Nuclear Power School is still the most challenging academic achievement of my life. A summary of the program can be seen here.
My main reason for joining the Navy was to figure out how a nuclear power plant worked (perhaps I should have read a book). Nuclear Power School satisfied this curiosity many times over. The coursework spanned from "Principles of Learning" to an 86 page written final exam (no multiple choice)... all essay, rote recitation of equations and definitions, hand drawn diagrams and conceptual regurgitation. The coursework was extremely challenging but the results were equally rewarding.
Heat transfer and fluid flow, reactor theory, the neutron life cycle, decay modes of various isotopes were all there. The biggest issue is that we were required to derive equations without having the calculus underpinnings that I realized were so critical to a fundamental mathematical understanding once I had completed a Chemical Engineering degree. We learned system by system, why each valve, heat exchanger and other feature was configured the way it was and we learned to diagram all essential systems in a nuclear plant from memory.
Why was it so challenging? I suppose it was a bit due to a lack of maturity, study discipline (you learn it fast) and some was likely due to a lack of advanced mathematical knowledge. The most likely reason was the sheer volume of material. This is a 6 month 45 hour (lectures) a week program that required 25-40 hours of study weekly outside of class All lectures and study had to be done in the same classroom with strict silence policies. I learned educational discipline here... and a lot of it. We were all 18-20 year-olds and had many other things on our minds but once we checked into the building we were there for pure learning. The only other educational experience I had that learning was my only responsibility was medical school (I didn't have a job in medical school).
I still consider Nuclear Power School as one of my grandest and highest-impact accomplishments of my life.