nan means Not a Number. It's an expression used in programs that's basically a fancy way of saying "That math is so out there, I'm not even touching that. In TPT, the only way to get to nan is basically to cause the program to try to do something it's not capable of. Causing the ambient heat grid to go to infinity will cause particles to become nan heat, for example.
Also most lasers use a heating system that isnt touching the actual emitter of the laser so to keep the beam emitter from being effected by the heating system