I don't think the overall shape would change (and it's cylindrical rather than spherical). Differences in rock density are trivial compared to the force being exerted. Density differences would likely cause the uppermost crust of the molten layer to be composed of the lighter sort of rock . . . which is good, since it's the denser rock that would hold the most radioactivity.

IIRC, dampers don't work that way. They affect whether or not the explosion happens and can stop radioactive decay while engaged. They have no control over the progress of an actual nuclear explosion, which is a macro rather than a micro event.