Earlier quoted context omitted.
A high school kid tried to build a nuclear reactor as a science project a while back, getting his mom's house designated as a superfund cleanup site. https://en.wikipedia.org/wiki/David_Hahn
He didn't create a nuclear reactor, this is a common misconception. It even says this in the wikipedia article. He basically got a bunch of radioactive stuff and put it together. He wasn't anywhere close to making a nuclear reactor let alone a nuclear weapon. For a weapon you need isotopes which he didn't have access to.
A real nuclear engineer with the knowledge he needed would also have said "no, don't do that and I won't help you." We are programming the knowledge into the ai agent. Giving ai a little discretion makes sense too.