Have a relative (also an ME) who recently went through Nuke Power School (submarine track) so will do my best to answer based on that experience.
First, a mid doesn't select NPS per se -- the service assignment is either submarines or nuke surface. If s/he wants to be a submariner, s/he should already have so indicated as this is a pre-selection process that starts 2/C year. IOW, you indicate you want to be a submariner and the submarine community decides if it wants you -- via interviews, etc. If DS hasn't already done this, he may still be able to select subs, but needs to sort that out asap with his leadership. Submariners almost always know well before service assignment that they'll be going subs b/c they've been pre-selected. Occasionally, USNA needs to get "volunteers" late in the process if the quota isn't met.
The other option is to go Nuke Surface. I believe (but am not sure), that the process is somewhat similar to subs in that there is pre-selection. However, I know that there are times when they "recruit" people for Nuke surface, even after service assignment as this happened to another relative (who decided to stick with regular SWO). More recent mids can weigh in on the above.
I believe that surface nukes first go to their ship and get qualified as a SWO, after which they go to NPS and more or less follow the path below. There were more than a few SWOs in relative's NPS class and most were LTs. As an aside, most did VERY well at NPS.
Once selected for the nuclear power program, submariners select their class date for NPS in Charleston. Class start dates range from June to October, though my family member's class was delayed until January. NPS lasts 6 months. After that, all nuclear-trained officers go to prototype (where they work on real reactors). Location for submariners remains in Charleston. Not sure where nuke surface goes. This school is also roughly 6 months long.
Near the end of prototype, submariners put in their preference for their boat. There are 6 geographic locations as well as 3 types of subs (SSN, SSGN, SSBN). My family member got 3rd choice in location and first choice in boat -- ended up with a fantastic boat in a great location (even if not first choice). The decision is mostly in the hands of Naval Reactors and a lot has to do with your performance at NPS and prototype. Nuke surface officers always go to carriers, but I don't know how their ship selection works.
After prototype, submariners go to sub school -- 3 months in Groton, CT. After that, they go to their boat. My family member who graduated in 2021 will go to the boat next week. Thus, training is about 15 months long but it can be "longer" if you select a later NPS date.
The commitment for nukes remains 5 years AD after graduation. There is no additional service obligation.
One of the big benefits for submariners is the extra pay. The career path also appeals to those with hyper-technical backgrounds, though humanities majors become submariners and do very well.
The first "sea tour" for submariners is 2.5 - 3 years long. After that, if they decide to stay in, they would go to a shore billet or to grad school. Grad school would involve an additional service obligation.