A nuke behaves similarly to TNT, however the explosion is a crater 5 times bigger than TNT.
Use the Nuke wisely...
Attempts at using the nuke in SMP using all different methods of triggering have shown it still doesn't work correctly and neither does ITNT.
Reinforced stone/glass is the most resistant material next to bedrock. Encasing a nuke in several layers should prevent most damage. Because of balance reasons, obsidian is nerfed down to a blast resistance of 60, removing its effectiveness.
It's not recommened to test over 15 nukes at once in a cluster on the same world you are building on, even if the testing place is very far away from your buildings. In the aftermath of all such big explosions, multiple water streams will emit from all affected nearby water blocks. Since the crater will be enormous if using 15+ nukes at once, there will also be a huge amount of water falling down in the hole, and also lava, from it's supply blocks, and a HUGE amount of loose items will be scattered around in the crater. Most computers can't handle it well, and aside from the explosion itself, the crater enviroment itself will mostly then cause a enormous lag. It will usually be better with time, and also getting a bit away from the crater may also get it better. I blew a 27 block nuke more than 300 blocks away from my two oil drilling towers (Also using buildcraft)on one of my new worlds. My small settlement is far away from the HUGE crater created, but I still suffers little to medium lag there, even if my computer is brand new. - []
Unlike Industrial TNT, placed nuke blocks can be breaked, just like ordinary TNT. Triggering nukes can be done in many ways, but many of them is surely fatal. A more safe, but hardy way, is to place a long, long redstone current to one or multiple nukes and detoneate from far away. The easiest way is to place Dynamite on a nearby block and detonate it with a Dynamite-o-mote from far away, but this can be very risky. You need first to right click the dynamite after placing it, and if do a small mistake, you might end up like...Coal Dust?
The Nuke is a secret recipe. Look for hints scattered around the Wiki :)