Referring to toolz: I don't think it's possible to get an explosion, animated or otherwise, to look that good in Bryce. I could be wrong, but it seems to me that Bryce just isn't cut out for that sort of thing. Metaballs sound like a really good idea to me, and could work if you could find the right volumetric cloud material (Bryce does come with some great ones), but personally I'd be scared off by the render times. Hope you have a renderfarm ;)