I'm still not seeing where this is fun to play with, though. If this is a solid, it won't be very useful for nuclear reactions - It won't be able to move/update according to nearby explosions.

We also already have several elements that function perfectly as nuclar bomb casings.

...However, none of them really hold up to our extreme explosion elements. It's usually not the pressure that blows casing up, but the temperature of all the debris flowing around. If you want something effective, have it block pressure and give it a temperature diffuse value of like 3 (about quartz rating)