Choosing a Rate in the Nuclear Navy isn't much of a choice. If you choose to go into the Nuclear Navy, your rate (EM, ET, MM) will be given to you. You will be allowed to input what rates you want, however that doesn't mean you will get it. The selection of your rate will be made when you are in Bootcamp. I got lucky and choose EM, ET, then MM and got EM. Most who choose MM first, get MM. Regardless of your rate, you will still learn all the basic principles of each rate. Approx 50% MM, 30% EM, and 20% ET.
MM work and operate the steam/water/lube oil systems in the plant and the machinery associated with it. They perform maintenance on pumps, valves, air conditioners, and turbines. They have the most in depth knowledge of mechanical engineering principles.
EM work and operate on the electrical generation equipment and motors. They perform maintenance on the electrical systems in the plant such as switchgears, motor control centers, and generators (turbine and diesel). Most in depth knowledge of electrical engineering principles.
ET work and operate on the instrumentation of the plant/reactor. They perform maintenance on the reactor plant control systems. In depth knowledge of nuclear engineering and control/process principles.
ELT/MM control the chemisty of the steam and reactor plant. They also are the radiation technicians that do the monitoring of cleanliness (contamination) and personal exposure controls. In depth knowledge of chemical engineering and radiation principles. ELTs are trained to be MM first. At prototype, they are choosen to be ELT and will stay in the pipeline learn the ELT job.
Most of time a Navy Nuke is on watch operating the plant when underway (w/ some maintenance). In port most of your time will be doing preventive maintenance and some corrective maintenance (w/ some watchstanding).
Another good post for information:http://www.nukeworker.com/forum/index.php/topic,5193.0.html