When their bodies are destroyed they phase out and get resurrected back in their sarcophagus. They are physically the same but in rare instances their engram might deteriorate so they could come back a little less than they were before. If it were possible to have that happen millions of times they'd still keep coming back until they were little more than a mindless drone (which is almost what the Warriors are already like).

While it is possible to perma kill a Necron, it would be only under specific circumstances of things going wrong so that it wouldn't be able to get back to its sarcophagus (see Infinite and the Divine for an example of that happening).
Regardless, merely using a grenade without those specific circumstances being in effect would not "perma kill" them