hey there
you could try taking a look at the tutorial on sidefx, the one about using volumetric shaders, it liikes like an atom bomb
then using pop's and a copy sop, you could connect the age of the particles into the size of the metaball for the volumetrics. this way, just like dust, its emmitted of a small area, and then explodes, thats what i would do.
http://sidefx.vislab.usyd.edu.au/houdini_video/by_topic/effects/index.html [sidefx.vislab.usyd.edu.au]
at the bottom
hope this helps
all the best
brian