Build a structure like the picture below. Make sure that the redstone dust is the last thing to place so that the piston won't extand. Then place a TNT in front of the piston and you will see that the piston extand and push the TNT. Then the TNT will become undestroyable and you can't ignite it through redstone signal.

If you try to break the TNT in Survival Mode, it won't drop.

This block can be pushed by pistons and sticky pistons. If you place a block near it, it will disappear and the block you just placed will move to the position of the undestroyable block.

Interestingly, if you put a piston in front of the TNT, you will find a undestroyable piston.