Not the poster, but I've personally found that old books from the 1950s and 60s about nuclear energy are very approachable. (Plus they have hilariously optimistic assessments of the future. It's cute, in a depressing way.)
"Principles of nuclear reactor engineering" by Samuel Glasstone is a commonly recommended text. If you do a little Googling you can find PDFs of it online, although out of respect for the authors I won't link them directly.
I have a 1955-ish print edition myself, and it's quite wonderful. The first third or so of the book requires no math and is the best overview of nuclear engineering that I've ever read, and then it starts to gradually ramp up the theory. At some point, unless you are coming directly from an undergrad physics program or a related discipline, you may find the math a bit daunting and decide to stop, but you'll still have a better understanding of the subject than probably anyone but a small percentage of humans.
The book does not, however, contain any information on nuclear weapons, such as you might find in a more modern text; at the time of its printing I suppose its inclusion would have gotten the book classified, since I don't think there was much available in the unclassified literature. So if that is an interest it may not be a good source. But this, too, adds a bit to the brave-new-world Atomic Era optimistic charm. I'd love to live in the world that Glassstone thought we'd be living in today from the vantage point of 1955.
I suspect he was being sarcastic. The fact that a 90 amu atom doesn't yield 131 amu atoms when it decays doesn't require you to know about anything beyond conservation of mass.
There are plenty of people whose nuclear physics isn't even up to that, so i think it was a reasonable question, even if the answer is "i paid attention in physics at high school and remembered it"!
The US Navy has multiple study guides to help prepare future nukes for school. You won't find any classified material or functionalities of the US Fleet, but it does cover the introductory work.
