Lithium would be a light blue metal.
It generates electricity when in contact with acid and is conductive. Flammable, but only ignitable via water. When exposed to tempatures exciding 2,000 degrees and pressures exciding 20, It explodes with 4,000 degrees, 40 pressure and a neutron.
It is a solid that blocks heat and pressure, but is transparent to neutrons.
The nuclear device Castle Bravo used a Lithium casing. The engineers of the bomb expected a 10-megaton blast. They had no idea that the Lithium would actually react and increase the explosive power to 15 megatons.
This would be interesting, especially if it has another special property, such as conducting spark really fast (much more than gold). The explosion idea is really good, filling in the long-empty slot for an element that actually explodes with power. And it's really good for, say, realistic batteries for electronics.
Somebody really should code this, because it has a very low chance of getting in otherwise.
It's designed mainly for a nuclear bomb casing that will contain the explosion long enough for effeciency, then explode itself. Batteries and water-activated fuse's are more of a secondary feature.
It would be nice if the casing exploded at a high degree, and it reacted with what was in the contianer.
Well, these points have already been addressed, with the container going off at 4000 degrees along with pressure. It also releases a neutron, so it will react with most fusion bombs. The pressure and heat will help turn on the effects of uran and boyl as well.
All in all the perfect example of a good casing. I say we should add this.
edit: Blocking heat and pressure to a higher degree than ttan is a nice feature as well. Currently, the max we can block without a wall element is 1668°c