More likely its for modeling nuclear weapons which I believe is the primary purpose of NSCI.

The story linked below about the French exascale plans explicitly states that it is the military applications division of the French nuclear energy agency (CEA) that is spearheading that effort. (I believe they work on both reactors and warheads.)

NNSA*, which is an office within the Department of Energy.

National Strategic Computing Initiative (NSCI) is the initiative within the DOE.

Serious question: what's left to model? They're already big enough to kill everyone, and we have heaps of them.

Often they model how the warhead ages, rather than how it explodes.

Yes, this is my understanding.