Couple of my friends were talking about motorcycles and a random though hit me...

What if yuo made a sphere of undestructable material..i dont know what material that is or if it exists...and you put an explosive with a timer in it(plutionium or whatever) and it explodes and has no chance of escaping the sphere...now i know the mass doesnt change when something goes from liquid to gas or whatever. But what if the mass does change(yet again i dont know if there are elements when comibed create a new one and change the overall mass but i think there is because i remember science class and there was something about that i think. So If the mass changes so much that it can't fit inside the sphere but also has no chance of penetrating the walls to get outside, what would happen ?