Interesting question and responses. Here's my two cents:
My undergraduate is in languages and philosophy. After college I knew I wanted to work in natural resources management. I used my humanities degree to study law and I completed a paralegal certificate. That has served me very well. I went on to complete a master's in environmental policy and management. During the course of all this, I started to specialize in water resources and I found a real passion for that field.
But I've run up against a bit of a glass wall, because this specialization requires experience in law and also engineering. The top professionals in this field tend to have multiple graduate degrees, and there are even some who have both a JD and an engineering PhD. A lot of my technical work to date has given me entry-level engineering experience, even though those projects had nothing to do with my previous education. So I'm looking at doing a second master's in hydrology as a long term goal. I can continue to work in law and policy, and should I be able to jump over to the other side of the fence and get an engineering degree, well, it would be hell on wheels
In sum and from my own background as a standpoint, I would not recommend working in energy (especially nuclear.) Make sure you balance your academic passions with a cold look at economic realities. There is (and will be) very little appetite or public support for nuclear power so long as oil and gas remains dependable and profitable. Alternative energies are fun, but economically they are every bit as uncertain as domestic oil production was in the 1980s. (I hope some people know that reference.)
The best thing about being an undergraduate is that you have much more opportunity to change your mind than you ever will have in the future. If you need to chart a slightly different course, stick with engineering, but make the change now. At worst you will spend an extra year in college, which in the long run, will seem very insignificant.
Whenever you are faced with a choice, ask yourself "will this choice open more opportunities for me in the long run, or will it limit my opportunities and restrict my options?" As a general strategy, do your best to remain skilled, but more importantly, remain versatile and prepared for both changes in the economy, and also changes in your interests as you gain experience after college. (I highly recommend working after college for 2-3 years before you begin a grad program.) I firmly believe that "life is what happens when you are busy making other plans" and that is what you really need to be prepared for.