with a huge, ground shaking thud. Ammunition explosions may tear the mech apart from within, but they don't "go nuclear" with a huge blinding white flash that destroys everything around them. They just explode normally, or fall over. If they do the Stackpole MW4 and MechAssault explosions in MWO, it will be a little silly. We don't need Stackpoles to have awesome looking destruction.
Edit:
tl;dr It's not canon for the mechs to explode with huge nuclear blasts. MW4, Mech Assault, and even games like Mech Commander were not true to canon in including this. Fusion reactors don't do that. Sorry to rain on the parade.
Edited by Furniture, 23 June 2012 - 07:32 PM.