The place Samantha intends to nuke in revenge for her daughter's death, which Carl immediately forbids.
After declaring that Tish is at fault for the death of Samantha's Daughter, Samantha states her intention plainly: after this is done she is going to plant the nuke she stole from Buster up at the college.
Carl refuses on the spot — she will not be nuking anyone — and redirects her to the far more useful subject of how to save Louis, which turns out to be exactly what she came to say.
The target of Samantha's threatened reprisal, invoked and immediately vetoed.
Also appears in books you've already read — spoiler-safe to their endings: